Engineers and Mechanics Encyclopedia 1839: Railways: Mechanical Resistance of Railways

Having now noticed the principal arrangements in several different kinds of railways, and the motive power employed, we shall proceed to inquire into the nature and extent of the effects produced. The resistance to the motion of carriages arises from three causes, whether travelling on the common road, or on railways; but they vary in their relative proportions according to the nature of the surface passed over. Thus the resistance to the motion of a carriage on the common road, arising from the obstructions or inequalities of the surface, to the rolling of the periphery of the wheels, is greater than that of the rubbing at the axles; while on a railway, owing to the smoothness of the surface, the contrary is the case.

According to the experiments made by Mr. Stephenson and Mr. Wood, the resistance at the periphery of the wheels on a good level railway does not exceed about a thousandth part of the insistent weight, while the same kind of resistance upon an ordinary turnpike road, according to our own observations, does not average less than a twenty-fifth part; or forty times that of the railway.

It is from the reduced amount of this, the first mentioned kind of resistance, that railways possess such great advantages for locomotion; for in the second kind, that of the axles, the difference of friction cannot be material, nor can the resistance from air, (the third kind,) be at all different, presuming, of course, that the opposed surfaces and the velocities are the same in each.

Mr. Palmer, in his description of his railway, justly remarks, that if some accurate means of ascertaining the resistance of roads and railways were on all occasions used, their improvement would be much advanced. The real value of either being then unequivocally compared, the amount of defect could no longer be a matter of mere opinion. The proprietors would then know whether an apparent inferiority arose from the difference of horses, or difference of circumstances; and it would be of great advantage to introduce a clause in contracts, which would determine the effect to be produced.

The methods by which resistance of roads and railways has been ascertained, have not been sufficiently accurate, or have been too inconvenient for general use. The dynamometers, which denote the resistance by the degree of extension given to springs attached to the carriage, are convenient as portable instruments, but do, not denote the measure with the necessary precision. The resistances are not equable, from the irregularities of the surface; neither does the force which draws the carriage continue equable. When horses are employed, those instruments are of no service whatever. The effect of the unequal force or resistance occasions a vibratory motion to the indicating point, and we can never have confidence in any result they exhibit. Similar defects are observable in all the instruments I have seen.

"Having had frequent occasion to ascertain these resistances, I constructed an instrument which, by removing the imperfection referred to, has been completely successful. The problem was to make such an instrument as would indicate very small differences, but which would not yield suddenly to a change of resistance. I therefore connected to a spring dynamometer a semicircular close copper vessel, containing water; at the centre is a spindle, on which an arm or fan is fixed, and which very nearly corresponds with the inside of the vessel.

“The springs are so connected with the spindle, that they cannot be acted upon without the area or fan turning upon its centre, and passing through water. In order to pass through the water, the latter must escape by its sides; and the space being extremely small, it cannot pass rapidly, but will yield to the smallest force.

"By way of exhibiting the difference of resistance upon different railways, I have attached a table containing experiments on several.

"The first column contains the articles conveyed; the second, the resistance in proportion to the weight; the third, the whole effect produced, i. e. including the weight of the carriage by one horse, or one hundred and fifty pounds, at two miles and a half per hour; the fourth, the usual effect, or the load conveyed, in pounds; the fifth, the same, in ordinary measures; the sixth, the inclinations, expressed by decimal fractions, on which a railway, whose resistance is equal to that specified, should be constructed, that the resistance of the loaded carriages downwards may be equal to that of the empty carriages upwards; the seventh, the effect produced under such circumstances; the eighth, the useful effect under the same, the weight of the carriages being deducted.

“In each experiment, the power is assumed at one hundred and fifty pounds, moving at the rate of two miles and a half per hour. In the inclinations, the weight of the horse itself, as part of the effect produced, is not taken into account, that the table may equally serve where mechanical force is applied. Some allowance must therefore be made where horses are used, but the difference in the inclinations given will be very trifling.

The following table was published antecedently to the formation of the Manchester and Liverpool railway, the resistance upon which, on a level plane, may be considered as a medium between the two last-mentioned results, that is, about a two-hundred and thirty-fifth part of the weight.

