COMPENSATION PENDULUM. 5^^ 



that I cdnnot account for ; when the air in the case was rati- 

 fied by 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 directl)' contrary to what I should 

 expect would have taken place. I afterwards found, that 

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

 extreme small de^gree, 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 errour 

 that would arise by such a d.minution of the arc, and al- 

 lowed for it accordingly ; but doubting whether the unlock- 

 ing of the swing wheel might not form a decrease of velo- 

 city in the pendulum, and have a greater tendency to retard 

 its motion, 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. Af- j^^ Smeaton's 

 ter several trials, I found the length of the unhammered ratio between 

 zinc rod to be about twenty-nine inches, whicTi agrees hammered"'i 

 pretty nearly with Mr. Smeaton's experiments; that is, in near the truth, 

 regard to the relative expansion of iron and unhammered 

 zinc. 



The zinc rod of the pendulum, which I here send to the Farther ham- 

 Society of Arts, was hammered three quarters of an inch "^''^""g ''^e 



„ ,, ,. . ... Till ^'"^ makes no 



per foot; and by making experiments with it as 1 had done alteration, 

 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 no- 

 thing is gained after hammering it to a certain degree; but 

 I cannot think, that any rule can be laid down to enable us Quantity of 



to iudge of the degree of expansion that will take place '',^'^*'^'°" '^y^ 

 ... • ^ r u ^ p .L • t ,lhe hammer 



With a determinate increase oi heat, trom the quantity that no rule. 



is extended by the hammer; much depends on the degree 

 of curvature and polish of the stake and hammer, and pro- 

 bably on the healing pf the rod at the time; for it is neces- 

 sary to heat it a little hotter than boiling water, otherv/ise it 

 will crack in hammering. 



In all these experiments it is to be understood, that Ball suspended 

 the ball of the pendulum was suspended by its centre ; '^J' ^'^ '^•*?'^®» 

 ]t)Ut if the batt be made to rsst oa it» lower edge, the 



expansion 



