1851 Great Exhibition: Official Catalogue: Class X.: Ordnance Survey Department
4. ORDNANCE SURVEY DEPARTMENT. — By Lieut.-Col. HALL.
Base-measuring Apparatus.—Two compensation bars, and one connecting compensation microscope.
These compensation bars and microscope form part of a base-measuring apparatus, invented by Major-General Colby, Royal Engineers, formerly Superintendent of the Ordnance Survey.
Description of Compensation Par.-1. The compensation bar consists of two bars of brass and iron, 10 feet 1:5 inch long, 0.5 inch broad, and 1.5 inch deep, placed 1.125 inch apart, supported on brass rollers, at one-fourth and three-fourths of their length, and firmly fixed together at their centres by transverse steel cylinders 1.5 inch in diameter, and being free to expand from or contract towards their centres independently of each other. At the extremity of, and at right angles to, each of these bars is a fiat steel tongue, 6.2 inches long, 1.1 inch broad, and 0.25 inch thick; projecting 3.25 inches on the side of the iron bar, and moving freely on conical brass pivots, riveted into the brass and iron bars, each axis being perpendicular to the surface of the tongue, allowing it to be inclined at slightly different angles to these bars, according to their expansion from, or contraction to, their centres. The centres of the two axes are at 0•5 inch and 2.3 inches from the end of the tongue next the brass bar. On the tongue, and flush with its upper surface, near the extremity, is inserted a silver pin, with a dot marked on it, as the compensation point.
The bars are placed in wooden boxes (made of well-seasoned straight-grained deal), to the bottoms of which are fixed the plates that hold the brass rollers on which they are supported, and having in the middle a vertical brass stay, screwed to the box, and passing upwards between two steel cylinders, to prevent the bars being moved longitudinally in their casing. To protect the tongue carrying the compensated point (which projects beyond the wooden box) from injury, nozzles are fixed to the boxes, having a small circular orifice with a lid on the upper side to allow the dot, or compensation point, to be seen.
On one side of the connecting steel cylinders, and attached to the brass bar only, is placed the longitudinal level, the lid of the box being furnished with a glass window and shutter, to enable it to be observed. Over the rollers which support the bars are two pieces of metal, for preventing any sudden jar from striking the bars against the lid of the box. At each end, on the outside of the bar-box a thick metal plate is screwed, for the purpose of firmly fixing a three-armed groove-stand, intended to support the tripod of the compensation microscope; and at each end of the box are two vane sights (which shut down with hinges into grooves), used for placing the bars approximately in line.
On both sides, at one-fourth and three-fourths of the length, are brass plates, with holes for receiving the screw which clamps the plate of the tripod-stand (technically called a camel) to the box, for the purpose of adjusting the bar in a longitudinal direction. The compensation bars are six in number; the weight of each bar, with its two brass ends, is 136 lbs.
Description of Compensation Microscope.-2. The compensation microscope consists of three microscopes, placed three inches from centre to centre, connected by two bars of brass and iron, 7 inches long, 0.6 inch broad, and 0.375 inch thick, 2.5 inches apart, firmly secured together by means of a brass collar and cylinder, forming part of the tube of the centre or telescopic microscope.
The two bars, carrying with them the outer microscopes, of two inches focal distance, being free to expand from, and contract towards, the central microscope,inde- pendently of each other; and thereby forming with it small angles of inclination similar to the steel tongues of the compensation bars. The compensated point of each is so adjusted as to be in the outer focus of its object glass. The microscopes revolve on the axis of the telescopic microscope in a tube fastened to a horizontal plate attached to a tripod-stand with levelling screws, and furnished with longitudinal and lateral adjusting screws. On one side, secured to the brass bar, is the spirit-level, for levelling the microscopes, and on the other, firmly fixed to the centres of the bars by a brass plate, is a telescope, embraced by a brass collar, with a small cylinder projecting from one side, which turns in a socket attached to the plate; thus affording it a vertical motion, allowing objects to be seen in opposite directions. The telescopic microscope is provided with an adjusting screw, for altering the focal distance within certain limits, as well as moveable object glasses of different focal lengths fitting into the lower end of the tube. The compensation microscopes are seven in number, the weight of each being 7 lbs.
[All the methods adopted in the measurement of base lines in trigonometrical surveys which had been in use previous to the survey of Ireland, depended more or less for their accuracy on the knowledge of the temperature of the bars, etc., used in measuring; but as the time which substances occupy in heating or cooling is dependent upon their nature, mass, etc., it did not appear that any application of thermometers would give the true temperature of a bar throughout its whole length, particularly when the temperature of the air itself was undergoing constant change.
This circumstance led to the application of the principle of compensation used in gridiron pendulums, to devising an apparatus for measuring a base line. As metals have different capacities for heat, and their surfaces have different powers of radiation, experiments were made to equalize the effects of varying temperature in the brass and iron bars; and for this purpose the brass bars were bronzed and varnished, and the iron bars were browned, lacquered, and smoked, and the amount of lamp-black so produced was gradually removed, on successive experiments, till the desired effect was obtained. — J. G.]