[ 8 ] 



between crofs ftaples of brafs i, 2, 3, 4, which touch only in 

 a point in front and rere (the ftaples having been carefully formed 

 for that purpofe) ; to the other rod is firmly fixed, by its centre, 

 the lens G, of twenty-four pounds weight, although it fhould in 

 ftridnefs be a little below it. This pendulum is fufpended by a 

 fhort fteel fpring on the gibbet at C, all which is intirely independent 

 of the clock. To the back of the clock plate I are firmly fcrewed 

 two cheeks nearly cycloidal at K, exadly in a line with the centre 

 of the verge L. The maintaining power is applied by a cylindrical 

 fleel ftud in the ufual way of regulators, at M. Now, it is 

 very evident that any expanfion or contradlion that takes place in 

 either of thefe exadly fimilar rods, is inftantly counteraded by the 

 other ; whereas in all compenfation pendulums compofed of different 

 materials, however juft calculation may feem to be, that can never 

 be the cafe, as not only different metals, but alfo different bars 

 of the fame metal that are not manufa£lured at the fame time, 

 and exadly in the fame manner, are found by a good pyrometer 

 to differ materially in their degrees of expanfion and contraction, a 

 very fmall change affeding one and not the other ; this, however, 

 is by their framers always tried by the pyrometer, and any defe6t 

 experimentally remedied by hammering or filing ; but it, notwith- 

 ftanding, requires a length of time, much expence, and an in- 

 ftrument not always accurate. It is very evident, that whenever 

 diirimilar rods are introduced to counteradl each other, diforders will 

 take place in the moft fufceptible, fometime before the remedy is 

 apphed by the moft taidy, and that this remedy or compenfa- 

 tion continues fometime after the occafion for it ceafes ; and 

 therefore in this country, where the temperature of the atmofphere 

 is fubjcd to fuch various and fuddcn changes ; when compen- 



fations 



