ELEMENTARY PRINCIPLES OF MECHANICS. 419 



for example," says Dr. Bostock, 1 "we begin by imitating the pronun- 

 ciation of the words, and use a direct effort to put the organs of speech 

 in the proper form. By degrees, however, we become familiar with 

 this part of the operation, and think only of the words that are to be 

 employed, or even the meaning that is to be conveyed by them. In 

 learning music, we begin by imitating particular motions of the fingers, 

 but at length the fingers are disregarded, and we only consider what 

 sounds will follow from certain notes, without thinking of the mechani- 

 cal way in which the notes are produced." In these, however, and in 

 all other cases that can be brought forward, it is difficult to conceive 

 how the effect can be produced without the agency of volition, obscure 

 it is true, but still in action. The case of reading is often assumed, as 

 confirming the view that invokes habit ; yet, if a letter be inverted, we 

 immediately detect it ; and although, by habit, we may have acquired 

 extreme facility in playing the notes of a rapid musical movement, no 

 doubt, we think, ought to exist, that an effort of volition is exerted on 

 each note composing it, inasmuch as there is no natural sequence of 

 sounds; and hence there appears no cogent.reason, why one should follow 

 rather than another, unless a controlling effort of the will were exerted. 

 With regard to the extent of muscular contraction, this must of course 

 be partly regulated by volition ; but it is also greatly owing to the length 

 of the muscular fibres. The greater the length, of course the greater 

 the decurtation during contraction. We shall see, likewise, that this 

 depends upon the kind of lever, which the bone forms, and the dis- 

 tance at which the muscle is inserted from the joint or fulcrum. 



Before passing to the examination of special movements, it will be 

 necessary to consider briefly certain elementary principles of mechanics, 

 most of which are materially concerned in every explanation, and with- 

 out some knowledge of which such explanation would, of course, be 

 obscure or unintelligible. Were we, as M. Magendie 2 has remarked, to 

 investigate narrowly every motion of the body, we should find the ap- 

 plicability of almost all the laws of mechanics to them. 



If we take a rod of wood or metal, of uniform matter throughout, 

 and support it at the middle, either like the beam of a balance, or on 

 a pointed body, we find, that the two ends accurately F . r 16g 

 balance each other; and if we add weights at corre- 

 sponding parts of each arm of the beam, that is, at parts 

 equidistant from the point of suspension, the balance 

 will still be maintained. The point by which the beam 

 is suspended, or at which it is equilibrious, is called its 

 centre of gravity; and, in every mass of matter, there 

 is a point of this kind, about which all the parts balance 

 or are equilibrious; or, in other words, they have all a 

 centre of gravity or inertia. The centre ofgravity, in 

 a mass of regular form and uniform substance, as in the 

 parallelograms, Figs. 168 and 169, is easily determined, 

 inasmuch as it must necessarily occupy the centre c ; but 

 in bodies that are irregular, either as regards density or Centre of Gravity 



1 Physiology, edit, cit., p. 774, Lond., 1836. a Precis, &c., edit, cit, i. 276. 



