24 SCIENTIFIC RECREATIONS. 



But if we go up into the air, the force of gravity will be diminished. 

 The attraction will be less, because we are more distant from the centre of 

 the earth. This decrease is scarcely, if at all, perceptible, even on very 

 high mountains, becatfse their size is not great in comparison with the mass 

 of the earth's surface. The rule for this is that gravity decreases in proportion 

 to the square of the distance. So that if at a certain distance from the earth's 

 surface the force of attraction be I, if the distance be doubled the attraction 

 will be only one quarter as much as before not one-half. 



Gravity has exactly the same influence upon all bodies, and the force 

 of the attraction is in proportion to the mass. All bodies of equal mass will 

 fall in the same time in a given distance. Two coins (or a coin and a feather 

 in vacuo) will fall together. But in the air the feather will remain far behind 

 the coin, because nearly all the atoms of the former are resisted by the air, 

 while in the coin only some particles are exposed to the resistance, the density 

 of the latter preventing the air from reaching more than a few atoms, com- 

 paratively speaking. The theory of weight and gravitation, and experiments 

 relating to the falling of bodies, may be easily demonstrated with ordinary 

 objects that we have at hand. I take a halfpenny and a piece of paper, 

 which I cut in the shape of the coin, and holding them side by side, I drop 

 them simultaneously; the halfpenny reaches the ground some time before 

 the paper, a result quite in accordance with the laws of gravitation, as one 

 must bear in mind the presence of air, and the different resistance it offers 

 to two bodies differing in density. I next place the paper disc on the upper 

 surface of the piece of money, and then drop them simultaneously. The two 

 objects now reach the ground at the same time, the paper, in contact with the 

 halfpenny, being preserved from the action of the air. This experiment is 

 so well known that we need not further discuss it ; but it must be plainly 

 evident that it is capable of development in experiments on phenomena 

 relating to falling bodies.* When a body influenced by the action of a force 

 acts, in its turn, upon another, the latter reacts in an opposite manner upon 

 the first, and with the same intensity. 



The Attraction of Cohesion is the attraction of particles of bodies to 



* M. A. G. has written us an interesting letter on the subject of similar experiments, 

 which we here transcribe : 



" When a siphon of seltzer water has been opened some little time, and the equilibrium 

 of tension is nearly established between the escaped gas and the dissolved gas, a vertical 

 stream of bubbles is seen to rise from the bottom of the apparatus, which present a very clear 

 example of the law of ascension of bubbles ; that is to say (putting out of the question the 

 expansion of the bubbles in their passage upwards), it is an inverse representation of the law 

 of gravity affecting falling bodies. The bubbles, in fact, detach themselves from their starting 

 point with perfect regularity ; and as the interval varies in one file from another, we have 

 before us a multiplied representation of that terrible law which Attwood's machine made such 

 a bugbear to the commercial world. I believe it is possible, by counting the number of 

 bubbles that detach themselves in a second, in each file, and the number which the whole 

 stream contains at a given instant, to carry the verification further ; but I must confess that 

 I have not done so myself." 



