SECT, xxxvn.] DISTRIBUTION OF NEBUUE. 441 



increase of density to apparently a solid nucleus may be 

 accounted for by supposing the general constitutions of 

 those nebulae 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 nebulae 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 propagated 

 through them. Consequently, since no general rotation of 

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

 ceived 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 nebulse 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, inter- 

 spersed with vacant intervals, which fills about an eighth 



