MECHANICS, is the science which treats of the laws of the equilibrium and motion of solid bodies ; of the forces by which bodies, whether animate or inani mate, may be made to act upon one ano ther; and of the means by which these may be increased, so as to overcome such as are most powerful. As this science is closely connected with the arts of life, and particularly with those which existed even in the rudest ages of society, the construction of machines must have been practised long before the theory upon which their principles depend could have been understood. Hence we find in use among the ancients, the lever, the pulley, the crane, the capstan, and many other simple machines, at a period when me chanics, as a science, were unknown. In the remains of Egyptian architecture are beheld the most surprising marks of me chanical genius. The elevation of im mense and ponderous masses of stone to the tops of their stupendous fabrics, must have required an accumulation of me chanical power, which is not in the pos session of modern architects. We are in debted to Archimedes for the foundation of this science : he demonstrated, that when a balance with unequal arms is in equilibrio, by means of two weights in its opposite scales, these weights must be reciprocally proportional to the arms of the balance. From this general princi ple the mathematician might have deduc ed all the other properties of the lever, but he did not follow the discovery through all its consequences. In demon strating the leading property of the lever, be lays it down as an axiom, that if the two arms of the balance are equal, the weights must be equal, to give an equi librium. Reflecting on the construction of the balance, which moved upon a ful crum, he perceived that the two weights exerted the same pressure on the ful crum as if they had both rested on it. He then advanced another step, and consid ered the sum of these two weights as combined with a third, and then the sum of the three with a fourth, and so On, and perceived that in every such combination the fulcrum must support their united weight ; and, therefore, that there is in every combination of bodies, and in every single body which may be considered as made up of a mumber of lesser bodies, a centre of pressure or gravity. This disco very Archimedes applied to particular cases, and pointed out the method of finding the centre of gravity of plane sur faces, whether bounded by a parallelo gram, a triangle, a trapesium, or a para.
bola. See CENTRE of gravity.
Galileo, towards the close of the six teenth century, made many important discoveries on this subject. In a small treatise on statics, be proved that it re quired an equal power to raise two dif ferent bodies to altitudes, in the inverse ratio of their weights, or that the same power is requisite to raise ten pounds to the height of one hundred feet, and twenty pounds fifty feet. It is impossible for us to follow this great man in all his discoveries. In his works, which were published early in the seventeenth cen tury, he discussers the doctrine of equa ble Motions in various theorems, contain ing the different relations between the velocity of the moving body, the space which it describes, and the time employ ed in its description. He treats also of accelerated motion, considers all bodies as heavy, and composed of heavy parts, and infers that the total weight of the body is proportional to the number of the parti cles of which it is composed. On this subject he reasons in the following man ner : " As the weight of a body is a power always the same in quantity, and as it constantly acts without interruption, the body must be continually receiving from it equal impulses in equal and successive instants of time. When the body is pre vented from falling, by being placed on a table, its weight is incessantly impelling it downwards ; but these impulses are de stroyed by the resistance of the table, which prevents it from yielding to them. But where the body falls freely, the im pulses which it perpetually receives are perpetually accumulating, and remain in the body unchanged in every respect, except the diminution which they expe rience from the resistance of the air : hence it follows, that a body falling free ly is uniformly accelerated, or receives equal increments of velocity in equal times. He then demonstrated that the time in which any space is described by a motion uniformly accelejated from rest.
is 40.11 to the time in which the same space would be described by an uniform equable motion, with half the final velo city of the accelerated motion, and that in every motion uniformly accelerated from rest, the spaces described are in the duplicate ratio of the times of description : after this he applied the doctrine to the ascent and descent of bodies on inclined planes. For a more particular account we may refer to Dr. Neil's " Physics."— Under the articles CENTRE of gravity,