It is also necessary that the reader should take into his consideration that the experiments given by Mr. Palmer, as respects his own railway, were conducted upon a well-made, full-sized model, while the others were probably, upon portions of rail considerably deteriorated by wear or neglect; for it is not otherwise possible to conceive so great a difference in the results, as are shown in the table; bearing in mind that they are all considered to be on a level, and that the surface material of all is iron.

Without being able to give any precise data for our opinion, our observation has from time to time led us to regard the ordinary resistance upon tram-roads to be not half that stated by Mr. Palmer; we therefore conclude, that the Surrey and Llanelly tramroads must have been in a very dilapidated state, or covered with dirt. One very important fact is, however, communicated with the following table, that of the great difference of resistance found upon the Cheltenham tramroad, by being merely slightly covered with dust, as it exhibits in a very strong light the superior advantages afforded by the edge-rail, in being so much less liable to the lodgement of dust.

Upon a reference to some of Mr. Wood's experiments, as detailed in his valuable treatise, we find that the results confirm our views as to the resistance upon plate rails. The rails he used were 4 feet long, 3.5 inches broad where the wheel runs upon them, and the height of the upright ledge 3 inches.

In an experiment made with two loaded carriages, each weighing 8,512 lbs., cast-iron wheels 39.5 inches diameter, 1.375 inch broad upon the rim which runs upon the rails, brass bearings 1.75 inch broad, and diameter of axle 2.625 inches, the resistance up a certain inclined plane was found to be 168, and down the same 126, making the mean resistance 147, which is equal to the 116th part of the weight moved.

With respect to edge-rails, it was usual, until recently, to estimate the amount of resistance at the two-hundredth part of the insistent weight; but the improvements which have of late years been made, both in the rails and the carriages, have reduced this resistance to about the 240th part of the weight; according to which, the following table has been calculated by Mr. Wood.

It becomes now an interesting point of inquiry to ascertain the extent of those several resistances to motion of which the foregoing table has given the total. For this purpose we are obliged again to resort to the ably conducted experiments of Mr. Wood; but as it would be impossible for us to give a detail of those experiments, or of the useful tables calculated there from, within the compass of our article upon this subject, we must content ourselves with a notice of the results derived there from, and to refer the reader who may be desirous of more precise information to the author's valuable work.

Mr. Wood found that the ratio of resistance to the rolling of the wheels upon a railway, was not increased by an increase of the weight in the carriage; and they were very nearly the same in velocity, varying from 5.50 to 14.45 feet per second; so that the resistance by the rolling of the wheels is an uniformly retarding force, both with respect to velocity and weight. Taking the resistance of the wheels as equal to the 1,000th part of the weight, and knowing the whole amount of resistance, we obtain that of the friction of the axles; applying this to the experiments detailed by Mr. Wood, the following results are given in that gentleman's work:-

We thus find that, in the above experiments, the resistance by the attrition of the axles amounts, in the most favourable case, to the 23d part of the insistent weight; or, taking the numbers 1 to 6, and the following experiments, equal to the 20th part of the weight; while, in some of the experiments on the empty carriages, the friction appears much greater; from whence we would be inclined to conclude, that the resistance is diminished by an increase of pressure. There is no subject in science, perhaps, on which there is a greater diversity of opinion than in the laws which govern friction; and the previous experiments, though sufficient, in many cases, for practical purposes, yet by no means tend to bring the inquiry into any more settled state.

In Nos. 1 and 6, and the following experiments, the ratio only varies (except in one instance) from the 19th to the 21st part of the weight: and as, perhaps, in the other experiments, the resistance of the wheels - the state of the axles - the construction of the carriages - or some other adventitious cause, might have operated to increase the friction, so as to induce us to leave these experiments out of the question, and take the former as the more correct amount; yet still this ratio is greater than shown by former experimentalists.

In, some experiments by Mr. Southern, in 1801, communicated to the Royal Society, and printed in the sixty-fifth volume of their Transactions, the friction of the axles of a grindstone weighing 3,700 lbs. amounted to less than the fortieth part of its weight. Now there does not appear any reason why, in well-constructed carriages, the resistance on the axles should be greater than in other machinery; and, therefore, we are obliged to conclude, either that the resistance of the wheels must be greater than we have assigned, or that there were some defects in the construction, either of the carriage or axles.

