Samuel Varley of Clerkenwell was a watchmaker, instrument maker, and scientist.
Born 24 Oct 1744.
1794 Samuel Varley founded the Chemical and Philosophical Society at Hatton House, one of the forerunners of the Royal Institution where his nephew Cornelius helped with chemical experiments and public lectures.
1800 Varley gave up his business and moved to Chevening to assist Lord Stanhope in his projects, including the stereotype and printing press. Stanhope made it worthwhile for Varley to leave his prosperous career in London, and they worked together for 16 years.
1817 The will of the late Lord Stanhope included this: 'To Mr. Samuel Varley, of Chevening, Kent, £1,000, and all my tools, machines, machinery, and instruments, mathematical and astronomical, chymical and mechanical, save and except such only as are bequeathed to Mr. Robert Walker. But in case the said Mr. Varley should not survive me, I give the said beforementioned £1,000 and said tool: and machinery, to Miss L. Varley, eldest daughter of said Samuel Varley ; but in case both of them should not survive, then the said £1,000, tools, and machinery, to go to Jane Varley, second daughter of the said Samuel Varley.' 
1822 Death notice: On Thursday, the 18th instant, aged 78, Mr. Samuel Varley, formerly superintendant and constructor of the mechanical apparatus requisite for the inventions and pursuits of the late Earl Stanhope. 
1822 Death notice: 'Thursday evening, at his residence in Newman street, Mr. Samuel Varley, in his 78th year. He was a man of extraordinary talent, very extensive acquirements, and sound judgment. Born in humble life, and brought up at a village in Yorkshire, he distinguished himself by his scientific pursuits, and was actually driven thence by the vulgar, under the opprobrious character of a Conjuror. In London (his retreat) be became a public Lecturer on Natural and Experimental Philosophy, in which capacity clearness and simplicity of his demonstrations gained him the attention of many who have since moved in the higher walks of science. For many years be was the scientific associate of the late Earl Stanhope, and has through life maintained the character of a Philosopher and a Christian.'
1825 Samuel Varley's method of cutting screws in the lathe was described by his nephew Cornelius Varley . The lathe spindle hadh several short threaded collars which provided a range of thread pitches (four are shown in the illustration accompanying the article). The lathe was equipped with a sliding flat bar located alongside the headstock, guided to move parallel to the lathe axis. Fitted to the left hand end of the bar was a guide pin, which could be pushed into contact with the selected threaded portion. The right hand end of the sliding flat bar carried a tool holder. This held a form tool (a chasing tool) which was manually pushed against the workpiece to cut the thread. The guide pin and the toolholder were supported on the underside. Varley probably developed the method before 1800.
Samuel Varley also developed a lathe for optical turning, grinding lenses, watch-jewelling, and other purposes. Cornelius Varley added some improvements, mainly related to the belt drive arrangements, and described the lathe in 1829. The toolholder was a prismatic bar carried in a swinging frame, providing accurate guidance for facing cuts. The position of the prismatic bar in the frame, and hence the depth of cut, was set by a screw with a graduated collar. On this lathe, Varley introduced taper fits to locate and secure the lathe chuck on the spindle and to hold the tool in its holder. Varley determined that the optimum taper was 4 degrees.
Sources of Information
-  'Reckoning with Matter: Calculating Machines, Innovation, and Thinking about Thinking from Pascal to Babbage' by Matthew L. Jones, Chicago, 2016
- Military Register, 26 March 1817
- Sun (London) - Monday 22 April 1822
- Star (London), 24 April 1822
-  Varley, Cornelius. "No. III. COPYING SCREWS BY THE LATHE." Transactions of the Society, Instituted at London, for the Encouragement of Arts, Manufactures, and Commerce, vol. 43, 1824, pp. 90–93 and Plate VJSTOR, www.jstor.org/stable/41325689. Accessed 15 Sept. 2020
-  Gill's Technical Repository, Volume 3 by Thomas Gill, 1829, p.307ff. & Plate VIII