ON PRESSURE AND EQUILIBRIUM. 65 



When a horse is walking, the centre of gravity is sometimes supported 

 only by two feet of the same side, yet for a time so short, that its declension 

 towards the other side is easily recovered, after the legs on that side have re- 

 sumed their activity. Some authors have thought it impossible that a qua- 

 druped should stand for an instant with both feet of the same side raised from 

 the earth ; but when a horse is walking fast, it may very often be observed, 

 that the print of the hind foot is considerably more advanced than that of the 

 fore foot, which has been raised to make way for it. 



From the general law of the equilibrium of the centre of gravity, we may 

 deduce the properties of levers of all kinds. It follows from the definition of 

 this point, that if two bodies be attached to a straight rod of inconsiderable 

 weight, they may be sustained in equilibrium, by a fixed point, or fulcrum, 

 which divides their distance into portions which are inversely as their weights. 

 And it is obvious that if any other equivalent forces be substituted for weights, 

 acting at the same distance from the fulcrum, and with the same inclination 

 to the rod or lever, the conditions of equilibrium will be precisely the same. 

 Also if either of the forces be transferred to an equal distance on the other 

 side of the fulcrum, and act there in a contrary direction, the equilibrium 

 will still remain. Hence we have two principal kinds of levers; the first, in 

 which the fixed point, or fulcrum, is between the points at which the forces or 

 weights are applied ; the second, where the forces aie applied, in contrary di- 

 rections, on the same side of the fulcrum. (Plate III. Fig. 47.) 



The demonstrations of the fundamental property of the lever have been 

 very various. Archimedes himself has given us two. Huygens, Newton, 

 Maclaurin, Dr. Hamilton, and Mr. Vince, have elucidated the same subject 

 by different methods of considering it. The demonstration of Archimedes, 

 as improved by Mr. Vince, is ingenious and elegant, but it is neither so ge- 

 neral antl natural as one of Dr, Hamilton's, nor so simple and convincing as " 

 Maclaurin's, which it may be worth our while to notice. Supposing two 

 equal weights, of an ounce each, to be fixed at the ends of the ecpial arms of 

 a lever of the first kind; in this case it is obvious that there will be an equi- 

 librmm, since there is no reason why either weight should preponderate. It 

 is also evident that the fulcrum supports the whole weight of two ounces, 

 neglecting that of the lever ; consequently we may substitute for the fulcrum 



VOL. I. K 



