THE SUN'S HEAT. 13 



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 min- 

 utes ; 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. Oi", to put it differently, if we could build up 

 a solid column of ice from the earth to the sun, two miles and a quar- 

 ter in diameter, spanning the inconceivable abyss of ninety-three mil- 

 lions 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 calcula- 

 tion 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 ^^-o^o-q. 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 sur- 

 face, 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 tempera- 

 ture ; but a small portion, perhaps -j^ of the whole, as estimated by 



