224 METEOKIC STONES. 



by any of the foregoing difficulties, if we attend to the fol- 

 lowing undoubted truths. 



As the attraction of gravitation extends over the whole 

 planetary system, a heavy body, placed at the surface of the 

 moon, is affected chiefly by two forces ; one drawing it 

 towards the centre of the earth, and another drawing it 

 towards that of the moon. The latter of these forces, how- 

 ever, is beyond all comparison greatest at or near the moon's 

 surface. But as we recede from the moon, and approach to 

 the earth, this force decreases, while the other augments ; 

 and at one point between the two planets these forces are 

 exactly equal so that a heavy body, placed there, must 

 remain at rest. If, therefore, a body is projected from the 

 moon towards the earth, with a force sufficient to carry it 

 beyond this point of equal attraction, it must necessarily fall 

 on the earth. Nor would it require a very great impulse to 

 throw the body within the sphere of the earth's superior 

 attraction. Supposing the line of projection to be that which 

 joins the centres of the two planets, and supposing them to 

 remain at rest ; it has been demonstrated, on the Is ewtonian 

 estimation of the moon's mass, that a force of projection 

 moving the body 12,000 feet in a second, would entirely 

 detach it from the moon, and throw it upon the earth. This 

 estimate of the moon's mass is, however, now admitted to be 

 much greater than the truth ; and upon M. De la Place's cal- 

 culation, it has been shown that a force of little more than 

 one half the above would be sufficient to produce the effect. 

 A projectile, then, moving from the moon with a velocity 

 about three times greater than that of a cannon ball, would 

 infallibly reach the earth ; and there can be little doubt that 

 such forces are exerted by volcanoes during eruptions, as well 

 as by the production of steam, from subterranean heat. We 

 may easily imagine such cause of motion to exist in the moon, 

 as well as in the earth. Indeed, several observations have 

 rendered the existence of volcanoes there extremely probable. 

 In the calculation just now referred to, we may remark, that 

 no allowance is made for the resistance of any medium in the 



