Proportion Compafs . 979 
a pinion of iix is a fixth part of its diameter ; for one of 
feven, afeventh; for one of eight, an eighth; for ten, a 
tenth ; for twelve, a twelfth, and fo on of all the movea- 
bles ferving for pinions. What is meant by a pinion in 
watch-making is that moveable which is fet in action by 
another of a greater number of teeth, and communicates 
its motion fucceffively to the laft of them ; hence all the 
pinions relatively to the fhape of the teeth of the wheels 
and pinions require an addition of the true diameter, and 
that is what is properly called the apparent diameter. 
§ 43. We muft, however, except the rochet pinions, 
becaufe the whole primitive radius nearly as far as the 
extremity of the tooth is run over by the point of 
the tooth, which is of a rochet form, as well as the 
tooth of the pinion. But whatever advantage thefe 
rochet 
du vrai, dedu£tion qui pour pignon de fix eft la fixieme partie de fon diametre, 
pour pignon de 7 la 7% pour 8 la 8% pour 10 la 10% pour 12 la i2 e ; ainfi du 
refte de tous les mobiles fervant de pignon; Ton entend dans l’horiogerie par le 
mot de pignon, ce mobile qui eft adlionne par un autre plus nombreux, et qui 
communique fuccelfivement lemouvement, jufqu’au dernier des mobiles; ce qui 
fait que tous les pignons, relativement a la nature des engrenages, exigent un 
excedent du diametre vrai, que Ton peut nommer a jufte titre, le diametre 
apparent, 
§ 43. Toutes fois, il enfant excepter les pignons a rochet, parce que tout le 
rayon primitif, jufqu’a rextremite de l’aile a tres peu de chofe pres eft parcotfru 
^ar la pointe de la dent egalement formee en rochet tout com me Faile du pignon ; 
6F2 mais 
