November, 1911. 



KNOWLEDGE. 



417 



Galactic circle are 

 hundreds of thousands 

 of nebulae. An immense 

 number of them are of 

 an exquisite double 

 spiral structure. In the 

 next article, in describing 

 the origin of the Galaxy, 

 we shall attempt to shoN\' 

 that these nebulae were 

 extruded during the 

 coalescence of the twi) 

 cosmic s\stems w hich this 

 theor\- suggests went to 

 the formation of the 

 system of stars that is 

 commonh- known as the 

 Universe. It will lu' 

 explained that the Milky 

 Way is the result of 

 what we have called 

 " Whirling Coalescence," 

 and that the two great 

 sheets of nebulous 

 matter that now clothe 

 the two poles of the 

 Galaxv were ejected 

 during the earlier periods 

 of coalescence by the 

 agencv \\e have named 

 " Axial extrusion." In 

 s t u d \- i n g . then, the 

 origin of these double 

 spiral and other white 

 nebulae, we have to 

 assume that the two 

 poles of the Galax\- were 

 at one time covered w ith 

 vast continuous sheets of 

 nebulous matter which 

 the action of " selective 

 molecular escape" had 

 largely robbed of its 

 light gases. Into such 

 a sheet of material, the 

 manv errant masses that 

 are sent out of systems 

 bv the agencies described 

 in "The Birth of Worlds 

 and Systems" would 

 penetrate, and would 

 gradually be volatilised 

 bv friction, and would 

 form cometic nebulae, 

 de\eloping into roughly 

 globular nebulous con- 

 densations. These 

 portions would be denser 

 than the general sheet 

 of nebulae. Any con- 

 densed masses that were 

 near one another would 



Taiin at lV</!-i->' Obsei-;iti'ry fchuary 7—S, /Q/v. 



Figure 5. Spiral Nebula, Canes Venatici. 



A probable case of oblique impact between two nebulae of 



extremelv diflerent volumes. A considerable. portion of the large 



nebula remains at the end of the spiral. 



Tai-cn al Vtrkcs Oi-s.-f^-atmy Mar fS, lolo. 



Figure 6. Spiral Nebula, M.64. Comae Beren. 



.A probable case of whirling coalescence of two previously 



existing nebulae. 



be influenced by mutual 

 attraction, and approach 

 one another. Lateral 

 attraction of other 

 nebulae would, as a rule, 

 prevent direct impact, 

 and hence the approach- 

 ing pair would move in 

 curved orbits and come 

 into grazing impact. 

 This graze may, of 

 course, be of any depth, 

 as already suggested in 

 the case of grazing suns. 



The Effects of 

 Depths of Graze. 



The mere margins 

 mav collide and, as they 

 do so, the impact would 

 greath" increase the 

 temperature, and hence 

 the luminosity, of the 

 colliding margin, and 

 then, after a long period, 

 a streak of light would 

 exist between a pair of 

 more or less globular 

 nebulae — a condition 

 that not i n f r e q u e n 1 1 \' 

 shows itself in connection 

 with double nebulae. 

 In certain positions we 

 should obviously ha\'e a 

 nebula somewhat of a 

 dumb-bell shape. 



If the graze be deeper, 

 we should have a vast 

 spindle - shaped nebula 

 produced as a third 

 body, only, unlike that 

 of the suns, it would 

 be of extreme tenuity. 

 But the kinematics of 

 the two are the same 

 and hence rotation will 

 ensue, and a spiral begin 

 to show itself in the 

 centre of such nebulae. 

 We know of many ex- 

 amples of this, that of 

 Leo being very beautiful. 

 As time rolled on such 

 an incipient spiral would 

 complete itself, and we 

 should have a double 

 spiral structure. Such 

 a definite structure as 

 this would not be 

 possible in the case of 

 the grazing impact of 

 dense bodies like suns, 