Under these circumstances, and considering the importance of obtaining the most correct information on the subject, Mr. Wood had an experimental carriage made, and fitted up with the utmost care; the axles and bearings of which were .of the best material, and were kept in use a considerable time before the experiments were made, to resister them as smooth as possible. The same wheels were used as in experiment 12, and the experiments were also made upon the same piece of railroad. Bearings of brass and cast-iron were both used to ascertain which gave the least friction; and the carriage was loaded with different weights to ascertain the relative resistance. The experiments, which were conducted with the utmost care, and repeated several times to obtain correct results, are given by Mr. Wood in a series of tables. We annex those which relate to the cast-iron bearings, as that metal evidenced in every experiment less friction than the above-mentioned alloy, to the amount of about one-thirteenth part.

From the facts contained in the first Table, the results in the second table are deduced.

From these experiments we find, that in a well-fitted up carriage, the whole resistance may be reduced to nearly the 500th part of the weight; and that, taking the resistance of the wheels upon the rails as equal to the 1,000th part of the weight, the friction of attrition at the axles amounts to no more than the 60th part of the weight, when the velocity of the surfaces in contact is equal to the progressive motion of the carriage.

The resistance from the above experiments being so much less than that previously found by practice in carriages on railroads, and in the proportion of 60 to 40 less than that found by Southern, Mr. Wood was induced to suppose there might have been some errors either in the experiments or calculations, though, in the prosecution of them, the utmost care was taken; and the uniformity of the result, in each of the experiments, almost proves that no error could have been committed. The degree of polish given to the axles, though nothing more than what was effected by using the best materials, and causing the carriage to be run up and down the railroad, with the axles lubricated with the best neat's-foot oil, may account for the great reduction compared with that of the former experiments; and, in this case, good neat's-foot oil was used and applied at the commencement of each experiment; whereas, in the former experiments, the grease commonly used on the axles of coal-waggons was used.

To put the question, however, beyond all doubt, and, at the same time, to ascertain more particularly all the phenomena of the friction of attrition, Mr. Wood had an axle fitted up, which was placed upon two chairs or bearings, by which the rubbing friction could be ascertained, independent of that of rolling. The axle was placed upon two bearings, at such a height from the ground as would allow a weight to descend 30 feet; a wheel was fixed in the middle of the axles 2 feet diameter, around which a cord was wound, to the end of which a weight was attached, and rings of lead were fastened upon the axle to vary the weight. In each experiment the cord was wound round the wheel, and the weight thus elevated precisely 30 feet from the platform; by withdrawing a pin the weight was then let free, and, falling 30 feet, unwound the cord, and put the axle and lead weights into rapid motion; the cord then detached itself, and left the axle to turn freely round, until the friction of the axles brought it to rest.

By a proper apparatus the time occupied, during each ten revolutions of the axle, was measured; as, also, the whole time, until it came to rest: by which means not only the absolute amount of frictions was obtained, but also the friction at different velocities; and by varying the weights from 1,331 lbs. to 4,140 lbs. the relative resistance, with different weights, was also ascertained. The principal object, however, of instituting this set of experiments, was to ascertain if the friction varied with the surface of bearing; and, if there was any, what size of bearing, subjected to a given pressure, produced the least resistance. With this view, bearings of 3, 4.5, and 6 inches, respectively, were used, the diameter of the axle being 3 inches, and, on each of which, the successive weights of 1,331, 2,465, 3,622, and 4,140 lbs. were placed. With these materials the number of experiments made were more than 600, and varied in every possible way, to leave no doubt as to the accuracy of the result; the weight, in each experiment, falling precisely 30 feet.

It is worthy of notice in this place, that a considerable variations in the number of revolutions was occasioned by different modes of applying the oil. The axle rested upon the chairs, without any cap or cover, as here shown, where A represents an end view of the axle, and B the chair. At the commencement of each experiment the axle was oiled in the usual way, with fine neat's-foot oil; but it was found, that unless the oil was continually feeding upon the axle as it turned round, the result was never the same, unless the oil was supplied in such quantities, that when the axle turned round, the oil was heaped up against it, as shown above, and thus kept up a continual supply to the axle. When that was not the case, although the axle was well oiled, yet, unless the oil was kept constantly feeding upon the axle as it turned round, a maximum effect did not take place. The following Table, being one of the series of experiments, will show the effect.

