MECHANICS. 



wards from the saddle, to a sufficient 

 height to clear the lower part of the 

 hoop with the feet. By the speed which 

 the rider has in common with the horse, 

 his body will, without any exertion on 

 his part, pass through the hoop, and he 

 will alight again in the saddle, on the 

 other side, in his descent. These are 

 striking instances of the composition of 

 motion. 



CHAPTER III. On the Force of 



Gravity. 



(15.) ALTHOUGH the force of gravity 

 cannot be considered, properly speak- 

 ing, as itself a first mover or mecha- 

 nical agent, yet it is the means of pro- 

 ducing and giving effect to so extensive 

 a class of first movers, that it becomes 

 necessary to explain the laws which 

 regulate its action, in order to render 

 the agency of some of the principal first 

 movers intelligible. 



The earth which we inhabit is a 

 mass of matter, nearly, but not ex- 

 actly, of a globular form, the diameter 

 being about 8000 miles in length. This 

 enormous mass possesses the property 

 of attracting towards its centre all 

 smaller bodies placed near its surface ; 

 so that if they be perfectly free to move 

 and opposed by no obstacle, they will 

 move in straight lines towards the cen- 

 tre of the globe, and will continue so to 

 move until they reach the surface. If 

 the part of the surface which they meet 

 be solid, or even a liquid specifically 

 heavier than the descending bodies, their 

 further approach to the centre will be 

 obstructed ; but in that case the attrac- 

 tion towards the centre will be mani- 

 fested by the force with which the bo- 

 dies press upon the resisting surface. 

 If the bodies thus supposed to have met 

 the surface in their approach towards 

 the centre happen to meet a liquid, as 

 the sea, and be specifically heavier than 

 it, they will still continue to approach 

 the centre, moving through the liquid 

 until, in fine, they be stopped either by a 

 liquid heavier than themselves or a hard 

 surface. All lines which are drawn 

 from points without a globe to its cen- 

 tre are evidently perpendicular to its 

 surface ; and hence bodies, in moving 

 towards the centre of the earth, at- 

 tracted by its influence, move perpen- 

 dicularly to its surface ; and when their 

 progress is obstructed by that surface 

 they press on it perpendicularly with a 

 force equal to that with which they are 

 attracted towards the centre. 



This attraction which the earth exerts 

 upon all bodies placed near its surface 

 is called terrestrial gravity : and the 

 force with which any body drawn 

 towards the centre is pressed upon an 

 horizontal plane, called the weight of 

 that body. 



It must be very obvious that all the 

 common effects of falling bodies, and of 

 pressures produced by weight, are per- 

 fectly accounted for in the preceding 

 observations. This attraction is by no 

 means peculiar to the earth, but is com- 

 mon to all material substances, whatever 

 be their form, quantity, or position. In 

 this respect the force of gravity differs 

 from magnetism, and other attractions 

 which are only resident in substances of 

 particular species. If the earth were a 

 large magnet, those peculiar substances 

 only which are affected by the loadstone 

 would have weight, or would fall to the 

 surface when unsupported. All other 

 bodies would rest indifferently in any 

 position in which they might happen to 

 be placed, and would move upwards 

 just as readily as downwards. But 

 every material substance is suscep- 

 tible of the attraction of gravity, and 

 what is more, it is susceptible of this 

 in the exact proportion of its mass. 

 Thus, if the mass of the earth were 

 doubled, it would exert a double attrac- 

 tion on all bodies placed near it, and 

 consequently the weights of all bo- 

 dies would in that case be doubled. If 

 its mass were tripled, the weights of all 

 bodies would be tripled, and so on. In 

 general, therefore, the attraction w r hich 

 the earth exerts on a body in its vicinity 

 is proportional to its mass. 



We have stated that gravity is an 

 attraction common to all material sub- 

 stances ; if so, then it may .be asked 

 why do not the various bodies placed 

 near the earth's surface attract the earth 

 towards them ? If a body be disengaged 

 at any height from the surface, it will be 

 drawn by the attraction of the earth, 

 and will consequently descend in a 

 straight line perpendicular to the sur- 

 face ; but since the body attracts the 

 earth, why does not the surface ascend 

 towards the body, being drawn by the 

 attraction of the body on the earth ; in 

 which case, the surface of the earth and 

 the body would meet at some place 

 intermediate between their first posi- 

 tions ? We answer that, in fact, this very 

 effect takes place. The surface of the 

 earth does approach the descending 

 body, and that descending body not only 



