50 Maintaining Power ofTime-Plecef. 



When the train of wheels in a clock is driven by a weight, the more immediate caufe of 

 variation confifts in the periodical irregularities of the wheels, the pinions, and the pivots, 

 which tranfmit the force diminirtied by fridlion and oblique aftions, which are by no means- 

 uniform : the remoter caufe of variation confifts in the oil becoming lefs fluid the longer 

 it is expofed to the air. But when a train is driven by a main fpring, the caufes of irre- 

 gularity are greatly increafed by the diminution of force in the fpring as it runs down, and 

 its friction in the box ; neither of which are fo regular as to admit of a gotod corre£llon 

 from the fpiral of the fuTee round which the chain, that a£i:s upon the movement, is drawn. 

 Perfeft equality of the maintaining power is not therefore to be expected. The only good 

 expedient, independent of the efcapement, appears to be that of caufing the tr in to go by 

 a weight or fpring, ferving only to wind up the laft wheel once during every revolution, 

 this wheel being urged by an appropriate weight or fpring. By fuch an expedient the 

 clock in effect becomes a time-piece with one wheel only, in which the irregularities are 

 minute, and their recurrence too frequent to produce any confiderable effedt on the vi- 

 brating part. 



When the irregularity of the firfl mover is propofed to be remedied by its mode of ap- 

 plication, it is clear that it muft give an increafe of velocity to fuch vibrations as are 

 performed through longer arcs, in order that the times may continue the fame. At- 

 tempts have been made to accomplifli this obje£t, by giving a proper figure to the face 

 of the pallets of the efcapement ; but it docs not appear that fuch attempts have been fuc- 

 cefsful. 



The third method of equalizing the effe£ls of the maintaining power with regard to time 

 18 founded on the confideration, that, if this impulfe eould be given in an inftant, the whole 

 of the vibration would be free, and refemble that of a difengaged pendulum or balance ; 

 and if the forces, by which the vibrating body returns to its point of quiefcence, were as 

 the fpaces to be moved through, all the vibrations would be performed in equal times, 

 whether long or fliort, or whether the maintaining power were greater or lefs. This dif- 

 poCtion is peculiarly adapted to the balance. For it is pofFible to form the pendulum fpring 

 and adapt the maintaining power to fo fmall a part of the vibration, that the times ftiall not 

 perceptibly vary, even when the power itfclf is greatly altered. 



Under the latter points of view the efcapement becomes an obje£t of great importance, 

 fince on this the manner of application of the maintaining power muft depend. It fcarcely 

 fccms ncceflary to fubdivide or arrange the principles on which the moft eminent artifts 

 have endeavoured to give perfection to their efcapements. The principles will perhaps 

 beft fliew themfelves in the form of remarks upon the fevcral conftruGions. 



One of the moft ancient efcapements is that which is at prefent applied in common ] 

 pocket watches. It is reprcfented Fig. i. Pi. IH. and is beft fuited to the long vibrations 

 of the balance, which was invented earlier than the pendulum *. A B denotes the rim oF 



a con- 



* Doftor Derham on Clockwork, p. 86. fourth edition, sffirms, that vibrations are no where mentioned 

 or alluded to in the clocks defcribed by the ancients : yet we learn from Vitruviiis that they poffclTed time- 

 pieces with wheel-work. Falconet, AcatUmie tin hfcriplicns vol. xx. thinks that all the mechanical time- 

 pieces made before the l6th century were ckpfydra: with wheel-work for reckoning the fmaller portions of 



dcfceiit. 



