340 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



If the expansion and contraction of the rod ib by heat and cold, in proportion 

 to the expansion and contraction of cd, were known, and the expansion and 

 contraction in perpendicular height of ie, then the length ib should be to the 

 length DC, as the contraction of the materials of cd is to the contraction of 

 the materials of which ib is constructed, added to the contraction in perpen- 

 dicular height of the materials of which ie is constructed. These lengths, in 

 this case, might be taken at once; but it is much more convenient to have the 

 power of fixing them by experiment, after taking them from measure as nearly 

 as may be. 



Dr. F. then gives a description of some complex machinery, assisted by se- 

 veral figures, for the purpose of raising the point i higher or lower in proportion 

 to ED, with the view to obtain an invariable pendulum. After which he adds, 

 on considering the several different methods of finding a measure of lengths 

 which could be always and universally ascertained, I am persuaded that the taking 

 the difference of the length of 1 pendulums, vibrating different times, appears 

 not only to be the most perfect, but the easiest attainable. Mr. Whitehurst 

 contrived an apparatus for the purpose of ascertaining this difference, an account 

 of which was read in the r. s., and afterwards withdrawn and published by the 

 author himself. After his death, I purchased this apparatus. 



There was no means in it whatever of keeping the pendulum of the same 

 length when the heat should vary; consequently it was impossible that any ac- 

 curate admeasurement of the different lengths of 2 pendulums keeping different 

 times could be ascertained. Mr. Whitehurst indeed had endeavoured to keep 

 his pendulum of the same degree of heat; but I know from many experiments, 

 among which some were for hatching eggs, how extremely difficult it is to main- 

 tain the same heat in any considerable mass, and that the means which may be 

 employed to keep it within 4 or 3° are almost totally inapplicable to pendulums; 

 so that his experiments must have been defective. I therefore endeavoured to 

 contrive a means of rendering the pendulum in his machine always of the same 

 length, whatever the heat might be, by some addition to it. I thought of the 

 principle, and formed the apparatus above-mentioned for this purpose. 



It would be improper for me to repeat what has already been laid before this 

 learned society; therefore I shall only mention briefly, that the frame of Mr. 

 Whitehurst's machine was formed of 2 pieces of very clean well-seasoned deal, 

 to which was fixed the apparatus for rendering the wire flexible of which his 

 pendulum was formed at a proper point, but there were no seini-c} lintlric pieces; 

 the 1 square pieces came together, so as to make the top of the penduUim at 

 their under surface; these pieces could be brought away from each other by a 

 screw, so as to leave the wire free. The use of this was, by a screw, to adjust 

 the pendulum to its proper length, which has in this apparatus a considerable ad- 



