Description of a new Compensation Pendulum, 23 



Now it is evident, that if any degree of heat or cold be 

 applied to this compound rod, the one of zinc expands 

 and contracts as much as the two iron ones together j the 

 distance from the point of suspension to the centre of oscil- 

 lation must remain the same. 



In proportioning the length of the bars, I made use of 

 Mr. Smeaton's table of expansion of metals in the 48th vol. 

 of Philosophical Transactions ; where he .shows, by experi- 

 ments made with a pyrometer, that the expansion of iron is 

 to that of unhammered zinc, with the same degree of heat, 

 as 151 to 353, and to that of zinc hammered, half an inch 

 per foot, as 151 to 373. This great expanding property of 

 zinc renders it in theory extremely fit for the purpose of 

 compensation in a pendulum, and I was desirous of know- 

 ing if it would answer in practice, and likewise the exact 

 proportion that was requisite to answer the intended purpose. 



I made two regulators whose pendulums were composed 

 of iron and zinc, as above described, with this difference, 

 however, that one had a detached escapement of a particular 

 construction ; the zinc bar was not hammered, the ball of a 

 lenticular form, and weighed twenty pounds, its arc of vi- 

 bration nearly five degrees. The other had a simple remon- 

 toir escapement, the zine bar was hammered half an inch 

 per foot, the ball, of spherical form, weighed forty-six 

 pounds, and vibrated two degrees and three quarters. 



These regulators were both placed in the same room, and 

 their cases firmly fixed to the wall ; the pendulums were 

 suspended from a stout brass cock, screwed to the back of 

 their respective cases. In the inside of each case, and im- 

 mediately behind the pendulum rod, was hung a thermo- 

 meter, for the purpose of comparing the degrees of heat. I 

 adjusted them to mean time nearly by corresponding alti- 

 tudes of the sun. After having compared them together 

 for several days, I found that the one which had the ham- 

 mered zinc bar went somewhat faster when the air of the 

 room was heated by a fire in the grate than the other did. 

 Hence I concluded that the difference of expansion of ham- 

 mered and unhammered zinc was greater than Mr. Smeaton 

 made it, at least it appeared so in this instance. 



B4 But 



