Description of a new Compensation Pendulum. 25 



and after many repeated trials with the lamp and tube, as 

 before, I found the length of the zinc bar to be 22 inches, 

 and consequently the length of the iron ones together 3f>"2 

 plus 22 equal to 61*2 inches, or. the expansion and con- 

 traction of iron to that of zinc hammered, half an inch per 

 foot, as 151 to 420. 



Haying thus far satisfied myself with the hammered zinc 

 bar. I proceeded to make similar trials with the one that was 

 unhammered : in doing which a circumstance occurred that 

 I cannot account for, that when the air in the case was ra- 

 refied bv means of the lamp and tube, the arc of vibration 

 would be about half a degree less than it was before the lamp 

 was applied, which is directly contrary to what I should ex- 

 pect would have taken place. I afterwards found that the 

 other pendulum was affected the same way, but in an ex- 

 treme small degree, which, without doubt, was in conse-> 

 quence of the ball being much heavier, and vibrating a 

 smaller arc. In taking the rate of the clock when the lamp 

 was in the case, I at first computed from theory the error 

 that would arise by such a diminution of the arc, and al- 

 lowed for it accordingly; but doubting whether the unlocks 

 ing of the swing wheel might not, from a decrease of ve- 

 locity in the pendulum, have a greater tendency to retard its 

 yibrations, I therefore thought the experiment would be 

 rendered more accurate if the maintaining power was in- 

 creased until the arc of vibration should be the same. After 

 several trials J found the length of the unhammered zinc 

 bar to be about twenty-nine inches, .which agrees prettv 

 nearly with Mr. Smeaton's experiment ; that is, in regard 

 to the relative expansion of iron and unhammered zinc. 



The zinc bar of the pendulum, which I here send to the 

 Society of Arts, was hammered three quarters of an inch 

 per foot ; and by making experiments with it as I had done 

 with the other two, I found the length of it to be twenty- 

 two inches, which is exactly the same length as the one that 

 was hammered half an inch per foot, so that it seems nothing 

 is gained after hammering it to a certain degree ; but I can- 

 not think that any rule can be laid down to enable us to 

 judge of the degree of expansion that will take place with a 



determinate 



