FUNDAMENTAI^ PROBI^KMS OF GEOLOGY. 217 



With IOC, 000, 000 or more known suns and an unknown number 

 of dark bodies moving in various directions with various velocities, 

 the possibility of collision is well recognized ; but, owing to the 

 vastness of the intervening spaces, the contingencies of collision for 

 an individual sun are small. However, no appeal is here made to 

 collisions as a source of the parent nebula of the solar system, but 

 only to an approach of the ancestral sun to another large body, and 

 this approach is not assumed to have been very close. This rather 

 distant approach is a contingency that may fairly be assumed as 

 likely to have been realized in fact. I have elsewhere discussed the 

 general effects of the close approach of celestial bodies'-'- to one an- 

 other, but the particular case of the supposed ancestral sun requires 

 special consideration. 



Our present sun shoots out protuberances to heights of many 

 thousands of miles, at velocities ranging up to 300 miles per second 

 and more. If it were not for the retarding influence of the im- 

 mense solar atmosphere, some of these outshoots would doubtless 

 project portions of themselves to the outer limits of the present sys- 

 tem, 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. 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 — suppose one several times the mass of 

 the sun, since it is regarded as a small star — the gravit}' which re- 

 strains this enormous elastic power will be relieved 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 is in- 

 creased. Such localized relief and intensification of pressure must, 

 it is believed, result in protuberances of exceptional mass and high 

 velocity. According to the well-known tidal principle, these ex- 

 ceptional protuberances would rise from opposite sides, and herein 

 lies the assigned explanation of the prevalence of two diametrically 

 opposite arms in the spiral nebulae. 



Nothing remotely approaching a general dispersion of the ances- 

 tral sun seems to be required. The present planets and their satellites 



* On the Possible Function of Disruptive Approach in the Formation of Me- 

 teorites, Comets, and Nebulae. Astrophys. Jour., vol. XIV, No. i, July, 1901, 

 pp. 17-40; Jour. Geol., vol. IX, No. 5, July-Aug. , 1901, pp. 369-393, 



