On the MUSCLES. 255 



mufcles have acted fully, or fhortened themfelves one-fifth of 

 their length, the point A will defcend to 1. But when the two 

 oblique mufcles have, by their action, brought the point A 

 down to 1, they have not loft one-fifth of their length ; for 

 the dotted lines reprefenting them muft be longer than the 

 lines IE or ID, becaufe the angle iIE being equal to the 

 two angles 1 1 A and 1 A I of the ifofceles triangle A 1 1, muft 

 be larger than a right angle, and therefore the fide 1 E muft 

 be longer than the fide I E ; that is, the oblique mufcles, after 

 bringing the point A down to 1, have not loft one-fifth of 

 their length ; or if they continue to act till they have loft 

 one-fifth of their length, they will bring the point A lower 

 down than can be done by ftraight mufcles, fhortened in the 

 fame proportion. 



To make this proposition ftill plainer, if poflible, by cal- 

 culation, I fhall fuppofe the oblique and ftraight mufcles in 

 T. 2. fig. 6. to be each five parts or five inches in length ; 

 that the bafes of the triangles B D and C D meafure four 

 inches > and that the perpendiculars, or altitudes of the tri- 

 angles, meafure three inches ; and let it be fuppofed, that 

 thefe mufcles, in action, can be fhortened one-fifth of their 

 length, the ftraight mufcles, on that fuppofition, can bring 

 A down to 1 only : But it is evident, that the oblique mufcles 

 will not be fhortened one-fifth of their length till the point 

 A has defcended to D, or to number 3 ; or the oblique muf- 

 cles will, with the fame degree of contraction, move the point 

 A three times farther than can be clone by ftraight mufcles cf 

 the fame length. 



In the next place, we may eafily demonftrate, that the ex- 

 tent of the motion produced by the co-operation of oblique 

 mufcles, increafes with their greater degree of obliquity. 



Thus, let us compare the extent of motion, produced by 

 the pair of oblique mufcles A D and A E, (T. 2. fig, 5,) with 



that 



