CHAMBERS'S INFORMATION FOR THE PEOPLE. 



rider springs right up from the saddle, and alights 

 on it again without falling behind the horse. 

 Additional illustrations of this law will be found 

 under METEOROLOGY and PHYSICAL GEOGRAPHY. 



TERRESTRIAL GRAVITY. 



It is a law, as we have seen, wide as the uni- 

 verse, that every particle of matter acts upon every 

 other particle by the force known as gravitation, 

 which draws them one towards another. We are 

 now to consider the laws of this force, and explain 

 the more important phenomena it gives rise to. 



The attraction of gravitation is always in pro- 

 portion to the mass of the attracting body. This 

 will be readily admitted. A force is exerted by 

 each atom, and the more atoms the more force. 

 It is not the volume or bulk of the body, but its 

 quantity of matter, that decides its effect. Gravi- 

 tation, again, acts at all distances, but with 

 diminished effect as the distance increases. The 

 decrease, however, does not go on in a simple 

 proportion ; twice the distance does not give 

 simply half the attraction. If, at the distance of 

 a foot, the force of attraction between two bodies 

 were a pound, at the distance of 2 feet, it would be 

 i of a pound ; at 3 feet, 1th of a pound ; at 4 feet, 

 Ath of a pound. This is expressed by saying 

 that attraction varies inversely as the squares of 

 the distances between the bodies. 



The distance between two mutually attracting 

 globes is measured from centre to centre. The 

 attraction of the earth, then, on a body at its 

 surface is exerted at the distance of about 4000 

 miles, which is the length of its semi-diameter; 

 at the height of 4000 miles above the surface, or 

 at the distance of two semi-diameters from the 

 centre, the attraction is reduced to one-fourth. 

 The moon being at the distance of sixty semi- 

 diameters, the earth draws it with a force which is 

 only a 36ooth part of what it would be, were the 

 moon at the earth's surface. It belongs to astronomy 

 to treat of gravitation as ruling the planetary 

 motions ; we have here to speak of the pheno- 

 mena it produces on bodies at or near the earth's 

 surface. 



The attraction of the earth for bodies on its 

 surface is so strong, owing to its overwhelming 

 mass, that it overpowers their attraction for one 

 another, and renders it in most cases inappre- 

 ciable. But a plumb-line suspended near a large 

 mountain is sensibly drawn from the perpen- 

 dicular ; and by a delicate instrument, called the 

 torsion-balance, the action of a large ball upon a 

 small one has been measured. The weight or 

 gravity of a body is another name for the force 

 with which it is drawn towards the earth. 



Falling Bodies. 



Perhaps nothing explains more phenomena 

 than the complete understanding of what takes 

 place when a body falls ; for half the motions in 

 the world are caused, directly or indirectly, by 

 falling. Falling is the best instance of a uni- 

 formly accelerated motion. Gravity not only puts 

 a body in motion, but continues to act on it with 

 equal force after it is in motion, and thus is uni- 

 formly adding to its speed. This acceleration is 

 seen in every body dropped from a height through 

 the air ; and ho less in a rock rolling down a 

 declivity, which continually gathers force as it 

 200 



descends, till its energy is sufficient to shatter 

 itself and everything it encounters to pieces. 



All bodies, heavy and light, would fall equally 

 fast if the resistance of the air were removed. A 

 ball of lead two pounds' weight does not fall faster 

 than a ball of one pound. A gold leaf, however, 

 will fall considerably slower than the same gold in 

 a solid state, because it exposes much more sur- 

 face to the air. That this is the cause of the 

 difference, is shewn by an experiment with the 

 air-pump, in which a guinea and a feather are 

 dropped in a vacuum, and fall to the bottom 

 together. 



It has been well ascertained by observation, 

 that when a body begins to fall from a state of 

 rest, it descends 16 feet i inch in the first second 

 of time. (This measurement varies slightly with 

 the latitude, as will be afterwards explained.) 

 Suppose that no fall had ever been observed 

 beyond that point, and that we had to find by 

 reasoning what the body would do during the 

 next second. The first consideration is, How far 

 would the body fall, if gravity were to cease ? in 

 other words, What velocity has it acquired? It 

 would not be unnatural to say 16 feet (we omit 

 the i inch), since that is the space it fell in the 

 second that is ended ; but reflection shews that 

 this cannot be the case. During the first half of 

 the second, it made very little way, and the greater 

 part of the distance was passed over in the last 

 half; 16 feet expresses the mean velocity, or the 

 velocity it had at the middle of the second, and 

 at the end it must have been moving with a 

 velocity just double the mean. The velocity 

 acquired, then, at the end of the first second is 32 

 feet ; and if gravity were to cease, it would, by 

 the law of inertia, move over 32 feet in the next 

 second. But gravity still acting, will make it fall 

 another 16 feet in addition to the 32, or 48 feet in 

 all ; and will create as much velocity in addition 

 as it created in the first second, so that if it were 

 to cease at the end of the second second, the body 

 would move 64 feet in the third second. Thus, 

 during the second second, the body will fall 

 through three spaces of 16 feet, and at the end of 

 it will have its velocity double of what it was at 

 the end of the first second. The whole space 

 fallen during the two seconds will thus be four 

 spaces of 1 6 feet. 



We arrive at this result by reasoning, and obser- 

 vation proves it correct. Without giving the 

 steps of the investigation for the succeeding spaces 

 of time, the results may be exhibited in the follow- 

 ing table : 



For example : The third column, headed 3 

 (seconds), informs us that at the end of 3 seconds 

 a falling body is moving (for the instant) at the 

 rate of 6 spaces of 16 feet that is, of 96 feet a 

 second ; that during the third second it falls 5 