In conducting these experiments, the first four were made with the axle oiled, so as to keep it constantly feeding on, as shown in the figure. The weight being drawn up was liberated, and falling 30 feet, the respective number of revolutions were made before the axle came to a state of rest; the second column being the time in oscillations of a pendulum vibrating 300 times in 157 seconds.

At the end of experiment 257, the oil which was resting upon the bearing, heaped up, as shown in the figure, was merely removed, as cautiously as possible, so as to allow that which surrounded the axle to remain; the weight was drawn up as before, and falling precisely the same distance, the number of revolutions was, in that experiment, 189. No additional oil being applied, the weight was successively drawn up and liberated as before, and the number of revolutions were found, as shown in the table, until the end of the 300th experiment, when the number of revolutions, by the same moving force, was only 37; during the whole of which period the axle was never touched, no oil was applied, and none removed.

At the end of the 300th experiment, the axle was again copiously oiled, so as to feed on during the whole of the 301st experiment, when the number of revolutions were 265. The oil was then removed as before, when the number regularly diminished until the 323d experiment, when it was again reduced to 36; and when, in the next experiment, the oil vas applied as before, the number was increased to 278, by the same weight falling precisely the same distance, which, in the previous experiment, only produced 36 revolutions.

The oil used should be very fluid, so as to present the least resistance to the bodies sliding over one another, yet of sufficient viscidity to prevent them coming actually in contact. The fine purified Plumbago, prepared as described under our article Plumbago, seems to us well deserving the attention of the experimentalist on the friction of running axles. It has, heretofore, been used only in a very impure, and, consequently, ineffective state.

The following Table, being the result of part of the experiments previously alluded to, having been made upon the friction of axles alone, the bearing surfaces and insistent weights of which being also more varied, will show the comparative effect of different sized bearings, with the finest neat's-foot oil.

From the various experiments made by Mr. Wood on the friction of carriages, he arrived at the following conclusions; viz.

"That in practice we may consider the friction of carriages moved along railways as a uniform and constantly retarding force.

"That there is a certain area of bearing surface compared with the insistent weight, when the resistance is at a minimum.

"That, when the area of bearing surface is apportioned to the insistent weight, the friction is in strict ratio with that weight."

The area of bearing surface in the axles of carriages, calculated to give the minimum of friction, he found to be one inch to every 98 lbs. of the insistent weight.

We shall now proceed to the consideration of the retarding effects of the air to the motion of carriages, which, although inconsiderable at low velocities, presents a great resistance at high velocities, and becomes, at length, so considerable by a farther increase of speed, as to constitute, comparatively speaking, the only cause of resistance worth mentioning.

The author of a series of papers that were published in the Scotsman some years ago, has elucidated this part of our subject with admirable simplicity. "During high winds (he observes) this resistance is so considerable, that means should be taken to lessen its amount, first, by making the vehicle long and narrow, rather than broad and short; and, secondly, by giving the front a round or hemispherical form.

Let us suppose, then, that there are two steam vehicles, each weighing with its engine, fuel, and lead, 15 tons (or 30,000 lbs.) The one a steam-waggon, for conveying goods, 5 feet high and 6 feet, wide, and having, of course, a front of 30 square feet, which, in reference to the pressure of the air, is reduced to 15 feet by giving it a rounded form. The other, a steam-coach, for carrying passengers, is 8 feet high, and 8 feet wide; or 7 feet high, and 9 wide, presenting a front of 60 square feet, but reduced to 30 by its rounded form.

Now, still it is found, by experiment, to press with a force of 16 grains upon a body presenting a front of 1 foot square, and moving at the rate of 1 foot in a second, and the pressure increases as the square of the velocity. Hence, our steam-coach, when moving at 4 miles an hour, in a still atmosphere, would encounter a resistance from the pressure of the air of 2.25 pounds; at 8 miles hour the resistance would be 9 lbs.; at 12 miles an hour, 20 lbs.; at 16 miles an hour, 36 lbs.; at 20 miles, 57 lbs. The steam-waggon, presenting only half the surface in front, would experience only half the resistance.

Note.- To affect minute accuracy in calculations of this kind, is a mere deception. Fractional quantities are therefore rejected. In point of fact, the resistance increases rather faster than in the simple ratio of the surface, and the resistance of a sphere is less than the half of that of its diametrical section. On the other hand, the resistance increases in a ratio rather less than that of the square of the velocity.

