54. Ph'ihtftphical Vfft of a ctmmin tyateb% 



50 great wheel 



IC— — 60 centre wheel 



8 64 third wheel 



8 48 contratc wheel 



6— —15 crownwheel 

 2 pivots. 

 Dial-work as ufual. 



Six fpirals on the barrel— to go 30 hours. 

 By the preceding general rule for afcertaining the beats per fecond in any watch, the cal- 

 culation of thefe numbers will be thus : 60X64X48X 15x3=5529600, and 8 x8x6 = 384J 



then " ■ = 14400, the beats in an hour, and-r — = 4 exaSly, for the beats per fecond ; 



which agreement with the rule is a proof of the accuracy of the numbers. 



Whilft r am upon this fubjedl, I Ihall take the liberty of cautioning medical gentlemen 

 againft an impofition which fome, I hope not many, watchmakers pradlife towards them in 

 the fale of watches ; and I the more readily make this caution public, becaufe the health of 

 thoufands of individuals is connedled with the impolltion, which is this, that a fecond-hand, 

 with a ftop, and an appropriate face, are fometimes put to a watch, the wheelwork of which 

 is not calculated to indicate feconds. The watch which is the fecond mentioned in this paper 

 as being in my own pofleffion, is one of this kind ; I bought it of a clock-maker, who had 

 it made in town, with his own name enamelled on the face, but unfortunately I kept it too 

 long before it^ imperfeftion was difcovered, fo that I am now under the neccflity of ufing 

 It. Upon enquiry, I found that more of the fame kind have been fold to medical gentlemen. 

 and others for the purpofe of afcertaining the number of pulfations of invalids in a minute, 

 in order that they may be treated accordingly. At firft I fufpe6led that the difagreement in 

 the motions of the fecond and minute hands, which I obferved might be owing to fome 

 inequality, orfhake, as the workmen call it, in the teeth and fpaces of the wheelwork ; b(jt 

 upon counting the numbers 1 afterwards detefted the real eaufe ; that part of the train which 

 lies between the axle of the centre wheel and the axle of the contrate wheel, on which the 



hands arc placed, viz. -2x -^ is equal to only 56,25 inftead of *6o, fo that3| feconds are 



in defeA in every minute, which is equal to a Whole revolution of the fecond-hand in every 

 16 minutes : hence, if the pulfations of any patient in a fever were really lao in a minttte^ 

 the determination by the fecond-hand of the watch in queftion would be only iiaj, confc- 

 quently the judgment of the phyfician or apothecary would be proportionably biafTed in draw- 

 ing acondufion from the pulfe upon the ftate of the fever, and would undoubtedly prefcribc 

 medicines accordingly. It is to be hoped, therefore, that the obfervations here made upon 

 the conftru6tion of a watch may, cxclufively of philofophical purpofes, prove ufeful in di-> 

 refting the choice of fuch gentlemen as may have occafton to purchafe a flop-watch, and conie< 



♦ J( a whetl «f «4 be fubftituted for either of thofe of 60 each, the feconds will be truly indicated.— P. 



quentljf 



