On thee MUSCLES. 255 
mufcles have acted fully, or fhortened themfelves one-fifth of 
their length, the point A will defcendto1. But when thetwo 
oblique mufcles have, by their action, brought the point A 
down to 1, they have not loft one- fifch of their length; for 
the dotted lines reprefenting them muft be longer than the 
‘lines IE or ID, becaufe the angle r1E being equal to the 
two angles 1 TA and 1A I of the ifofceles triangle ArI, mutt 
be larger than a right angle, and therefore the fide 1E 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 ad 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 propofition ftill plainer, if poffible, ise 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 BD and CD 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 contraétion, move the point 
A three times farther than can be done by ftraight mufcles of 
the fame length. 
Iw 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. © 
Tuus, let us compare the extent of motion, produced by 
the pair of oblique mufcles A D and AE, (T. 2. fig. 5.) with 
that 
