420 



MUSCULAR MOTION. 



Fig. 169. 



Centre of Gravity. 



Fig. 170. 



form, it has to be determined by rules of calculation, to be found in all 

 works on physics; but which it is unnecessary to adduce here. 



The nearer the centre of gravity is to the soil on which the body- 

 rests, the more stable is the equilibrium. In order that the Figures 

 168 and 169 shall be overturned from left to right, 

 the whole mass must turn upon e as upon a pivot; 

 the centre of gravity describing the curve c b, and 

 the whole mass being lifted in the same degree. In 

 Fig. 168, the curve is nearly horizontal, owing to the 

 narrowness of the base and the height of the centre 

 of gravity. In Fig. 169, on the other hand, whose 

 base is broad and the centre of gravity low, the 

 curve rises considerably; the resistance to overturn- 

 ing is consequently nearly equal to the whole weight 

 of the body, and the equilibrium necessarily firm. 

 The condition of equilibrium of a body resting upon a plane is such, 

 that a perpendicular, let fall from the centre of gravity, shall fall within 

 the points by which it touches the plane. This per- 

 pendicular is called vertical line or line of direction, 

 being that in which it tends naturally to descend to 

 the earth; and the space comprised between the points 

 by which the body touches the soil is called base of 

 sustentation. We can now understand, why a wagon, 

 loaded with heavy goods, may pass with safety along 

 a sloping road; whilst, if it be loaded to a greater 

 height with a lighter substance, it may be readily 

 overturned. When the wagon is loaded with metal, 

 the centre of gravity is low, as at c, Fig. 170; the 

 vertical line c p falls considerably within the base of 

 sustentation ; and the centre describes a rising path; 

 but in the other case the centre is thrown higher, to a; and the vertical 

 line falls very near the wheel, or on the outside of it, and consequently 

 of the base, whilst the centre describes a falling path. 



Of two hollow columns, formed of an equal quantity of the same 

 matter, and of the same height, that which has the largest cavity will 

 be the stronger ; and of two columns of the same diameter, but of dif- 

 ferent heights, the higher will be the weaker. 



All bodies tend to continue in the state of motion or of rest, so as 

 to render force necessary to change their state. This property is called 

 the inertia of motion, or of rest, as the case may be. When a carriage 

 is about to be moved by horses, considerable effort is necessary to over- 

 come the inertia of rest; but if it moves with velocity, effort is required 

 to arrest it, or to overcome the inertia of motion. We can thus under- 

 stand why, if a horse start unexpectedly, it is apt to get rid of its 

 burden; and why an unpractised rider is projected over his horse's head 

 if it stops suddenly. In the former case, the inertia of rest is the 

 cause of his being thrown ; in the latter, the inertia of motion. The 

 danger of attempting to leap from a carriage, when the horses have 

 taken fright, is thus rendered apparent. The traveller has acquired 

 the same velocity as the vehicle ; and if he leaps from it, he is thrown 



Condition of Equi- 

 librium. 



