606 



SCIENCE 



[N. S. Vol. XL VI. No. 1199 



would have been the result if the bodies in 

 question had not actually collided, but had 

 passed very close to one another? It can 

 be demonstrated mathematically that such 

 an approach would entail the formation of 

 two prominences, on each body one at the 

 point of least distance apart, and one dia- 

 metrically opposite. If the approach be 

 close enough, these prominences may be 

 drawn out into the form of two long arms 

 composed of discrete particles. As these 

 bodies pass, each, by the pull of its grav- 

 ity, will communicate to the other a rota- 

 tory motion which will result in the coil- 

 ing of the arms. These will be composed 

 of large numbers of comparatively small 

 particles, each of which is revolving in a 

 regular orbit around the central nucleus 

 of the system. These particles, resembling 

 in their constitution meteorites, have been 

 named planetesimals, and hence the name 

 of the hypothesis. 



Now, while the whole is rotating and has 

 the form of a spiral swarm, the tendency 

 will be for the planetesimals to come to- 

 gether and form a series of nuclei in the 

 arms, which, as they grow by accretion, be- 

 come solidified and form the planets. In 

 our present stage, most of them have been 

 gathered in. A few are still falling as 

 meteors, but the addition from this source 

 to the size of the earth is quite insignifi- 

 cant. 



This is the famous Planetesimal Theory. 

 It explains the phenomena better than any 

 other which has yet been suggested. But 

 it may be that this, too, will eventually go 

 the way of past theories, and its place 

 taken by another newer one. It is too 

 much to believe that we have now reached 

 finality, and that our hypothesis outlines 

 the actual physical facts of our earth's 

 history. 



Certain recent observations already 

 suggest a somewhat different organization 

 of the universe than that on which this 



theory is based. It has been pointed out 

 by Campbell that while the gaseous neb- 

 ulas are to be found mainly in the direc- 

 tion of the Milky Way, the spiral nebulae 

 are never seen in these parts of the heav- 

 ens but are numerous in directions at right 

 angles. Now the stellar system is looked 

 upon as being of a discoidal shape, and 

 what we call the Milky Way is merely the 

 direction of greatest depth and conse- 

 quently of closest distribution of the stars. 

 It follows that at right angles to this we 

 look through the stellar system and out 

 into infinite space, and it may be that the 

 spiral nebulas which are to be seen in these 

 directions are not within our stellar sys- 

 tem at all. Measurements of their motion 

 towards and away from us indicate that 

 they are moving at very rapid rates, prob- 

 ably as great as 500 miles per second, a very 

 much greater speed than that of any 

 known star. And yet, when their relative 

 positions in space are compared with those 

 they occupied fiJteen years ago, scarcely 

 any change can be observed. That is to 

 say, the nebulse are either all moving di- 

 rectly towards or away from the earth, 

 which is incredible, or, although they have 

 a lateral motion of enormous rapidity, 

 they are so far away that the distance 

 traveled in fifteen years is imperceptible to 

 us. How great their distances may be we 

 can not comprehend, even though it were 

 expressed in figures. From here to the ut- 

 most confines of our stellar system is esti- 

 mated as being of the order of 15,000 light 

 years, that is the distance light will travel, 

 going at the rate of 186,400 miles per sec- 

 ond, in 15,000 years. And if this theory be 

 correct, the nebula are so far away that, 

 though as large probably as our stellar 

 system, they seem to us scarcely larger 

 than one of the planets. We may there- 

 fore look upon them as other steUar sys- 

 tems like our own. And if there be on a 

 planet within one of these spirals, astron- 