"Let us assume, according to what we have already stated, that a power of 150 lbs. would just put the steam-coach in motion; then, if we allow an additional power of 33 lbs. for acceleration, making 183 lbs. altogether, we find that if the air did not oppose its progress, it would move over 43 miles in one hour. Now, since it is propelled only by a force of 33 lbs., as soon as the resistance of the air pressed it back with a force of 33 lbs., the acceleration would cease, and the motion become uniform. This would take place within 12 or 15 minutes, and, when the velocity had risen, to 14 or 15 miles an hour. With the steam-waggon, presenting only half the front, the velocity would become uniform at 22 miles an hour. Hence we see, that if we had a perfect calm in the atmosphere, we could impel 15 tons along a railway with a velocity of 15 of 22 miles an hour (according to the extent of surface the vehicle presented) by a force of 183 lbs."

The intelligent author next proceeds to compare the resistance on a railway with that in a canal or arm of the sea, in a calm atmosphere. Although this mode of treating the subject is somewhat irregular, yet it places the matter in such a striking and interesting point of view, that we think the digression will be excused. The force required to impel a vessel weighing, with her load, 15 tons, through water at different velocities, would be us follows:-

• At 2 miles an hour 50 pounds.
• At 4 miles an hour 200 pounds.
• At 6 miles an hour 450 pounds.
• At 8 miles an hour 800 pounds.
• At 12 miles an hour 1,800 pounds.
• At 16 miles an hour 3,200 pounds.
• At 20 miles an hour 5,000 pounds.

To ascertain the power required to move a waggon on a railway weighing 15 tons, we have merely to add to the power necessary to overcome the friction (150 lbs.) a few pounds more to balance the resistance of the atmosphere at the velocity proposed. For the steam-coach with 30 feet front, it would be as follows:-

• At 2 miles an hour 150 pounds
• At 4 miles an hour 153 pounds
• At 6 miles an hour 155 pounds
• At 8 miles an hour 159 pounds
• At 12 miles an hour 170 pounds
• At 16 miles an hour 187 pounds
• At 20 miles an hour 208 pounds

We may now combine the two tables into one, and exhibit the results in horse power, as well as pounds, reckoning one horse power equal to 180 lbs.

We see from this Table the astonishing superiority of the railway over the canal for all velocities above 4 miles an hour. Nearly three times as much power would be required to move an equal mass at 6 miles per hour on a canal as on a railway; 5 times as much power would be required at 8 miles an hour; 10 times as much at 12 miles; 15 times as much at 16 miles; and 24 times as much at 20 miles an hour.

It is evident, also, that an addition of power, too trifling to add any thing material to the weight of the vehicle, would raise the terminal or uniform velocity from 4 miles an hour to 20; and that, speaking practically, it would cost no more to command a velocity of 20 miles an hour on a railway than a velocity of one. Except for the chances of injury to the railway or the vehicles, there would not be the smallest reason for conveying goods, even of the coarsest kinds, at 4 miles rather than at 20 miles the hour.

But a perfect calm in the atmosphere is very rare, and vehicles intended for daily and constant use must be prepared to contend with the strongest winds. The power must therefore be increased to such an extent as to enable the vehicle to travel at its wonted pace in all weathers. Now, according to Mr. Smeaton, a "hard gale" is found to sweep along the surface of the earth at the rate of from forty to fifty miles an hour. This velocity, which would be increased to sixty or seventy by that of the steam-coach when travelling at twenty miles an hour, would produce a resistance of six hundred pounds upon the thirty feet of front of the steam coach, or three hundred pounds upon the front of the steam waggon. With a speed of eight miles an hour, the coach and waggon would encounter a resistance about one-half less.

The vehicles, however, should not be constructed entirely with a view to extreme cases; and, except for the conveyance of mails and other similar purposes, an average velocity of twenty miles an hour for vehicles of the weight and description mentioned would be secured by a power varying from 200 to 500 pounds; that is, from one fifth to one tenth of the power required to produce the same effect on water.

We see, however, that the resistance of the air, which, in vulgar apprehension, passes for nothing, comes to be the greatest impediment to the motion of the vehicles, and may, in some cases, absorb five parts in six of the whole power. Let it be remembered, at the same time, that this aerial resistance rises into consequence solely because the high perfection of the machinery (the vehicle and the road) almost annihilates every other. The atmosphere equally opposes the progress of the stage-coach, the track-boat, and the steam-boat; but the motion of these vehicles is comparatively so slow, and the power of impulsion required to overcome the other impediments so great, that the resistance of the air is disregarded.

