great shell of ice, one inch thick and 186,000,000 miles in diameter, its rays would just melt the whole in the same time.
If, now, we suppose this shell to shrink in diameter, retaining, however, the same quantity of ice by increasing its thickness, it would still be melted in the same time. Let the shrinkage continue until the inner surface touches the photosphere, and it would constitute an envelope more than a mile in thickness, through which the solar fire would still thaw out its way in the same two hours and thirteen minutes; at the rate, according to Herschel's determinations, of more than forty feet a minute. Herschel continues that, if this ice were formed into a rod 45·3 miles in diameter, and darted toward the sun with the velocity of light, its advancing point would be melted off as fast as it approached, if by any means the whole of the solar rays could be concentrated upon it. Or, to put it differently, if we could build up a solid column of ice from the earth to the sun, two miles and a quarter in diameter, spanning the inconceivable abyss of ninety-three millions of miles, and if then the sun should concentrate his power upon it, it would dissolve and melt, not in an hour nor a minute, but in a single second; one swing of the pendulum, and it would be water; seven more, and it would be dissipated in vapor.
In formulating this last statement we have, however, employed, not Herschel's figures, but those resulting from later observations, which increase the solar radiation about twenty-five per cent., giving fifty feet, and not forty feet, as the thickness of the ice-crust which the sun would melt off of his own surface in a minute. An easy calculation shows that to produce this amount of heat by combustion would require the hourly burning of a layer of anthracite coal about sixteen feet (five metres) thick over the entire surface of the sun—four fifths of a ton per hour on each square foot of surface—at least eight times as much as the consumption of the most powerful blast-furnace known to art. It is equivalent to a continuous evolution of more than seven thousand horse-power on every square foot of the sun's whole area. As Sir William Thomson has shown, the sun, if it were composed of solid coal and produced its heat by combustion, would burn out in less than six thousand years.
Of this enormous outflow of heat the earth of course intercepts only a small portion, about 2200000. But even this minute fraction is enough to melt yearly at the equator a layer of ice something over one hundred and ten feet thick. If we choose to express it in terms of "power," we find that this is equivalent, for each square foot of surface, to more than sixty tons raised to the height of a mile; and, taking the whole surface of the earth, the average energy received from the sun is over fifty mile-tons yearly; or one horse-power, continuously acting, to every thirty square feet of the earth's surface. Most of this, of course, is expended merely in maintaining the earth's temperature; but a small portion, perhaps 1000 of the whole, as estimated by