108. =The Centre of Gravity and Attitudes.=--The object of various attitudes assumed under different circumstances is to keep the centre of gravity over the base of support. A man with a load on his back would not assume the position of Fig. 63, but that of Fig. 62, so that the centre of gravity of his load may be directly over his feet. So a man carrying any thing in front leans backward, as in Fig. 64. In ascending a hill a man appears to lean forward, and in descending to lean backward; but in fact he is in both cases upright in reference to the plain on which the hill stands, as may be seen in Fig. 65. A perpendicular line drawn from his centre of gravity strikes the ground midway between the feet, that is, in the middle of the base, and if prolonged would go straight to the centre of the earth. When one rises from a chair he draws his feet backward, and then bends his body forward to bring the centre of gravity over the feet. Unless this is done, it is impossible to rise, at least deliberately, as you will find by trying the experiment. A man standing with his heels close to a wall can not stoop forward and pick up any thing, for the wall prevents him from moving any part of his body backward, and therefore when he stoops forward, the centre of gravity being brought far in advance of the base, he loses his balance and falls. A man who did not understand this undertook to stoop in this way to pick up a purse containing twenty guineas, which he was to have if he succeeded, the forfeiture in case of failure being ten guineas. Of course his centre of gravity made him lose his wager. 109. =Rope-Dancers, Tops, etc.=--Great skill is exhibited by the rope-dancer in supporting the centre of gravity. Similar skill is seen in feats of balancing, as, for example, in balancing a long stick upright on the finger. In these cases the centre of gravity is very little of the time directly over the point of support. It is kept in constant motion nearly but not quite over this point, this unstable equilibrium, as it is called, being vastly less difficult to maintain than stable equilibrium; that is, keeping the balance in one unvarying position. It is the motion of the top that makes it to stand upright upon its point--a very beautiful example of unstable equilibrium. The centre of gravity revolves around a perpendicular line, at exceedingly little distance from it at first, but greater and greater as its motion becomes less rapid, till at length the centre of gravity gets so far from this line that the top falls. For a similar reason an intoxicated man may not be able to keep himself up if he undertake to stand still, and yet may do so if he keep moving. As in the case of the top, his centre of gravity must be kept in motion, or he must fall. CHAPTER VII. HYDROSTATICS.