Mr. Squire on Compensating Pendulums. 39 



I have also found the variation in the arcs of vibration of a 

 clock with a recoiling scapement, a heavy weight, and a light 

 pendulum, to be still greater than in the one above mentioned, 

 with a dead beat scapement, a light weight, and heavy pen- 

 dulum. 



From these cases it certainly appears reasonable to sup- 

 pose that the ci/cloidal checks would be of service, and I see 

 no good reason why they are not now used ; as surely a mo- 

 del of so small a portion of the cycloid that would be neces- 

 sary for the involute arc of vibration, performed by a seconds 

 pendulum, could easily be made. But otherwise, I think Mr. 

 Farnham's plan, of facing the pallets with segments of cylin- 

 ders, to be the best that has ever been invented, for rendering 

 the arc of vibration constant, provided that under such circum- 

 stances a perfect dead beat scapement can be preserved. 



I come now to speak of the pendulum simply in itself; and 

 from the nature and importance of this appendage to a clock 

 it requires more than ordinary consideration. Now as it is 

 well known that all metals expand with heat and contract with 

 cold, for this reason the simple pendulum, when constructed 

 wholly of such materials, is found to be useless where great 

 accuracy is required. Accordingly, many ingenious compound 

 pendulums have been invented, for the purpose of counteract- 

 ing the effects arising from these causes, so that under every 

 degree of atmospheric temperature the centre of oscillation 

 may remain constant. Hence we have the angular, the lever, 

 the conical, the gridiron, the mercurial, and a great many 

 other compound pendulums : but I believe the last two are 

 those mostly used in our best astronomical clocks; and if pro- 

 perly constructed, so that a just compensation is invariably 

 preserved, they must be considered the best of the kind that 

 have ever been invented. Valuable as these pendulums doubt- 

 less are in a philosophical point of view, yet they are found to 

 be too expensive for general use. For this I'eason, I would 

 recommend the wood pendulum in preference to all others, 

 as combining in one important unity, cheapness and utility ; 

 and I am moreover convinced from experience, that a pendu- 

 lum with a deal rod, previously prepared and properly ma- 

 naged, will perform, under similar circumstances, equally well 

 as the best compound pendulum that has ever been invented. 



For this purpose take a straight-grained well-seasoned piece 

 of deal, perfectly free from knots, and which in the rough may 

 be about one inch in breadth, half an inch in de])tli, aiul three 

 feet and a half in length : let V be exposed for a considerable 

 time to a gradual increasing heat, till at last its surface be- 

 comes a little charred ; it may then be pUnied to its proper 



breadth 



