VOL. XLII.] PHILOSOPHICAL TRANSACTIOKS. - 665 



its extremity is reduced to ik ; and the extension it produces, reaches from a 

 to F, if we carry those levers as far as they will go, which is never necessary 

 In short, it will be in your power to give to this lever an extension as great as 

 you please, joined to a very small elevation. To this end you need only set 

 more backward the lever's point of rest, along the perpendicular marked in 

 fig. 7. Now this is precisely what the spur does, which we have added to our 

 ambe ; the holes it is pierced with, as well as the mortise of the foot, are 

 placed in different degrees, as the points 1, 2, 3 ; and these holes, as has 

 been said, are the places of the pin which forms the lever's hinge or point 

 of rest. 



The gradation of those holes therefore enables us to augment at will the ex- 

 tension, while the elevation diminishes in the same proportion ; but if we want 

 the elevation to diminish more or less than in the foresaid proportion, for in- 

 stance, to make a great extension, and a very small elevation, there is nothing 

 easier for it than this machine. We need only push the spur 1 3, which is 

 moveable, towards the end of the lever to l, and stop it there : then the end 

 of the lever al, being very short, it has but little room to play ; on the con- 

 trary, to have a great elevation, we need only bring back the said spur to m, 

 or I, or still farther ; the farther we remove from the end of the lever, the more 

 it will have room to play, and the more considerable will be its elevation. It 

 is true, the power of the lever will decrease in the same proportion ; but this 

 power is so great, that losses like this ought to be reckoned for nothing. 



We have it therefore in our power with this sort of ambe, to make, as occa- 

 sion requires, such extensions and counter-extensions as we please ; and we 

 may also vary all the degrees of the elevation, which shall be necessary to give 

 to the bone to be reduced ; and these are the perfections which have been 

 hitherto required in this machine. 



Commonly, when the bone of the arm is sufficiently stretched and raised, so 

 as to be on a level with the cavity of the articulation, those bones replace them - 

 selves as it were, because this level is not always exact ; on the contrary, the 

 extension and counter-extension being never regular enough to hinder the sca- 

 pula, which is a moveable part, from following a little the head of the bone, or 

 its extension, it mostly happens that this head bears pretty strongly against 

 the edge of the cavity, and consequently does not fail to fall into the said cavity, 

 as soon as it has only passed its edge, and even before it has met the level, or 

 the axis of the hollow of the articulation ; but it is otherwise after an extension, 

 a counter extension, and an elevation so regular as those which may be per- 

 formed by this machine ; it may happen, that after the 3 preceding operations, 

 the head of the bone, without having touched the edge of the cavity, will be 



VOL. VIII. 4 Q 