There have been various propositions to construct the periphery and tire of carriage wheels, so that they may roll not only upon the common road, but also on iron railways: but if this were allowed, they would be kept in order but a very short time, owing to the injurious effects that would be produced by the gritty mud taken off the gravelled road and ground upon the rails. The temptation to use a railway in this manner is great; for the load which required a horse on the common road might be drawn by a man on the railway; thus enabling them to go with a greater speed, and yet with less injury to the horses.

Mr. Wood justly observes, that the object of all railroads being to present to the wheels of the carriages a smooth, straight, and level surface, all depressions or displacement of the rails therefore defeat the object for which such a road is formed; and, consequently, their formation must be on the principle of forming and preserving such a level and uninterrupted surface. The nature of the foundation upon which we have generally to form a railway renders this a task of no ordinary difficulty. Perhaps it is almost impossible to form an absolutely perfect railway according to the above principles; we most therefore endeavour to approximate as nearly as possible towards such a perfection.

Two modes of effecting this suggest themselves; either to form the joinings of the rails to the chairs in such a manner that the stone supports can adapt themselves to the yielding of the foundation, without disturbing the parallelism of the rail; or, that the stone supports be made of that size, and be so embedded upon the foundation, that the weight of the carriages shall not be capable of disturbing them; in which latter case the joinings of the rails to the chairs must be such that the actions of the carriages has not the power of deranging the continuity of the rail.

To carry the former of these modes into practice, and to preserve the continuity of the rail with ease and freedom, the stone should be capable of moving round, or assuming any degree of inclination to the line of the road that might occur in practice, without either straining the pins or distorting the ends of the rails. To effect this, if the pin be made the centre of motion, the underside of the rail should be a portion of the circumference of a circle, formed from the pin as a centre; the base of the chair could then be either the apex of a curve, or a circular cavity corresponding with the exterior semicircular surface of the rail. The stone might then be depressed an either side, without straining the pin or deranging the joints: or we might otherwise make the bearance of the rail upon the chair or pedestal the centre of motion; in such case the pin-hole should be a circular slit or opening formed from the bearing upon the chair as a centre; the pin being made exactly to fit this cavity in a perpendicular direction, would prevent the rails from starting upwards out of their proper position, and the semicircular slit would allow it to turn longitudinally; when the stone then became depressed towards one side, the chair could snore round without injuring the pin, or deranging the joints of the rails.

Innumerable forms of joinings might be devised, every one of which might, in some degree, effect the purpose intended; the essential consideration being to secure a continued and permanent parallelism in the rails, under every derangement that may take place in the supports on which they rest.

"It is not enough (adds Mr. Wood) that the bearing be such that the rails are all in the same plane, when the stones on which they rest are in good order, or in their proper position, parallel with the line of road: the parallelism of the rails should be preserved, when, by the yielding of the ground, or from any other cause, the stones are displaced from their proper position, and are made to form a considerable angle with the line of road. It would not have been necessary to have been thus diffuse on this point, had I not found that several, even of the most modern forms of chair, were evidently formed contrary to this principle; many with a view of causing the mode of joining, to keep the support or stone in its proper position, rather than allowing it to adapt itself to the unavoidable yielding of the ground on which it rests; but the least consideration will evince the futility of this, especially when tine yielding of the ground causes the stone to rest entirely on one side; it will at once be seen, that when the carriages come upon the rails, something must yield and give way, by the great strains thrown upon the fastening from the oblique actions of the weight.”

Mr. Stephenson has, in forming the greatest part of the Liverpool and Manchester railway, adopted the latter mode, and has endeavoured to obviate those difficulties and imperfections, by making the blocks very large, and embedding them firmly upon the surface of the road; in the hopes that the weight of the carriages will have no effect in displacing them. Where stone is readily obtained, though expensive in the first formation, this mode will, no doubt, be frond ultimately to be the most beneficial, especially if proper care is taken to keep the surface on which the stones rest dry, and free from water.

Upon public lines of road, where the traffic is considerable, it is highly advisable to avoid the necessity of any interruption, by having displaced blocks to set right again; and therefore it becomes the more necessary to secure their permanent stability in the first formation.

