110 



height is the doing of work. One foot-Bb, or the work done 

 against the force of gravity in Hfting lib through a distance of 1 

 foot, is the engineer's unit of work. A long series of experiments 

 by Dr. Joule of Manchester, has established the fact that a cer- 

 tain relation exists between a unit of work and a thermal unit, 

 one of the latter being equivalent to 772^ of the former, thus 

 proving that heat is but one of the several forms of energy. 

 Now the heat received by the earth at its equator would, if 

 wholly converted into work, every year raise fifty-three tons 

 through a mile for every square foot of surface, which is equival- 

 ent to over 16,000 tons raised every minute through the same 

 height for every square mile of surface. It must be borne in 

 mind, also, that the earth receives but a scanty quota of the whole 

 supply of heat poured forth by the sun into space. For no less 

 than 2,200,000 earths could be placed on an imaginary sphere 

 constructed on the earth's orbit as a great circle. Defining one 

 horse-power to be the doing of 33,000 foot-lbs. in one minute, 

 each square yard of the solar surface is equivalent to an engine of 

 108,000 H.P. or about the total H.P. of the engines of eleven first- 

 class iron-clads. At the distance of the earth this amount corresponds 

 to 1 H.P. for every 25 square feet when the sun is in the zenith. 



6. The Tempeeatube of the Sun. (a) By Newton's Law. 



Knowing the radiant energy of the Sun, it might not at first 

 sight seem a difficult thing to gauge the corresponding tempera- 

 ture. All that we require to know is the law that connects 

 radiation and temperature, But this connecting link between 

 the two quantities is still in the region of doubt and uncertainty. 

 Sir Isaac Newton's opinion, ably championed in modern times 

 by Father Secchi and Capt. Ericsson, is that temperature like 

 radiation follows the law of inverse squares. Thus the intensity 

 of radiation on one square foot at two feet from the source of the 

 rays is only one-fourth the intensity at a distance of one foot, 

 while at three feet it would have fallen to but one-ninth part of 

 the original value. And so with temperature, if the temperature 

 at a distance of three feet is found to be 60°, according to this 

 law the temperature of the source is 9 x 60° or 540°. Sir Isaac 

 Newton calculated the temperature of the Sun, by observing the 

 distance of the comet of 1680 from the Sun, calculating its tem- 

 perature, and then by applying the law of inverse squares, passing 

 to the temperature of the Sun itself. It will be instructive to 

 follow him in this investigation. First he exposed a clod of dry 

 earth, a very absorptive material, to the direct rays of the noon- 

 day Sun. This was his "pyrheliometer," and he found by its 

 means that the excess of temperature of the earth thus exposed 

 above its surroundings was 60° F. Next he ascertained that the 

 distance of the comet fronj the centre of the Sun on the 8th of 



