7 2 THE POPULAR SCIENCE MONTHLY. 



William Herschel was, that it was a very common thing for 

 double nebulae to make their appearance in his gigantic telescope. 

 Now, it is difficult for us to imagine that these double nebulae, like 

 their allied systems of stars, should not be in motion ; and if we 

 imagine a condition of things in which one swarm is going around 

 a larger one in an elliptic orbit, and occasionally approaching it 

 and mingling with it, we shall have at one part of the orbit the 

 centers nearest together ; so that a greater number of particles of 

 meteoritic dust will be liable to encounters at this time than at 

 others. Hence we shall get a cause of increased temperature of 

 a periodic kind ; there must be variable stars in the heavens and 

 there are. 



As a third possible condition we have the known movement of 

 these swarms of dust through space. If we take note of the known 

 movements of the star which forms the center of our own system, 

 we can learn that these movements may be gigantic. We know 

 that the sun is traveling nearly half a million of miles every 

 twenty-four hours toward a certain region ; we know that other 

 stars are moving so quickly that Sir Robert Ball has calculated 

 that one among them would travel from London to Pekin in 

 something like two minutes. We have, therefore, any amount 

 of velocity. Now suppose that without the formation of either a 

 single or a double system, such as we have considered by the ordi- 

 nary condensation of an initial single or initial double swarm we 

 have what we may call a " level crossing " at which two or more 

 streams of meteoritic dust meet. There, of course, we shall have 

 a tremendous cause of collisions. Have we such instances in the 

 heavens ? Again I appeal to Mr. Roberts's photographs of the 

 Pleiades ; we see in them four nebulae which have been stated to 

 surround four of the stars. But if we look at the nebulae more 

 carefully, we find that distinct stream-lines are seen in each in 

 certain directions ; we have interlacing, the meeting of these 

 streams at some angle or other, and in each such region we have 

 the locus of one of the chief stars. 



This may be considered to be an irregular cause of a produc- 

 tion of high temperature ; but so long as such an action as that 

 continues, an apparent star will be seen, distinct, of constant 

 light, and not to be discriminated, without such photographs as 

 these, from those stars which have been produced by more ordi- 

 nary sequences connected with the more ordinary processes of 

 condensation. 



If, however, the above explanation be the true one, we should 

 expect to find cases in which we may see such an action beginning 

 or ending suddenly ; the action will be less constant and durable 

 that is to say, the supply of these streams of meteoritic dust 

 may not be continuous ; it may be smaller, and then the effect 



