7* 



ON PENDULUMS. 



portion of charcoal in different parcels of it, is much more 

 various than that of zinc is in brafs: perhaps, for this reafon, 

 it might be better to ufe Come of the fimple metals in the place 

 of thefe, in their conftruction. 

 Difficulty of ad- Another difficulty in forming thefe pendulums with accu- 

 ju men:. jaC y, arifes from the want of a good method of proving them. 



The adjuftment by the going of a clock is (hewn to be imper- 

 fect in the paper on the tubular pendulum, and that propofed 

 in its place, the ufe of the pyrometer, is equally defective, 

 for the following reafons : 

 Defeds alledged Pyrometers are unfit for meafuring the effedte of the heat 

 and cold of the atmofphere on any fubftance, becaufe their 

 own machinery, and particularly the part fupporting the fub- 

 ftance under trial, is liable to be affected by the fame caufes : 

 and in this cafe the index will (hew the fum of the alteration 

 of the fubftance, the part of the apparatus between its two 

 extremities, and the machinery of the pyrometer, inftead of 

 that of the fubftance alone: and in applying artificial heat to 

 the matter under trial, it is extremely difficult to communicate 

 that heat equally to all its parts at the fame time, and fo as 

 not to operate on the pyrometer itfelf, without which the ex- 

 aclnefs of a compound pendulum could not juftly be tried 

 by it. 



Laftly, another error is produced in the computation of the 

 aberration of all pendulums, from not taking into account the 

 dilation and contraction of the fmall fteel fpring by which 

 they are ufually fufpended. 



As all the ofcillatory pendulums yet made public are influ- 

 enced in their notation of time by the expanfion of their fub- 

 ftance, and as the beft contrivances to compenfatc this are 

 only an approach to perfection, an increafe of probability but 

 no certainty, it is therefore an object worthy attention to in- 

 veftigate other methods for effecting the fame purpofe : for 

 which reafon I (hall here beg leave to fuggeft the recontider- 

 ation of a fpecies of pendulum of a different nature, which 

 has never been condemned on any juft ground that I could 

 hear, and which in fact is fo little known, that the application 

 of a firnilar movement to other purpofes, is by many fuppofed 

 to be a late invention. The pendulum which I allude to, is 

 that treated of in the fifth Part of the llorologium Ofcillatorium 

 of the well known Huygens, publifhed in 1673, which he 



calls 



Other pendu- 

 lums. 



