“A method of converting circular into rectilinear motion with an extremely low velocity has been discovered by M. Prony. It is by means of this contrivance that we are enabled to avoid the necessity of using screws of an unusually fine thread, to procure a slow adjusting motion. The disadvantages arising from the use of such screws were formerly very great; the rapid wear and consequent inaccuracy of the usual micrometers was an instance of the ill consequences produced by that circumstance. It is also capable of many other practical and useful applications. The original idea of the inventor is extremely simple and elegant.

AB is an axis or spindle divided into three portions ab, cd, ef. The two screws ab, ef are of the same thread: they pass through the two fixed supports CD, in each of which there is a nut; the spindle has an horizontal motion, and at each revolution of the screw moves over a space equal to one of its threads; the portion cd; is formed into another screw, the thread of which may either be a little finer or a little coarser than that of the screws ab, ef, and the difference may be small at pleasure. A nut M is introduced, in which the threads of a micrometer are fixed: this nut being checked by the block EF, is not at liberty to turn with the spindle AB, which it would otherwise do: but at each revolution of the spindle moves a space equal to one of its own threads, its rate of motion is therefore compounded of the actual and relative advance of the spindle AB: so that it really moves but the amount of the difference between those motions.

This is M. Prony’s simple and ingenious solution of this problem.

1. ↑ M.M. Lanz & Betancourt, translated from the original French by Lanz, Philippe Louis (1817) Analytical essay on the construction of machines, London: R. Ackermann, S. text pg. 14−15, drawing pg. 181 Plate 1 fig D3