38S PHILOSOPHICAL TRANSACTIONS. [aNNO 1778. 



frame ; and the effects of the humor on ivory being inconsiderable, he wanted, 

 in order to measure them correctly, to destroy the effect of heat on the frame, 

 which he did, as in the compound pendulum, by the expansion of a rod of 

 brass in a contrary direction. But to do this it became necessary to determine 

 the proportion between the dilatations of brass and glass by heat, which thus led 

 him to pyrometry. 



Mr. De L. had heard the ingenious Mr. Ramsden say, that he had a notion 

 of a pyrometer different from all that had been invented ; and knowing his great 

 skill in philosophical and mechanical matters, applied to him, and pressed him 

 to execute his idea. The multitude of his other engagements prevented his 

 complying with this request ; and he advised Mr. De L. to look no farther for 

 the proportions of the expansions of brass and glass than to Mr. Smeaton's ex- 

 periments, which he considered as the best that had been made. On Mr. 

 De L. desiring him to explain by what means he thought of being able to cor- 

 rect the faults of the ancient instruments, he did so, and told him that he pro- 

 posed measuring the expansions of bodies by the micrometer of a microscope ; 

 by which means he should obviate the greatest mechanical difficulties. He 

 added that he had made a first trial of his method a long while ago, and was 

 assured of the success. This idea struck Mr, De L. and being very desirous of 

 following it, he determined to undertake the execution of it himself. Hence, reflect- 

 ing that he did not want absolute measures, and that it was enough to find the 

 proportions of dilatibility between two different bodies, he was led by that idea, 

 he says, to a very simple method, which made all the difficulties vanish, and 

 gave him the confidence to undertake the work. Afterwards indeed, he says, he 

 went much farther than he expected in the absolute measures themselves. Mc. 

 De L. then delivers a very long, tedious, and desultory discourse, on a variety 

 of miscellaneous topics, whence no satisfactory or useful conclusions are derived. 