On the Manchester and Liverpool railway, the rails are each five yards in length, and weigh thirty-five pounds each yard. The rails are supported every three feet upon stone blocks, each block containing nearly four feet of stone. Two holes, six inches deep, and one inch diameter, are drilled into each block, and into these are driven oak plugs; and the cast-iron chairs, to which the rail is immediately fastened, are firmly spiked down to the oak plugs, forming a construction of great solidity and strength. On the embankments, where the foundations may be expected to subside, the rails are laid on oak sleepers; thus there are thirteen miles of the rail resting on oak, and the remaining eighteen miles on stone sleepers.

There are two double lines of rails, four feet apart, elevated above the ground rather more than an inch. The sectional form of the rail is represented in the subjoined cut at r; this figure is designed to exhibit the mode adopted by Mr. Stephenson in joining the rails to the chairs, which is deserving of notice. In passing the bars through the rollers, a lateral projection is rolled upon one side of the rail; and, on one side of the cheek of the chair, a cavity is cast, equal in size with the projection, as seen at a in the annexed figure. On the opposite side of the chair another cavity b is cast, for the purpose of receiving an iron key. When the rail is laid into the chair, the key is driven into the cavity b, which, pressing against the side of the rail, forces the projection a into the cavity on the opposite side, and thus effectually secures the rail from rising up.

Mr. Losh has a different mode of effecting this object. In this plan the projection is rolled on both sides of the rail, as shown in the annexed section; one of these projections enters a cavity, a, in the chair as in Mr. Stephenson's. On the other cheek of the chair a longitudinal cavity is cast to receive a key, but, as shown in the figure, it is a double one, acting at the same time upon the upper part of the projection on the rail, to force it down upon the chair, and against the side of the rail, to steady it, and force the projection on the other side of the rail into the cavity. By this mode of keying, if the rail works loose upon the chair, by driving the key, it can again be tightened.

The plan of fastening the rails by keys is infinitely, preferable to pins; as the latter are apt to work loose, and to secure them again permanently has been found a difficult task.

An opinion haying been extensively promulgated by the advocates for cast-iron rails, that those made of wrought iron, from their softness and fibrous texture, were liable to exfoliate and wear away fast, an investigation into the facts was very generally instituted, the result of which was decidedly favourable to the eligibility of the malleable rails. Mr. George Stephenson's report on this subject, corroborated as it is by other indubitable testimony, is deserving of attention.

"In my opinion," says Mr. Stephenson, "Birkinshaw's patent wrought-iron rail possesses those advantages in a higher degree than any other. It is evident that such rails can at present be made cheaper than those that are cast, as the former require to be only half the weight of the latter to afford the same security to the carriages passing over them, while the price of the one material is by no means double that of the other. Wrought-iron rails, of the same expense, admit of a greater variety in the performance of the work, and employment of the power upon them, as the speed of the carriages may be increased to a very high velocity without any risk of breaking the rails; their toughness rendering them less liable to fracture from an impulsive force, or a sudden jerk. To have the same advantages in this respect, the cast-iron rails would require to be of enormous weight, increasing, of course, the original cost.

"From their construction, the malleable iron rails are much more easily kept its order. One bar is made long enough to extend over several blocks; hence, there are fewer joints or joinings, and the blocks and pedestals assist in keeping each other in their proper places.

"On this account, also, carriages will pass along such rails more smoothly than they can do on those that are of cast-iron.

"The malleable iron rails are more constant and regular in their decay, by the contact and pressure of the wheel; but they will, on the whole, last longer than cast-iron rails. It has been said by some engineers, that the wrought-iron exfoliate, or separate in their laminae, on that part which is exposed to the pressure of the wheel. This I pointedly deny, as I have closely examined rails which have been its use for years, with a heavy tonnage passing along them, and on no part are such exfoliations to be seen. Pressure alone will be more destructive to the cohesive texture of cast iron than to that of wrought iron. The true elasticity of cast iron is greater than that of malleable iron; i. e. the former can, by a distending power, be drawn through a greater space, without permanent alteration of the form; but it admits of very little change of form without producing total fracture.

