5(^ Enumeration and Defirlption of Machines to meaftire Tiiiif, 



I 



n. 



On the Methods of obviating the Effcils of Heat and OJd in Time-Pieres. 



N every mechanical inftrumcnt for meafuring the .flow of time, fome natural power is 

 uppliid to produce motion ; and the portions of this motion being ufually regiflered by an 

 index, are taken to denote certain correfpondent portions of time. The finiplefl inftru- 

 iHcnts of this nature are the hour-glai's, which is ftill in ufe; and certain clocks, regulated by 

 the flow of Hind or water, upon the fame principle as the hour-glafs; of which a con- 

 fiderable number arc defcribed in Ozanam's Mathematical Recreations. Of tliefc it is 

 unnccefl'.iry to fay more in this place, than that the pafllxge of fand through an aperture is 

 far from being uniform, from a variety of obvious caufes ; and that the flow of water not 

 on!v varies from circumftances capable of hydrodatical eflimation, but alfo from its fluidity, 

 which thefe very inflrumcnts have (hewn to be greater or lefs in proportion to its heat. 

 The clock and watch, which confift of an aflemblage of wheels acled upon by a weight or 

 fpring, and regulated by a pendulum or balance, aie fo much fuperior in their performance 

 to every ancient inftrument applied to this purpofe, that the attention of men of fcience has 

 long been confined cxclufively to thefe inflruments. 



When a larger wheel drives a fmallcr, the number of revolutions performed by the latter 

 vlll be more numerous in a given time than thofe of the driving wheel in the inverted 

 proportion of their diameters, or the numbers of their teeth. Hence it may eafily be de- 

 duced, from the infinity of aflumable ratios, th.at combinations or trains of wheels may 

 be made fo that t!ie flow defcent of a weight, or other equivalent agent, may caufe 

 certain wheels in the fyftem to revolve with any determinate velocity, provided the rate or 

 velocity of the firft mover be determined. It is an objeiS of mechuuical Ikill to adjuft a 

 train of wheels, applied to the folution of a problem of this nature, in the befl: manner 

 pclfible. For the frictions will increafe the more complex the train, the lower the num- 

 bers of the teeth, and the larger the pivots. Thefe are not, however, the objc£ls of the 

 prefent diflertation. 



When a train of wheels is a£led upon by a weight, this lafl, inftead of defcending 

 with the accelerated velocity produced by the force of gravitation, is continually clieckcd by 

 a refiilance from the train, chiefly arifing from the funi of the friflions of the parts 

 againft each other, and in a certain dej;ree from the rcfillance of the air, which againft 

 the quicker moving partt is confiderable. Thefe retarding caufes fpcedily increnfe, until 

 they counterbalance the whole force of gravitation ; when the weight of courfe being de- 

 prived of the caufe of acceleration, defcends uniformly. Here then we oblervc an uni- 

 form motion, produced fnnply by the application oi" raechanifm, to impede the free defcent 

 of a weight : but a mere train of wheels, when a£led upci '.)y a large weight, fulFcrs it 

 very foon to run through any moderate height ; and if the wjight be diminiflied, the re- 

 Cftinces from fii£lion and imperfe<Slion of work being far from uniform, and occurring 

 at certiin periods of the rotations, are liable to (lop the machine altogether. For this 

 T'-sfon it has been found expedient to increafe that part of the rcfillance which arifes from 

 the , by the addition of a fly at the end of the train. The common jack for roafting 

 n.t.t, whidt is to be fccn in every kitciien, is an engine of this kind, in which the weight 



deicends 



