2^6 THE POPULAR SCIENCE MONTHLY. 



under three hundred planets, and an enormous multitude of comets. 

 It will probably not be an overestimate if we suppose that the com- 

 ets are ten times as numerous as the planets. The case, then, stands 

 thus : The solar system consists of some thousands of different bod- 

 ies ; these bodies move in orbits of the most varied degrees of eccen- 

 tricity ; they have no common direction ; their planes are situated in 

 all conceivable positions, save only that each of these planes must pass 

 through the sun. Stated in this way, the present condition of the 

 solar system is surely no argument for the nebular theory. It might 

 rather be said that it is inconceivable on the nebular theory how a 

 system of this form could be constructed at all. Nine tenths of the 

 bodies in the solar system do not exhibit movements which would 

 suggest that they were produced from a nebula : the remaining tenth 

 do no doubt exhibit movements which seem to admit of explanation 

 by the nebular theory ; but, had that tenth not obeyed the group of 

 laws referred to, they would not now be there to tell the tale. The 

 planetary system now lives solely because it was an organism fitted 

 for survival. It is often alleged that the comets are not indigenous 

 to the solar system. It has been supposed that the comets have been 

 imported from other systems. It has also been urged with consider- 

 able probability that perhaps many comets may have had their origin 

 in our sun and have been actually ejected therefrom. I do not now 

 attempt to enter into the discussion of these views, which are at pres- 

 ent problematical ; let me pass from this part of the subject, with the re- 

 mark that, until the nature and origin of comets be better understood, 

 it will be impossible to appraise with accuracy the value of the argu- 

 ment for the nebular hypothesis which has been based on the uniform- 

 ity of the directions in which the planets revolve around the sun. 



There are, however, other circumstances in the solar system which 

 admit of explanation by the nebular theory. It is a remarkable fact 

 that the Earth, Mars, Jupiter, and Saturn are all known to rotate 

 upon their axes in the same direction as their revolutions around the 

 sun. The nebular theory offers an explanation of this circumstance. 

 It does not appear that this common rotation of the planets is abso- 

 lutely necessary for the stability of the system. Should it further be 

 proved that there is no other agency at work which would force the 

 planets to rotate in the same direction, then it must be admitted that 

 the nebular theory receives very substantial support. 



There is another way in which we can examine the evidence on 

 behalf of the nebular hypothesis. There are certain actions going on 

 at present in the solar system ; and by reasoning backward from 

 these present actions we are led to believe that in extremely early 

 times the condition of things may have resembled that which is sup- 

 posed by the nebular hypothesis. Let us begin with the consideration 

 of our sun, which is, as we know, daily radiating off light and heat 

 into space. This heat is poured off in all directions ; a small portion 



