818 



APPLIED MECHANICS. 



[AXIXAL POWER. 



longitudinal contraction of this muscle pulls the tondon, 

 and thus erases th radius t<i turn ui-.ii tlio elbow-joint, 

 and raise the weight at it* extremity. 





Besides the direct movement of the bones as levers, 

 such as we have described, there are numerous modified 

 motions, sideways or rotatory, which are all effected by 



rig. 100. 



the contraction of muscles specially arranged for their 

 respective purposes. For instance, in order to turn the 

 baud upside down, it is necessary to make the arm rotate 

 or turn upon its axis. This is effected by muscles placed 

 spirally round the bones of the arm. By diminishing 

 the length of one of these, the bones are caused to twist 

 round. And so for every movement of every part of the 

 frame, a special muscle is provided to effect the required 

 motion by simple contraction. 



The force or contracting energy of some of these 

 muscles is enormous, as may readily be believed, when 

 it is remembered that they are attached to the short end 

 of a losing lever, and yet are capable of sustaining great 

 weights at the extreme end, where the weight acts with 

 immense leverage. 



Some of the muscles are not placed exclusively under 

 the control of the will, btft act regularly without any 

 effort of volition. Such are the muscles which effect the 

 action of the heart, and organs of respiration. Some 

 muscles, again, seem to be put in action by an effort of 

 will uf which we are not conscious. Such are those of 

 the eye to direct and focus it, and those of the tongue, 

 mouth, and throat in speaking. Although we will to 

 peak or articulate certain sounds, we never think of the 

 particular, muscles that we must put in action, in order 

 to effect the articulation desired. So in walking and 

 other actions which have become familiar by habit, no 

 thought is given to the special muscular movements re- 

 quired. It seems to be enough to will to walk, and the 

 muscles obey. It is only when fatigue and exhaustion 

 render their exercise painful, that we have to employ 

 special act* of volition in order to effect our purpose. 



In making use of muscular power aa a moving force, 

 it is important to call into exercise those muscles which 

 require the least effort of will for their movement, and 

 to apply them in the way least calculated to fatigue. 

 The animals chiefly employed the horse, the ox, and 

 the camel can only be used in particular ways, as for 

 .bearing and dragging loads. The hone and the ox are 

 best adapted for dragging, as neither of them is capable 

 of sustaining great weights. The camel, on the other 



hand, moves with facility under a very considerable load 

 placed upon its back. The elephant is employed in 

 tropical countries both for carrying and dragging, and 

 appears to have enormous strength for 

 either purpose. When human power is em- 

 ployed, it may be exercised in various ways. 

 The operation in which man seems capable 

 of exerting the greatest power, for the long- 

 est period, is that of rowing. In moving 

 the oar against the resistance of the water, 

 almost every muscle of the body is called into 

 operation. The spine, bent forwards in the 

 back-stroke, is thrown back as the pull is 

 taken ; the shoulders and arms are also 

 thrown bock, the legs are stiffened against 

 the foot-board, and thus immense power 

 is applied to the oar, without excessively 

 straining any one or two particular muscles. 

 In excavating earth or clay, conveying loads 

 up steep inclinations by a wheelbarrow, 

 and generally in the operations for which 

 navigator* arc employed, a large amount of power is de- 

 veloped. The muscular frames of these men, and their 

 long practice in particular operations requiring great 

 strength, enable them to continue for long pe- 

 riods at work without much fatigue ; and to 

 one watching minutely their mode of work- 

 ing, while there is little appearance of effort, 

 yet there is manifest a regulated play of 

 the muscles, and a sort of contrivance for 

 doing the work with the least possible 

 movement, which indicate that every one 

 of these muscular movements is the result 

 of habit, without the need for a special 

 effort of will. 



In pumping, as in rowing, numerous 

 muscles are exercised, and accordingly 

 this kind of labour can be continued for 

 a long period. There is, however, a monotony about 

 the operation, and a want of visible effect to satisfy 

 the mind of the workman, which tend to make him 

 weary of his work. In rowing a boat, excavating, 

 lifting or moving a load, there is a visible progress made, 

 and the workman has a satisfaction in observing the 

 change effected by his exertion, and can reckon the 

 advance he has made, and the amount still remaining 

 to be done ; but in the continuous discharge of a body of 

 water from a pump, drawn probably from an unseen 

 source, and flowing into an unseen reservoir, or into 

 one so large that additions to its contents produce no 

 visible effect, there is nothing to reckon by, no progress 

 apparent ; and the workman accordingly becomes fatigued, 

 more from want of mental satisfaction, than from actual 

 weariness of body. It is true that cases frequently occur 

 where seamen and passengers on board a leaky vessel, 

 work for many days continuously at the pumps. But in 

 such a case, life is at stake; and the excitement of 

 danger, fear of death, and hope of safety, are impulses 

 to exertion capable of overcoming much bodily fatigue 

 and mental lassitude. In all operations demanding 

 manual labour, it is good economy to let the workman 

 see the effect of his efforts : let his mind be satisfied, 

 and his body will the longer withstand fatigue. Where 

 labour is used as a punishment, it is certainly an aggra- 

 vation of the penalty to make the labour of a kind that 

 manifests no result. The perpetual walking up the 

 rounds of a treadmill, or perpetual turning a winch to 

 drive unseen machinery, is therefore a penal labour of 

 a most severe character, although the actual muscular 

 exertion which either demands is not great. Prisoners 

 working at the treadmill always prefer grinding corn to 

 "grinding air," as they express the act of simply turn- 

 ing the wheeL 



As rotatory motion is generally that kind of movement 

 best applicable to machinery, being most easily modified 

 in direction and velocity, labour at the winch in the kind 

 of manual exertion chiefly used. Before steam machinery 

 became common, the number of operations effected by 

 manual labour was very much greater than it is now. 



