THE ORIGIN OF THE EARTH, 55 



In harmony with this conception is the fact that the known spiral 

 nebulae are distributed on the sides of the supposed stellar disk or flat 

 spheroid of which the Milky Way is the assigned equatorial girdle. 

 If spiral nebulae occur on the outer border of the Milky Way, as our 

 view seems to imply that they should, they are doubtless beyond the 

 limits of distinct vision. 



Whatever may be the value of this particular view of the causes of 

 close approach, it is a consistent, if not necessary, assumption that, 

 in a multitudinous stellar system characterized by diverse rates of 

 motion in diverse directions, close approaches of suns to one another 

 are contingencies likely to be realized in fact. 



The special consequences of close approach. — Our present sun 

 shoots out protuberances to heights of many thousands of miles, at 

 velocities ranging up to 300 miles and more per second. If it were 

 not for the retarding influence of the immense solar atmosphere, some 

 of these outshoots would doubtless project portions of themselves to the 

 outer limits of the present system, and perhaps, in some cases, quite 

 beyond it, for the observed velocities sometimes closely approach the 

 controlling limit of the sun's gravity, if they do not actually reach it. 1 

 This illustrates the enormous explosive elasticity resident in the sun's 

 interior. The expansive potency of this prodigious elasticity is held in 

 restraint by the equally prodigious power of the sun's gravity. 



If, with these potent forces thus nearly balanced, the sun closely 

 approaches another sun, or body of like magnitude, possibly one several 

 times the mass of the sun, since the sun is regarded as a small star, the 

 gravity which restrains this enormous elastic power will be reduced 

 along the line of mutual attraction, on the principle made familiar in the 

 tides. At the same time the pressure transverse to this line of relief 

 will be increased. Such localized relief and intensified pressure must 

 bring into action corresponding portions of the sun's elastic potency, 

 resulting in protuberances of corresponding mass and high velocity. 



Dynamic considerations seem to make it clear that the sun might 



1 A body falling from infinity to the sun's surface, and thus acquiring the maxi- 

 mum velocity which the sun's attraction can give to it, would strike the sun's sur- 

 face at 382 miles per second, which is technically known as "the velocity from infinity," 

 or the "parabolic velocity," since a body shot from the sun with this velocity would 

 take a parabolic path and never return. This, therefore, represents the maximum 

 power of control of the sun, £ nd the velocity of some protuberances is nearly equal 

 to it. 



