16 M. Arago cwi Double Stars. 



^nd that point, somewhat lower, at which the ball would really 

 penetrate the wall, is the measure of the effect which gravitation 

 produces in the interval of a second, upon a body which moves 

 with so great a horizontal velocity. The experiment gives for 

 this distance 4 metres 9 c. ; — precisely the distance which the 

 bullet taken up so high, and then abandoned to itself, falls ver- 

 tically in the same time. 



Let us now place the wall at a somewhat greater distance from 

 the cannon. Let us suppose that the bullet does not reach it 

 till at the end of two seconds. The point which this bullet will 

 strike, we shall find much farther below the fixed point than in 

 the preceding experiment. But the distance betwixt the two 

 points will be exactly equal to the vertical descent of a body, 

 which, left to itself, is, for two seconds, subjected to the attrac- 

 tion of gravitation. 



In general terms, the attractive power of the earth produces 

 precisely the same effect upon a body at rest, and a body in mo- 

 Hon, when this effect is measured in the direction correspond- 

 ing to that in which the attraction is excited. 



The moon will now furnish us with an additional opportunity 

 of verifying this last law, and that of the diminution of the at- 

 tractive force in the ratio of the square of the distances. The 

 moon, in truth, in the eyes of an astronomer or geometrician, is 

 nothing more than a projectile, which, at the creation, has been 

 launched with a force sufficient to circulate indefinitely around 

 the earth, as would now happen, without the presence of an at- 

 mosphere, to a bullet projected horizontally from off the sur- 

 face of our globe with sufficient velocity. 



Let C, for example, be the point T ____jl 



occupied by the earth, round which 

 the moon revolves from right to left, 

 and A be the position of this lu- 

 minary. At the instant of quitting 

 the point A, the moon is moved in 

 the direction of a small element of 

 its curvilineal orbit, which passes 

 through the point A, that is to say, 

 in the direction of the straight line, the tangent A T. It is not, 

 however, in the point T that the moon will meet the radius CT, 



