A System of Physical Education, Theoretical and Practical (extract)

'i I i I I I I 1 I I I I li 160 EXERCISES OF PROGRESSION. (SECT. II. force is in relation to the size of the object, and as the earth is incomparably larger than any object on its surface, it has the power of neutralizing the individual attractive force of any such objects, to draw them towards its own centre, and thus to hold them on its surface. To enable any object thus attracted to retain its position, it is not necessary that its entire surface should be brought in contact with that of the earth, because a principle by which this force is governed is, that it is directed from centre to centre,-from the centre of the object attracting to the centre of the object attracted,- not the actual centre of its bulk, but that of its substance, the spot around which all the remainder is, as it were, grouped equally on every side. It is, therefore, only necessary that one point should adhere to the ground, but it is necessary that this point should be in a direct line below the central spot. This spot is called the centre of gravity, and that part of the surface coming in contact with the ground is called the basis of support. Everything, therefore, on the earth's surface, animate or inanimate, is held by this central grasp, and whenever the centre of any object is changed, whether by its own action or by extraneous influence, the line of attractive force is also changed, and the process is to pull that part toward the ground which has now become the centre. To meet the demands of this inevitable law, animated creatures, with whom locomotion, implying a constant shift- ing of the centre of gravity, is a necessity to existence, are endowed with other forces, the chief of which is muscular contraction. But this muscular power is itself subject to and governed by certain :fixed laws, and one of the most important of these is that it shall be exerted but for a limited continuous space ; and thus, unless the relaxation