214 CARNEGIE INSTITUTION OF WASHINGTON. 



remarkable nebula in Canes Venatici there are curved streamers, like 

 the tails of comets, stretching outward from some of the knots in the 

 arms. If these are indeed streamers driven outward from the knots 

 and 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 hvo dominant arms that arise from diametrically 

 opposite sides of the nucleus and curve concentrically away. No 

 single-arm spiral of the watchspring 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 yiebulous 

 knots or partial concentrations on the arms and more or le.ss 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 irregularit5^ 



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 /><?/-- 

 tnits independence of parts. It is, therefore, conceived that the in- 

 numerable 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 C2i\\e.<l planetesimal nebnlcc. 



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

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

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

 nebula 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 can not be taken as 

 precise examples of the solar evolution. Because of these vast 

 dimensions and of the probable feebleness of control of the central 

 mass, which often appears to be itself quite tenuous, a rapid motion 

 can not well be assigned to the arms. Seen from the immense 



