274 PHILOSOPHICAL TRANSACTIONS. [anNO 1751. 



the year 1734, that he again resumed them on the following occasion. A gen- 

 tleman desirous to make some experiments concerning the expansion of metals, 

 employed him to make an instrument like one invented by Mr. Muschenbroek 

 for that purpose, which he calls a pyrometer. On looking over Mr. Muschen- 

 broek's experiments, he not only found the difference between the expansion of 

 some of the metals much greater than he expected, but, as he thought (if they 

 were to be depended on) sufficient to answer his former purpose. This led him 

 to consider the structure of the instrument which Mr. Muschenbroek made use 

 of in his trials, and on examination he thought it liable to some objections, 

 which would probably make the result of experiments with it very uncertain. 

 He therefore endeavoured to contrive one of a different construction, that might 

 be more to be depended on. Such an instrument he some time afterwards com- 

 pleted : and though it was not in every respect so accurate as he could wish, he 

 is fully persuaded that such experiments as are carefully made with it, may be 

 depended on, as very near the truth. Having made a great variety of experi- 

 ments with this instrument on bars of different metals, as nearly of the same 

 dimensions as possible, he found, on a medium, their several expansions by the 

 same degree of heat to be as follows: 



Gold Silver Brass Copper Iron Steel Lead 

 73 103 95 89 60 56 149. 



Thus finding so great a difference between the expansion of brass and iron, he 

 immediately determined to make a pendulum after the manner above described, 

 composed of those two metals, and also ordered a clock to be made, with the 

 utmost aire and exactness, with which he intended to make the experiments. 

 These were both finished in the beginning of the year 1738; and having no 

 reason to doubt of success, he showed the pendulum to the late Mr. Machin, 

 and gave him a drawing and description of it, in order to its being communicated 

 to the Royal Society; but objections were made to it, of which the only one that 

 appeared to have any weight was, that it had been found by experiment, that 

 two bars of different metals, screwed together so as to be in contact with each 

 other, would not expand regularly and smoothly, but by jerks. In order to exa- 

 mine into the force of this objection, he directed two bars of equal dimensions 

 to be made, one of brass, the other of iron, of about 2 feet in length, fastened 

 together after the same manner as the two rods of the pendulum, which he 

 intended to place so, that, by acting very near the centre of an index of a con- 

 siderable length, even the smallest alteration in the bars would be made sensible, 

 and by the motion of the index, he should be able to form a judgment, whether 

 the rods moved regularly and freely, or not ; but before this was put into execu- 

 tion, he contrived, by fastening the two bars to the back plate of a clock, not 

 only to make them answer the end above proposed, but at the same time to 



