This interesting Machine, see [Plate 12], [fig. 11], consists of a screw divided into three parts, a, b, c; the first, a, is a mere cylinder to centre the screw at that end: c is a screw of (suppose) 20 threads to the inch; and b another screw of 21 to the inch. D E represents the frame of the Machine, the part E being the fixed nut of the screw C, while the piece f g, forms the moveable nut of the screw C, carrying a finger g, along the graduated bar, E g D. If now, the screw be turned once round by the button H, it will have moved to the left 1⁄20 of an inch; while the nut and it’s finger g will have progressed on it’s screw 1⁄21 of an inch to the right: and the difference 1⁄20 - 1⁄21 = 1⁄420 of an inch is what the nut f has really moved to the left, along the bar E g D. If therefore, the rim of the button be divided into 100 parts, one of these will represent 1⁄42000 part of an inch by this Micrometer: and I need not add, that this minute portion may be rendered still more minute at pleasure. The means of doing this are evident: It is only making the screws b and c nearer alike in fineness, or number of threads per inch; as 29 and 30, 30 and 31, &c.
I hope it will be understood, that I do not give any of these Machines as the only examples I could furnish of the application of the principles on which they are founded. This very Machine is not a Micrometer only; it might be (if made in proper forms and dimensions) a vice, a press, or other power Machine. It has been already hinted, that change of form must remain to be considered hereafter.
I have chosen to bring forward this Machine at an early stage of the work, because it has, inadvertently perhaps, been ascribed to another person. I refer to an article in the celebrated programme of M. Hachette, of Paris; with which is combined an essay on the composition of Machines, by Messrs. Lanz and Betancourt. In the article D 3, at page 10 of that work, are the following words:
“M. de Prony a trouvé une maniere de transformer le mouvement circulaire, en un autre rectiligne dont la vitesse soit aussi petite que l’on voudra;” and further on—“l’idée en est extremement simple, heureuse; elle est d’ailleurs susceptible de plusieurs applications utiles aux arts.” And in page 11, are these words—“C’est ainsi que M. de Prony est parvenu à une solution aussi simple qu’ingenieuse du probleme qu’il s’etoit proposé.”
For the sake of my English readers, I subjoin a translation of these passages: “Mr. de Prony has found (or invented) a manner of transforming a circular movement into a rectilinear one, of which the velocity shall be as small as may be desired;” and further on “This idea is extremely simple and happy: and is besides, susceptible of several useful applications to the arts.” And in page 11, are these words—“Thus has Mr. de Prony given a solution as simple as it is ingenious, of the problem which he had proposed to himself.”
The above account appears in 1808, and M. de Prony does not prevent or disavow it. Perhaps he had forgotten the circumstance: and perhaps he did not know of this publication: but I solemnly declare that I shewed HIM this Micrometer, executed, fourteen years before! that is, while he and M. Molard were making their report on the Machines proposed for the Water-works at Marly. I certainly wish to accuse no body in this affair: but if I did not state the fact as it is, I should, myself, be stigmatized as a plagiary! I am forced, therefore, to take my stand on the adage—“Fiat justitia ruat cœlum.”
In closing the first Part of this Work, I cannot but express my gratitude for the unexpected degree of support, with which my numerous Subscribers have honoured me. I presume to offer these pages as a fair Specimen of what they may expect in the four succeeding Parts,—namely, as it regards the execution: for the materials of what remains, include objects of greater importance than those preceding. If I have been fortunate enough to raise any favourable expectations in the minds of my present readers, I hope they will express those feelings; and thus induce others to join in bringing to a useful close, a work which is at least intended to produce unmixed public utility. From criticism, I expect candour: and should my intentions, though pure, be misrepresented—should envious tongues or pens assail my labours, or asperse my character, I will defend both, after I can use my Book as my shield—that is, after I have fulfilled my Engagements with my Subscribers: of whom (in expectation of meeting them again within three months) I now respectfully take leave.
J. W.