DYNAMICS. 



within the same bulk. The former is 

 said to be more denae, while the latter is 

 said to be more rare ; density and rarity 

 being opposite qualities. Therefore, if 

 we could always ascertain the number of 

 atoms, or of minute parts, contained in a 

 mass, we should be able to appreciate the 

 density; because, under the certainty 

 that, in proportion as more or less atoms 

 are comprised within a given space, so 

 must the mass be more or less dense ; 

 i. e. heavier or lighter. 



In computing density we therefore take 

 the rectangle of the mass, and, ascertain- 

 ing the dimensions, discoverby its weight 

 how many atoms, or particles, it contains. 

 Hence is derived our table of specific gra- 

 vities, or the comparative weights of va- 

 rinvis bodies of unequal densities. (See 

 HYDROSTATICS.) From this it will be 

 seen, that bodies of similar substance 

 and form, but differing in bulk, are 

 to each other in proportion to the mag- 

 nitude of their respective masses; while, 

 on the other hand, bodies of similar 

 form, and equal in bulk, are, to each 

 other, in proportion to their respective 

 densities. 



Forces are considered according to the 

 quantities of motion they are capable of 

 producing; but, as we cannot measure 

 those forces, we are under the necessity 

 of ascertaining the power by means of 

 the effect. Thus we can correctly ascer- 

 tain the force of gunpowder by the ef- 

 fect produced by the shot ; or we can fully 

 explain the force with which the spring 

 of a watch acts, by finding what re- 

 sistance it is capable of overcoming; 

 or we may compute the strength of a 

 horse, by witnessing the weight he can 

 draw. But it must be obvious we could 

 not discover, per se, either the strength of 

 the gunpowder, the elastic powers of 

 the spring, or the muscular vigour of the 

 animal. 



Force and velocity are, in fact, synoni- 

 mous terms; for the impetus given to 

 the shot fired from a cannon, estimated 

 by the bulk of the shot, and the distance 

 to which it may be projected, or the im- 

 pression it may make on an opposing ob- 

 ject, completely supply the result of our 

 research ; remarking, that, this being a 

 diminishing force, its action will be 

 strongest at the moment of expulsion, 

 and gradually less as it recedes from the 

 origin of motion, until it finally acquires 

 a state of rest. The spring in a measure 

 partakes of the same diminishing tenden- 

 cy ; but as it may be held in equilibrio at 

 any period of its exertion, it cannot be 



classed with the former, though, rigidly 

 speaking, it is assuredly a diminishing 

 force ; for, as we see in clocks and watch- 

 es, springs will in due time arrive at a 

 state of rest, or inaction. The animal 

 power is subject to so many anomalies, 

 that it is next to impossible to treat of it 

 with any strict adherence to calculation ; 

 because, in so doing, we are compelled 

 to banish what we know to be the effects 

 of labour, and to consider the power as 

 always equal, and always maintaining the 

 same physical ability. Here, indeed, we 

 find theorists generally proceeding upon 

 a wrong basis ; and, of course, rarely 

 correct in their conclusions. We find 

 them estimating the powers of horses, 

 &c. as though their limbs had no flexibi- 

 lity, their muscles no relaxation, and as if 

 their shoulders were insensible to pres- 

 sure ; in fact, they generally consider the 

 animal as a fulcrum of wood, iron, &c. 

 The absurdity of such a calculation must 

 be obvious. 



But to proceed. We consider force to 

 be either equal (or permanent), accele- 

 rating (or gaining in power), or dimi- 

 nishing (or losing in power.) Thus, the 

 motion of a well-regulated clock may be 

 considered as an equal force ; because, in 

 equal periods, it proceeds over equal 

 spaces. A weight falling from a height 

 is an accelerating force ; because it gra- 

 dually accumulates velocity in proportion 

 to the space through which it falls ; and 

 a shot fired from a cannon is a diminish- 

 ing force ; because it constantly and gra- 

 dually loses velocity, until, at length, it 

 ceases to move. Dissimilar bodies will 

 move through the same space in exact 

 ratio with their own squares, and their 

 relative impulses ; but if two bodies, of 

 equal bulk and density, be set in motion 

 oppositely, by the same momentum, or 



Eower, they will hold each other in equi- 

 brio : and if two such bodies so acted 

 upon should meet, they will mutually ob- 

 struct each other's progress. Of this we 

 may frequently see instances in the game 

 of billiards. But if two bodies of differ- 

 ent density be acted upon by forces 

 that correspond with their masses re- 

 spectively, the greater will overcome the 

 lesser ; as will also a body impelled by a 

 greater force than one 01 equal density, 

 to which it may come in opposition. 

 Thus, a pistol shot meeting a fives' ball 

 will cause it to deviate from its course, or 

 to recede. 



Uniform, or perfectly equal, motion 

 does not exist naturally. It is, perhaps, 

 not to be found any where i though our 



