MOMENTS. 



249 



Fig 



. 134. -Drawn from an instantaneous photo- 

 graph of cheese maggots. Some are curling 

 round to fix the tail to the head. The upper 

 one has already done this, is straining to 

 extend itself, and is just about to unfix itself 

 and jump into the air. 



contraction of the finger, supported during the initial stage of the contraction, 



and then suddenly released — 



snapped, in fact. The same @*\ 



movement is made while the ^ 



plate travels from B to 1!' in 



ahout one-fourth of the time. 

 Amongst many lower 



animals this "hold and let go" 



method of increasing velocity 



characterises their most impor- 

 tant movements. The elater 



(click - beetle, SjpringMifer), 



when he falls upon his back 



is unable to right himself in 



the manner usual with insects, 



on account of the shortness of 



his legs. lie bends his pro- 

 thorax backwards, and hooks a 



spine into a depression of his 



mesothorax. He then contracts 



his extensor muscles, suddenly 



lets go, and springs many inches 



into the air. 



The cheese maggot bends its body round and inserts a hook attached to 



its tail into a grooved plate, like a file, near 

 its head. It then contracts its straightening 

 muscles, the hook is released suddenly from 

 the file, and the maggot is projected with 

 great velocity many times its own length. 

 (Fig. 134). 



Moments. — Let us suppose that the 

 upper arm and trunk are fixed in space, the 

 upper arm being raised, and that the lower 

 arm and hand are fixed together so as to 

 form a single rigid body free to move at the 

 elbow-joint upon the upper arm (Fig. 135). 

 The centre of gravity of the lower arm 

 and hand (the weight of which is w) is at 

 the point g, and the line of action of gravity 

 will be in the direction of the downwardly 

 pointing arrow. The triceps will exert a 

 force pulling in the direction of the arrow 

 descending from m. The point r is a point 

 through which passes the axis of rotation 

 of the lower upon the upper arm. 



If the two forces m and w are balancing 

 each other, and if we then shift the force 

 w nearer to the hand, it will overcome the 

 force m, and the limb will sink. Shift it 

 towards r, and it will itself be overcome 

 by m, and the limb will rise. This "value" 

 of a force, is termed the " moment " of the 

 force, and is measured by the product of 

 the force, and by the distance of its line 

 of application from the point considered 



(in this case r). If the distance gr be five times as great as the 



distance rm, and if the force w be 5 lb., and the force m 25 lb., 



V 



V 



o 



Fig. 135.— The letter g marks the 

 position of the common centre of 

 gravity of the hand and lower arm, 

 the letter r the position at which 

 the lower is supported by the upper 

 arm, and m the position at which 

 the triceps muscle is attached. The 

 arrows represent the forces acting 

 at these positions. 



