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of space in a whole county covered with such cannon, all pointed upward and all being discharged at once. The result would compare with what is going on inside the photosphere about as a boy's popgun compares with the cannon.

Perhaps, from a practical point of view, the most comprehensive and important problem of science is: How is the sun's heat kept up? Before the laws of heat were fully apprehended this question was not supposed to offer any difficulties. Even to this day it is supposed by those not acquainted with the subject, that the heat which we receive from the sun may arise in some way from the passage of its rays through our atmosphere, and that, as a matter of fact, the sun may not radiate any actual heat at all—may not be an extremely hot body. But, modern science shows that heat cannot be produced except by the expenditure of some form of energy. The energy of the sun is necessarily limited in quantity and is continually being lost through radiation.

It is very easy to imagine the sun as being something like a white-hot cannon ball, which is cooling off by sending its heat in all directions, as such a ball does. We know by actual observation how much heat the sun sends to us. It may be expressed in the following way:

Imagine a shallow basin with a flat bottom, and a depth of one centimetre, that is, about four tenths of an inch. Let the basin be filled with water, the latter then being one centimetre deep. Expose such a basin to the