“Malleable iron, however, is susceptible of a very great change of form, without diminution of its cohesive power; the difference is yet more remarkable, when the two substances are exposed to pressure; for a force which in consequence of its crystalline texture would crumble down the cast-iron, would merely extend or flatten the other, and thus increase its power to resist the pressure. We may say, then, that the property of being extensible, or malleable, destroys the possibility of exfoliation as long as the substance remains unchanged by chemical agency. A remarkable difference, as to uniformity of condition or texture in the two bodies, produces a corresponding want of uniformity in the effects of the rubbing or friction of the wheel. All the particles of malleable iron, whether internal or superficial, resist separation from the adjoining particles, with steady equal forces. Cast-iron, however, as is the case with other bodies of similar formation, is both harder and tougher in the exterior part of a bar than it is in the interior. This, doubtless, arises from the more rapid cooling of the exterior. The consequence is, that when the upper surface of a cast-iron rail is ground away by the friction of the wheel, the decay becomes very rapid.

"The effects of the atmosphere in the two cases are not so different as to be of much moment. On no malleable iron railway has oxidization or rusting, taken place to any important extent.

"I am inclined to think that this effect is prevented, on the bearing surfaces of much used railways, by the pressure upon them. To account for their extraordinary freedom from rust, it is almost necessary to suppose that some diminution takes place in the chemical affinity of the iron for the oxygen or carbonic acid. The continual smoothness in which they are kept by the contact of the wheels, has the usual effect of polish, in presenting to the destroying influence a smaller surface to act upon. The black oxide or crust, which always remains upon rolled iron, appears to act as a defence against the oxidizing power of the atmosphere, or water. This is the reason why the rail does not rust on its sides."

According to Mr. Wood, practice seems to have established the fact since the above was written, that there is no waste or destruction from oxidation or exfoliation, and that the wear is less than its cast-iron, subjected to the same action.

A more severe test of comparison in the wear of wrought and cast-iron, exists in wheels made of the two materials; locomotive engine-wheels of the latter material generally become, by wear, unfit for use in nine months, while the wrought-iron tires have worn its some cases three years, and are not yet unfit for use.

One phenomenon in the difference, in the tendency to rust, between wrought-iron laid down as rails, and subjected to continual motion by the passage of the carriage over them; and bars of the some material, either standing upright or laid down, without being used at all, is very extraordinary.

A railway bar of wrought-iron, laid carelessly upon the ground alongside of one in the railway in use, shows the effects of rusting in a very distinct manner. The former will be continually throwing off scales of oxidised iron; while the latter is scarcely, at all affected.

The first cast-iron rails were by far too weak. Scarcely any of the rails laid down twenty years ago are in existence; this is partly owing to the increased weight now carried upon the rails, and partly to the mistaken policy in the saving by the lightness of rails, to keep the cost below that of the wooden way.

It seems necessary that the rails should be made considerably stronger than merely to support the weight they have to carry. The blows they are subjected to, from the unevenness of the road, transferring the weight alternately from one side of the carriage to the other, and the side shocks from projections upon the sides of the rails, all have a tendency to snap in two the cast-iron, or bend the malleable iron rails. We shall have occasion to introduce some more remarks on this part of our subject, but as they have relation to a more advanced stage of improvement than had been attained at this period of time, we shall here resume our chronological narration of the progress of invention.

Many ingenious contrivances have been devised to enable (what has been termed) a carriage to carry its own railway. The generality of these inventions have been turned to very little account; partly, in some cases, from their inherent defects; and partly, in others, frees their being only useful under circumstances which rarely occur, in countries like our own, wherein mechanical skill and industry have done so much to mend our ways. Nevertheless some of these contrivances exhibit such admirable combinations of parts that they are ultimately rendered subservient to other uses than those which their inventors designed them for. It not infrequently happens that the general benefit is more advanced by an original clever invention, that has failed in accomplishing the object intended, than in one of the little every-day ameliorations which perfectly succeeds. Original combinations of genius, founded upon correct scientific knowledge, see are disposed to venerate as the result of a power that has been bestowed upon us by the beneficent Author of nature, to imitate, for our particular uses, his glorious works. We are therefore indisposed to pass by unnoticed such inventions, because they might have failed in their object on first application; and believing, with the late Sir Humphry Davy, that "a history of failures invariably shortens the road to success," in mechanics as well as chemistry, we hesitate not to give the matter insertion without more apology.

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