ACTION OF GRAVITY. 59 



A piece of paper, or a feather, falls more slowly than a 

 piece of lead or wood, and if the height is considerable 

 we shall moreover find that the lead reaches the ground 

 sooner than the piece of wood. This difference in the 

 velocity of falling bodies is due to the resistance of the 

 air; the force of gravity does not act differently on 

 different bodies, but acts equally upon all kinds of 

 matter. A body of small specific gravity, or generally 

 every body which possesses a large external surface 

 in proportion to its mass, meets with a greater resist- 

 ance by the air of our atmosphere, and falls therefore 

 more slowly than a body of greater specific weight and 

 smaller surface. Glass cylinders have been used for 

 experiments on falling bodies of various kinds, and it 

 has been found that when the air, and hence its resist- 

 ance, was removed, all bodies, whether small or large 

 and whatever their specific gravity, fall with equal 

 velocity. 



Cut a round disc of thin paper, a few millimetres 

 smaller than a crown piece or a bronze penny, and place 

 the paper disc upon the coin. Hold the coin perfectly 

 horizontal with the thumb and forefinger at opposite 

 points of the edge, about O m 5 above the table and let it 

 fall. The paper disc will not fall behind, but will reach 

 the table together with the coin, for the latter pushes 

 the air aside, and the paper disc is unresisted in its fall. 

 The velocity of bodies which fall freely is so great that 

 the motion can only be observed with some difficulty, 

 but its laws may be studied conveniently by means of 

 the machine described in the preceding article. The 

 descent of the weight is slower, but it is regulated by 



