286 Sir William B. Grove [April 20, 



Neptune, as you know, was discovered by the effect of its pulling 

 force on another planet, the latter being deflected from its normal 

 course. When this pulling force is not counterbalanced by other 

 forces, or when the objects pulled have not sufficient resisting power, 

 they fall into each other. Thus, this earth is daily causing a bom- 

 bardment of itself by drawing smaller bodies — meteorites — to it, 

 20,000,000 of which, visible to the naked eye, fall on an average into 

 our atmosphere in each twenty-four hours, and of those visible through 

 the telescope, 400,000,000 are computed to fall within the same period. 

 Mr. Lockyer has recently given reasons for supposing the luminosity 

 of nebulae, or of many of them, is due to collisions or friction among 

 the meteorites which go to form them ; but his paper on the subject 

 is not yet published. You must get from Mr. Lockyer the details of 

 his views. I hope he may, at one of these evening meetings, give 

 you a resume of them from the place I now occupy. 



What is commonly called centrifugal force does not come from 

 nothing ; it depends upon the law that a body falling by the influence 

 of attraction, not upon, but near to, the attracting body, whirls round 

 the latter, describing one of the curves knowTi as conic sections. 

 Hence a meteorite may become a planet or satellite (one was supposed 

 to have become so to this earth, but I believe the observations have 

 not been verified) ; or it may go off in a j^arabola as comets do ; or 

 again, this centrifugal force may be generated by the gradual accre- 

 tion of nebulous matter into solid masses falling near to, or being 

 thrown off from the central nucleus, the two forces (centrifugal and 

 centripetal) being antagonistic to each other, and the relative move- 

 ments being continuous, but probably not perpetual. Our solar system 

 is also kept in its place by the antagonism of the surrounding bodies 

 of the Kosmos i3ulling at us. Suppose half of the stars we see, i. e. all 

 on one side of a meridian line, were removed, what would become of 

 our solar system ? It would drift away to the side where attraction 

 still existed, and there would be a wreck of matter and a crash of 

 worlds. It is very little known that Shakespeare was acquainted 

 with this pulling force. He says, by the mouth of Cressida — 



" But the strong base and building of my love 

 Is as the very centre of the earth, 

 Drawing all things to it " — 



a very accurate description of the law of gravitation, so far as this 

 earth is concerned, and written nearly a century before Newton's time. 

 But in all probability the collisions of meteorites with the earth 

 and other suns and planets are not the only collisions in space. I 

 know of no better theory to account for the phenomena of temporary 

 stars, such as that which appeared in 1866, than that they result from 

 the collision of non-luminous stars, or stars previously invisible to us. 

 That star burst suddenly into light, and then the luminosity gradually 

 faded, the star became more and more dim, and ultimately disappeared. 

 The spectrum of it showed that the light was compound, and had pro- 



