•4< , ' On the Trains of Watches. 



II. 



Third Commmunicathn on thi Trains of Watches. By the Rev. W, Pearson. 



To Mr. NICHOLSON. 

 SIR, 



W, 



HEN I tranfmitted to you the two papers, contained in the third Volume of your 

 Journal, refpe£llng the philofophical ufes and trains of watches, I had not an opportunity 

 of fatisfying myfelf with regard to one material confideration in the conftruftion of thofe 

 delicate machines ; namely, whether or not the diameters of the wheels ought to be fuc- 

 ceffively diminiflied from the center wheel to the end of the train, in proportion as the 

 maintaining power becomes exhaufted ; four beats in the fecond were then fixed upon, for 

 the new trains recommended, for no other reafon but becaufe this feemed a convenient 

 number to be divided and fubdivided without a remainder, and was alfo very nearly equal 

 to what one of the watches, under examination at that time, was governed by : fince, 

 however, I came to refide near London, I have inquired of fome eminent watchmakers 

 what number of beats in a fecond, is deemed in pra£lice rhoft defirable for preferving a 

 fteady equable motion in the going of a watch, and the information I have received, par- 

 ticularly from Mr. lies, of St. Martin's Court, is, that about fve is found to be the 

 beft number of any : now as the number of beats of a watch in a fecond has been fhown to 

 depend upon what may be called the third, or laft portion of a train, (in a watch which 

 fliews feconds) with the contrate wheel, its pinion, and the balance wheel j and as live 

 beats in a fecond require higher numbers, in either one or both of thefe wheels, than four 

 do, the inference is, that, allowing 15 to be the beft pra£tical number for a balance 

 wheel, the contrate wheel muft neceflarily be larger than I have before given it, in order 

 to produce the defired effeft of five beats in each fecond. 



Hence arifes the necefTity of my troubling you with a third communication on this fub^ 

 jeft, which, I am perfuaded, you will readily lay before the public, not only becaufe 

 watches are in the hands of every reader, but alfo becaufe you are aware, that they can 

 be made to meafure fraftional parts of a fecond for as little expence, as they arc gene- 

 rally conflru£ted now that no attention is paid to the exa£t meafure of a beat. 



In order to make a watch beat exadly five times in each fecond, whilft 15 is the num- 

 ber of the balance wheel, the contrate wheel and its pinion muft be to each other in the 

 ratio of * 10 to i j fo that if 6 be the pinion, 60 will be the wheel; if 7 then 70; if 8, 

 80, &c. and as experience feems to point out the expediency of retaining 15 as the moft 



f When the beats of a watch are four per fecond, the ratio between this contrate wheel and its pinion is 

 ■«nly as 8 to 1. 



3 - eligible 



