THE ORIGIN OF THE EARTH. 47 



curved by motion, as in the case of comets' tails, they testify very 

 definitely to a rotatory movement. 



A notable and seemingly very significant feature of these nebulae 

 is the presence of two dominant arms that arise from diametrically op- 

 posite sides of the nucleus, and curve concentrically away. No single- 

 arm spiral of the watch-spring type has been found. There are often 



Fig. 16. Fig. 17. 



Fig. 16. — A typical spiral nebula in Piscium, Messier 74, with very symmetrical arms, 

 pronounced nucleus and knots, and a relatively limited amount of nebulous haze. 

 (Photo, from Lick Observatory.) 



Fig. 17. — A suggestive nebula in Ceti, Messier 77, in which the proportions between 

 the apparent masses of the central and the outlying parts is somewhat analogous 

 to those assigned the solar nebula; so also are the proportions between the knots 

 and the nebulous haze, and the number of the knots is also similar, but the volume 

 of the central portion is disproportionately large. (Photo, from Lick Observatory.) 



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 

 knots or partial concentrations on the. arms, and more or less outside 

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

 tributed, and in some of the forms, regarded as youngest, dark spots 

 and lines emphasize the irregularity (see Fig. 12). 



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 inde- 



