Astronomy and Seismology. 153 



In 1872, the comet was something like 200,000,000 miles away 

 from the bodies that we met as we passed through them on the 

 27th of November, giving us a brilliant shower. Thirteen years 

 later we passed through the group again, and then we were some- 

 thing like 300,000,000 miles ahead of the group. So that some 

 of the particles, leaving the comet between 1840 and 1870, had 

 fallen behind and others between 1840 and 1885 had gained. 



What should separate those particles? What are the forces 

 which carried off those particles so many miles — 200,000,000 miles 

 on the one hand and 300,000,000 miles on the other, in round 

 numbers? The force that acts on thern must be a force acting in 

 one plane, that is, the plane of the orbit of the comet. Any force 

 acting in other planes would have scattered the group and we 

 would not have met them as a single definite group at the times 

 named; but if it acts in the plane, only scattering them on the 

 plane, they would be together as we saw them. 



In that plane, it must be either an impulsive force acting once 

 or it must be a constant force acting continually. The only 

 bodies in that plane are the comets and the sun, and if the force 

 is a continuous force it must be from the comet or from the sun. 

 It is almost inconceivable to suppose that the comet could have 

 sent them off, either impulsively or continuously, in such a way 

 as to give us the distance of 200,000,000 and 300,000,000 miles 

 in the course of thirty years; it would require far more than any 

 velocity that we can give in our terrestrial experiments, and we 

 have no reason to suppose that there is any such power of impul- 

 sion. Moreover, if the impulsion came from the comet, they 

 would go in all directions and their character, as being in a plane, 

 would have been entirely lost. 



We are then thrown back on this one hypothesis, that the sun 

 is the source of that force. In other words, we are led to extend 

 the idea that I gave you in the beginning, and which is accepted 

 by astronomers, that the material which goes off from the comet, 

 after it leaves it, is subject to a force like that of attraction but 

 differing in its intensity. In the case of the tail, it is a repulsive 

 force. To satisfy these conditions of separation, part in one 

 direction and part in the other, from the comet, we must have an 

 attraction in the one case exceeding the attraction of gravitation 

 and, in the other, an attraction less than the attraction of grav- 

 itation. In other Words, these little bodies of hard matter that 

 go off from the comet and follow very nearly in its train are 

 acted on not in proportion to this mass like the force that acts on 

 the planets in their orbits. 



I see no escape, myself, from this conclusion. What it means, I 

 must leave to you to decide. Our experiments make it very im- 

 probable that the attraction of matter differs in any way from 

 proportion to the mass. It looks to me as though the more 

 natural explanation is that, in some way, the materials which go 

 off from the comet carry with them a load of electricity, or 

 something of that kind, by which they have a permanent repul- 



