1 6 EVOLUTION OF THE EARTH 



constitution its temperature must rise as it contracts, even 

 though at the same time it is radiating heat. The temperature 

 of the interior must furthermore be higher than the tempera- 

 ture of the surface, because of the greater compression with 

 depth, as is illustrated in the different strata of the terrestrial 

 atmosphere. In those convective or slow boiling movements 

 which are necessary in the sun and other stars in order that 

 they should be able to maintain their surface radiation, there 

 is then a constant liberation of energy from the depths and 

 a system of balanced motion which if disturbed could lead in 

 any star to an explosive blowing out of material from it on an 

 enormous scale. 



The tide-generating force varies directly with the mass of 

 the disturbing body and also with the radius of the body dis- 

 turbed. It varies approximately inversely with the cube of 

 the distance between the centers. The deforming force is, 

 furthermore, greatest in the interior because the tidal forces 

 acting on the zone at right angles to the line of attraction 

 have a component which tends to squeeze in the points d, d' 

 of Figure i toward the center. The gravitative control is 

 accordingly weakened along the line a, b, c, and is strengthened 

 in the directions at right angles. Now apply this principle 

 to the gaseous balanced nature of a star, and it is seen that the 

 expansion in the line a, b, c is no longer exactly balanced by 

 the gravitative compression, and the unbalancing is greatest 

 in the center, where also is the region of highest compression 

 and highest temperature. The effect is as if one squeezed a 

 syringe bulb with orifices for exit at both ends, a bulb, however, 

 like an air rifle, filled with gas compressed to an explosive 

 degree. 



Tidal disruption of the ancestral sun. The sun is occa- 

 sionally observed to shoot out streams of gas, known as solar 

 prominences, to heights of nearly 300,000 miles, and at 

 velocities ranging up to 300 miles per second. Such phe- 



