Proportion, Compajs. 983 
can only begin fucceffively upon the central line, as like- 
wife the operation of the rife in fo many required de- 
grees between thefe two moveables, the proof of which 
may be evidently demo nitrated by the inftrument made 
with two parts of a circle, in which the arc of the rife, 
run over by each of the movers, is diftinguifhable to one 
half, one quarter, and even one eighth of a degree. It 
would certainly be fuperfluous to point out the manner 
of operating with fuch a machine, fince all thofe who 
have any notion of the method of ufing mechanical and 
mathematical inftruments will immediately fee what is 
meant. 
§ 47. But fhould any body doubt of the reality, the 
utility, or the importance of this rule, the means of fa- 
tisfaciion are offered by the experiment of the degrees of 
rife 
a roue, l’attouchement, ou la menee ne peut du moins que commencer fuccef- 
fivement fur la ligne centrale, ainfi que l’operation de la levee en aufant de de* 
gres requis, entre ces deux mobiles, <lont la preuv-e fe peut demontrer d’une ma- 
niere evidente par l’inftrument a deux portions de cercle, ou Tare de levee par 
couru par chacun des mobiles, fe diftingue jufqu’a £, J, et meme a -| e , d’un 
degre pres; il feroit fans doute tres fuperflu d’indiquer la maniere d’operer fur 
la dite machine, car toutes les perfonnes qui auront quelques connoiffance et 
quelque pratique des inftrumens de mechanique ou mathematique verront au pre- 
mier coup d’oeil de quoi ils’agit. 
§ 47. Au refle fi quelqu’un vouloit revoquer en doute la realite, Tutilite, et Hm- 
portance de cette regie, on offre les moyens a tous ceux qui le defireront, d’en 
1 faire 
