416 DISTRIBUTION OF NEBULAE. SECT. XXXVI. 



clusters contain thousands of stars, and are frequently in the 

 poorer parts of the sky, as if in the course of ages the stars had 

 been attracted towards a centre. 



The existence of every degree of ellipticity in the nebulas from 

 long lenticular rays to the exact circular form and of every 

 shade of central condensation, from the slightest increase of 

 density to apparently a solid nucleus may be accounted for 

 by supposing the general constitutions of those nebulas to be 

 that of oblate spheroidal masses of every degree of flatness from 

 the sphere to the disc, and of every variety in their density and 

 ellipticity towards the centre. It would be erroneous, however, 

 to imagine that the forms of these systems are maintained by 

 forces identical with those already described, which determine the 

 form of a fluid mass in rotation ; because, if the nebulas be only 

 clusters of separate stars, as in the greater number of cases there 

 is every reason to believe them to be, no pressure can be propa- 

 gated through them. Consequently, since no general rotation of 

 such a system as one mass can be supposed, it may be conceived 

 to be a quiescent form, comprising within its limits an indefinite 

 number of stars, each of which may be moving in an orbit about 

 the common centre of the whole, in virtue of a law of internal 

 gravitation resulting from the compound gravitation of all its 

 parts. Sir John Herschel has proved that the existence of such 

 a system is not inconsistent with the law of gravitation under 

 certain conditions. 



The distribution of the nebulas over the heavens is even more 

 irregular than that of the stars. In some places they are so 

 crowded together as scarcely to allow one to pass through the field 

 of the telescope before another appears, while in other parts hours 

 elapse without a single nebula occurring. They are in general 

 only to be seen with the best telescopes, and are most abundant 

 in a 'zone whose general direction is not far from the hour circles 

 O h and 12 h , and which crosses the Milky Way nearly at right 

 angles. Where that nebulous zone passes over the constellations 

 Virgo, Coma Berenices, and the Great Bear, they are to be found 

 in multitudes. 



The nebulous system is nearly divided into two parts by the 

 Milky Way. One- third of the whole visible nebulous contents of 

 the heavens forms a broad irregular mass, interspersed with 

 vacant intervals, which fills about an eighth of the surface of the 



