ANIMAL MECHANICS. 



289 



The point 5 is the observed centre of the glenoid cavity of 

 the shoulder joint, and ST is the actual axis round which the 

 wing rotates. This axis of rotation is placed at a distance of 

 4.83 inches from the vertex 0 ; and, as we have just seen, 

 the axis PQ, corresponding to the imaginary straight bone 

 A"B", lies at a distance from 0, equal to 5.70 inches. The 

 distance, therefore, between the actual axis, ST, and the 

 calculated axis, PQ, is 0.87 inch. By substituting in equa- 

 tion (64), for x, the intercept 0.87 between ST and PQ, we 

 find 



f — - — ) = 0.9232, 



or 



if = 1.872; 



but 



n 2 * 2.028. 



Therefore 



n 2 - t/ 2 1 5 6 1 

 w 2 = 2028 = 13 ' 



or, the work done by the rotation of the wing of the Albatross 

 round its observed socket differs by j\th part from the cal- 

 culated possible maximum derivable from the imaginary 

 straight bone A"B'. 



In Proposition C, we have seen that, in a triangular 

 muscle, the position of maximum effect of the axis of rotation 

 corresponds with the point of bisection of the bisector of the 

 vertical angle ; and it may be seen, from the preceding cal- 

 culation, that, in a quadrilateral muscle with straight bones, 

 the axis of rotation of maximum effect nearly corresponds to 

 the point of bisection of the portion of the bisector included 

 within the quadrilateral. For the distance from 0 to this 

 point of bisection is 



s b + h' 8.13 + 2.67 . . 



o = ■ = 1 = c.40 inches;- 



2 2 3 H ' 



U 



