28 
HAROLD’S DISCUSSIONS. 
heated if not a molten spheroid. Besides the condi¬ 
tion of the lowest rocks, it has been found that every¬ 
where as we descend into the crust the temperature 
rises about one degree on an average for every fifty- 
five feet. At that rate a temperature of three thou¬ 
sand degrees, sufficient to melt all known rocks, would 
be reached at a point thirty-two miles below the sur¬ 
face. 6. The nebulse present different forms. If 
you look at the Milky Way on a clear night, you will 
see whitish spots like thin, airy clouds. These and 
many others have been examined by Sir William 
Herschel. He observed that some appear stationary, 
others are drawn out into a spiral shape, as if revolv¬ 
ing about a given point, still others have the appear¬ 
ance of separating into two or more masses, etc. 
6. Some of the distant stars, which are suns like ours, 
seem to have separated, forming double or even triple 
and quadruple stars. These appear to revolve around 
each other; sometimes both are seen, and then only 
one, as if one were moving behind the other. In 1815 
Biela’s comet divided and became twins; seven years 
later they reappeared and had separated 1,404,000 
miles. 7. Mercury, it is claimed, is still enveloped by 
dense clouds, as if it were so hot that its water does 
not settle to form oceans. Jupiter, which is more 
than a thousand times as large as the earth, bears evi¬ 
dence that it is still very hot, if not glowing. It is 
possible that the sun may give off further rings and 
form other planets. Prof. George Darwin, in a 
study of the tides, established the birth of the moon 
by mathematical reasoning. He shows that the earth 
