ORAVITATTOH.] INTKODTJCTION TO PHYSICS. 



Numerous instances are daily occurring wherein the inertia of matter plays a prominent part. The jolting 

 motion of a carriage passing over a rough road ; the apparent projection of a passenger forward or backward, according 

 as a boat in which he may be at rest is suddenly moved in the reverse direction, <fcc. are cases wherein the body 

 does not, by its inertia, partake of the motion, and therefore remains still. In the case of a collision on a railway, 

 the passengers continue to move at the velocity they had already acquired, and are not able to bring themselves to. 

 rest : but the carriage in which they were travelling loses its motion, owing to the inertia of the object against which 

 it had been impelled, and which opposes its further progress. 



Having thus mentioned those properties which are common to every kind of matter, we proceed to inquire into 

 the powers by which two or more masses mutually act on and attract each other. Before the time of Sir Isaac 

 Newton, to whom is accorded the merit of having first proved the universal nature of the law of gravitation, many 

 philosophers held the opinion that there was a great attractive force acting on all matter. Hooke, a contemporary 

 of Newton, gave expression to such views, but in an unintelligible and enigmatic form. Newton, however, availed 

 himself of some rough observations of the moon's revolution in her orbit, to test the idea of universal gravitation. 

 By an acute intelligence, and deep mathematical acquirements, he was enabled, despite every obstacle, to make 

 certain of that which had previously been mere conjecture. He not only proved the fact that masses mutually 

 attract each other, but also demonstrated the amount of such attraction, and the variations to which it was subject 

 at different distances of the attracting bodies. 



GRAVITATION. The attraction of gravitation, so far as our earth is concerned, may be defined as that force in 

 obedience to which all bodies tend to fall towards its centre. This force varies according to the distance from the 

 nontui ; and as the poles are about thirteen miles nearer to it than the equator, it follows that a body will weigh 

 more at the poles than at the equator. 



The amount of this variation is inversely as the square of the distance ; or, more plainly, if any object be 

 attracted with a force at any specified distance from the centre of the earth, at double the distance the force will 

 be diminished to one-fourth ; at three times the distance, to one ninth ; and so on. 



The amount of attractive force which two bodies in space can exert will be directly proportional to the mass of 

 matter they contain ; hence the greater mass will draw the smaller to it. But each has an attraction for the other ; 

 and if their mnnmn do not greatly differ, they will, by their mutual attraction, remove each other from any supposed 

 position they may have occupied, on coming within the sphere of each other's action. It is literally true, that 

 the earth is attracted by every object raised from its surface, but as such bears no assignable proportion to the mass 

 of the earth, the effect of course is infinitely small. If, however, a ball be attached to a flexible cord, and suspended 

 near a max of metal, the attraction of both the earth and metal will act on it, and, instead of hanging in a line 

 perpendicular to the earth's surface, it will be drawn to one side. By comparing the attraction which a known mass 

 of metal baa, in causing a ball thus to diverge from the perpendicular, a rough idea of the density of the earth may 

 be arrived at. This is, however, more perfectly ascertained by means of a pendulum, which, as it is continually falling 

 towards the earth, when in motion, must necessarily vary in the number of its vibrations, according to its distance 

 from the centre of the earth. At London, a pendulum of the length of 39. 139 inches, vibrates sixty times per minute ; 

 whilst at the Poles, the number of vibration* would be much more numerous, and at the Equator they would be less. 



By thus experimenting, it has been found that the weight of the earth is equal to that of about six globes of 

 water of the same size, or, in other words, the earth weighs nearly six times as much as its own bulk of water. 



One effect of the attraction of gravitation is, that bodies falling towards the surface of the earth, move towards it 

 with an accelerated motion, that is, the nearer they approach the earth, the more space do they pass through in each 

 >eood of time. In the case of a body being projected from the surface of the earth, its upward motion is gradually 

 retarded until it comes to rest, and again returns to the earth, by virtue of the attraction of the latter. The 

 acceleration and retardation here spoken of, are directly consequent of the law that the force of gravitation varies 

 inversely as the square of the distance of the attracted from the attracting body. It has been found that a mass 

 will fall, in one second of time, through a space of 1G,', 7 feet ; but, neglecting fractions, a general rule may be 

 enunciated to the effect, that the space through which a body will fall during any number of seconds, may be 

 ascertained by multiplying 16 feet by the square of the number of seconds of time during which it has fallen. 



If, for instance, it were desired to know the depth of a mine, or the distance of the top of a tower from the 

 ground, it would be only necessary to count the number of seconds of time which a stone takes in falling to the base ; 

 and by multiplying the square of the number of seconds by 16 feet, the depth sought for would be at once 

 ami (allied. 



The nile just named gives only the total space through which a body would fall towards the earth during a given 

 number of seconds. If it be desired to kuow the space passed through in each second, a different course must be 

 adopted. It has been found, that by multiplying 16 feet successively by I, 3, 5, 7, 9, <fec., the space passed through 

 in the first, second, third, fourth, fifth, <tc. , second is at once obtained. The reader will therefore perceive, that the 

 lotai space passed through increases in geometrical progression, whilat the relative space in each second increases in 

 an arithmetical ratio only. The following table will more fully explain the rules just laid down : 



of Second! during 

 tlhich > tdy hu fallen. 



1' 

 2* 

 3* 

 4* 



6* 

 6' 



Space poMcd throngh 

 during each Second. 



16 feet. 



48 



80 

 112 

 144 

 170 



Total ipoce pawed through, 



16 feet 

 64 

 144 , 



256 

 400 

 676 



