PLANETESIMAL THEORIES OP THE EARTfl's ORIGIN. 191 



nebulae are estimated to number at least ten times the whole number 

 previously known. From the superior number of spiral nebulae it is 

 •a safe inference that their peculiar forms represent some prevalent 

 process in celestial dynamics. This is in itself a reason why research 

 should turn to them, by preference, for the origin of the present 

 solar system. . 



" A notable and seemingly very significant feature of these 

 nebulae is the presence of two dominant arms that arise from 

 diametrically opposite sides of the nucleus and curve concentrically 

 away. No single arm-spiral of the watch-spring type has been 

 found, so far as I am aware. There are often more than two arms 

 in the outer part, and there is much irregularly dispersed matter, 

 but even in the more scattered forms the dominance of two arms is 

 •discernible. 



" A second feature of note is the presence of numerous nebulous 

 Icrwts or partial concentrations on the arms and more or less outside 

 them. So, also, the more diffuse nebulous matter is unequally 

 •distributed, and in some of the forms, regarded as youngest, dark 

 spots and lines emphasize the irregularity. 



" All these features go to show that these forms are controlled, 

 not by the support of part on part, as in a continuous body or in a 

 mass of gas or even in a definite swarm of quasi-gaseous meteorites, 

 but by some system of combined kinetic energy and gravity which 

 ^permits independence of parts. It is, therefore, conceived that the 

 innumerable solid or liquid particles which the continuous spectrum 

 implies, revolve about the common center of gravity as though they 

 were planetoidal bodies. If this were certainly known to be the 

 case, these might well be called planetesimal nehidce. 



" It is clear from the tenuity of these nebulae, as seen from the side 

 •of the spiral, that they are disk-like, and this is directly shown to be 

 so when they are seen obliquely. In their disk-like shape these 

 nebulae conform to the mode of distribution of matter in the solar 

 system. Within the area of their disks, also, the distribution is 

 irregular, as it is in the solar system— a fact too much overlooked 

 by reason of our predilection for symmetry, under the influence of 

 the symmetrical Laplacian conception. 



" All of the more familiar spiral nebulae have dimensions that 

 vastly transcend those of the solar system, and they cannot be 

 taken as precise examples of the solar evolution. . . . It is to 

 be hoped, however, that the present rapid progress in the perfection 

 of instruments and of skill will soon bring within the reach of 

 successful study some of the smaller spiral nebulae that represent 

 the solar system more nearly in mass and proportions. 



" With this much of knowledge and of limitation of knowledge 

 relative to existing nebulae, the construction of a working hypothesis 

 required not a little resort to supplementary deductive and 

 hypothetical considerations. The inference that a spiral nebula is 



