MECHANICS. 



741 



the particles only of one or of several bodies, conies 

 within the department of chemistry. Those motions 

 which affect masses are the appropriate subject of 

 the second part of mechanics. 



All motions are found to take place in conformity 

 to a few universal principles. Deduced from obser- 

 vation, and confirmed by experiment, these principles 

 have often been placed at the beginning of treatises 

 on mechanics, under the name of the laws of motion. 

 If not expressed in this manner, the truths they 

 declare, making an essential part of the principles of 

 the science, are necessarily introduced under some 

 other form. Their comprehensiveness suits them to 

 our purpose, and they are here quoted in the lan- 

 guage of Newton. 



I . " Every body perseveres in its state of rest, 

 or of uniform motion in a right line, unless it is 

 compelled to change that state by forces im- 

 pressed thereon." This is called the law of inertia, 

 and expresses the entire indifference of matter to 

 motion or rest. The proposition that a body will 

 never begin to move of itself needs no proof. It is 

 the conclusion of universal observation. Wherever 

 we observe motion, we conclude that there is a 

 power in action to produce it. The other part of 

 the law, that motion is, in its nature, as permanent 

 as rest, and that it is in a right line, is far from being 

 a self-evident, or even an obvious truth. Limited 

 observation would lead to the conclusion that all 

 matter has a tendency to rest, and such has long 

 been, and still is, a common error. The same 

 limited observation led some of the ancient astronom- 

 ers to imagine that all bodies, when forcer! into a 

 state of motion, naturally moved in curved lines. 

 There is, however, abundant proof of the perman- 

 ence of motion ; and if friction and the resistance of 

 the air, the two most universal obstacles to the 

 motion of bodies near the surface of the earth, could 

 be entirely removed, instances of permanent motion 

 would be still more numerous. In proportion as 

 they are removed, or as bodies are beyond their 

 influence, we observe a tendency in motions to be- 

 come more and more permanent. A marble, rolled 

 on the grass, soon stops; on a carpet, it moves 

 longer ; on a floor, still longer ; and on smooth, level 

 ice, where the wind is not unfavourable, it continues 

 very long in motion. In a vacuum, where the resist- 

 ance of air is not felt, two windmills, whose pivots 

 have equal friction, and which are set in motion by 

 equal forces, continue to move equally long, what- 

 ever be the position of their vanes. In the air, the 

 one whose vanes cut the air, will move much longer 

 than the one whose vanes are opposed to it. A pen- 

 dulum in a vacuum, having only the stiffness of the 

 riband by which it is suspended to overcome, will 

 vibrate for a whole day. A spinning top, in the 

 same situation, retarded only by the friction of its 

 point, continues spinning for hours. In all these 

 cases, the continuance of the motion is proportioned 

 to the diminution of friction and resistance. We 

 can hardly avoid the conclusion, that a body once 

 put in motion, would, if left to itself, continue to 

 move with undiminished velocity. 



The heavenly bodies, moving in free space, subject 

 to no opposing influence, keep on in their path with 

 a velocity which has remained unabated since first 

 they were launched from the hand of the Creator. 

 They move not, indeed, in straight lines, but in 

 curves, as they are drawn towards each other, and 

 towards a centre, by the universal force of gravity. 

 (See Gravity.) This force does not diminish their 

 velocity, but deflects them continually from the right 

 line in which they tend to move. If this central 

 force were suspended, they would all shoot forward 

 into space, and the harmony of their motions would 



cease. Some force similar to this central tendency 

 is always in action, whenever we see bodies move in 

 curve lines. The stone, to which a boy gives accu- 

 mulated force by whirling it round in a sling, is, for 

 a time, kept in its circle by the central force repre- 

 sented by the string; when let loose, it darts for- 

 ward in the air, turning not to the right or left, until 

 the atmospherical resistance destroys its motion, or 

 the force of gravity bends it to the ground. A 

 full tumbler of water, placed in a sling, and 

 made to vibrate with gradually increasing oscil- 

 lations, may, at last, be made to revolve in a 

 circle about the hand, each drop tending to move 

 out in a straight line from the centre, and there- 

 fore remaining safe in the tumbler, whose bottom 

 is always farthest from the centre. In a corn 

 mill, the grain is poured gradually into a hole in 

 the centre of the upper mill-stone. The weight 

 of the stone pulverizes the corn, while its circular 

 motion throws it out, as fast as it is ground, into a 

 cavity around the stone. When a vessel, partly full 

 of water, is suspended by a cord, and made to turn 

 rapidly round, the water, in its tendency to move 

 out in a straight line, recedes from the centre, and 

 is gradually heaped up against the sides of the ves- 

 sel, sometimes even leaving a portion of the bottom 

 dry. Water, moving rapidly in the stream of a river, 

 or the tide of the sea forced violently through a 

 narrow passage between opposite rocks, not unfre- 

 quently forms a whirlpool on the same principle. 

 Bent out of its course by a projecting ledge, it 

 departs, as if reluctantly, from a straight line, and 

 heaps itself up towards the circumference of the 

 circle in which it is compelled to move. To this 

 cause, too, it is owing, however little we might 

 expect such a consequence, that a river, passing 

 through an alluvial soil, and once turned from its 

 onward channel, continues to pursue a meandering 

 course to the sea. Driven, by any cause, to one 

 side, it strikes the bank with all its violence, is 

 repelled, and rebounds with the same force to the 

 opposite side, continually wearing the two banks, 

 and leaving a larger space on the inner side of the 

 bends. The force with which a body constrained to 

 move in a circle, tends to go off in a straight line, is 

 called the centrifrugal force. Advantage is taken 

 of it in many processes of the arts, and in all cir- 

 cular motions of machinery. The clay of the potter 

 is placed on the centre of a swiftly revolving table, 

 and while his hand shapes it, the centrifugal force 

 causes it to assume the desired dimensions. A 

 globe, or sheet of molten glass, is in a similar 

 manner made to expand itself. The legs of a pair 

 of tongs, suspended by a cord, and made to revolve 

 by its twisting or untwisting, will -diverge in pro- 

 portion to the velocity of the revolution. The steam 

 governor of Watt is constructed and acts on this 

 principle. Weights are attached to two rods, to 

 which circular motion is communicated by the 

 machinery which is to be governed. If the motion 

 be so rapid as to cause these rods to diverge from 

 each other beyond a certain angle, they act upon a 

 vitlve which partly closes, and diminishes the supply 

 of steam. With a slower motion, the rods collapse, 

 and the valve is opened. In consequence of the 

 centrifugal force occasioned by the rotation of the 

 earth, the weight of bodies at the equator is dimin- 

 ished the 289th part. If the earth revolved on its 

 axis in 84 minutes, the loose parts near the equator 

 would be projected from the surface. 



Another consequence or particular of the law of 

 inertia, is, that motion is communicated gradually. 

 A force which communicates a certain quantity of 

 motion in one second, will impart double the quan- 

 tity in two seconds. A ship does not yield at once to 



