THE SUN'S HEAT. zi 



brilliantly incandescent, the conduction of heat from within through 

 solid matter of even the highest conducting quality known to us would 

 not suffice to maintain the incandescence of the surface for more than 

 a few hours, after which all would be darkness. Observation confirms 

 this conclusion so far as the outward appearance of the sun is con- 

 cerned, but does not suffice to disprove the idea which prevailed till 

 thirty or forty years ago that the sun is a solid nucleus inclosed in a 

 sheet of violently agitated flame. In reality the matter of the outer 

 shell of the sun, from which the heat is radiated outward, must in 

 cooling become denser, and so becoming unstable in its high position, 

 must fall down, and hotter fluid from within must rush up to take its 

 place. The tremendous currents thus continually produced in this 

 great mass of flaming fluid constitute the province of the newly de- 

 veloped science of solar physics, which with its marvelous instrument 

 of research the spectroscope is yearly and daily giving us more and 

 more knowledge of the actual motions of the different ingredients, 

 and of the splendid and all-important resulting phenomena. 



Now, to form some idea of the amount of the heat which is being 

 continually carried up to the sun's surface and radiated out into space, 

 and of the dynamical relations between it and the solar gravitation, 

 let us first divide that prodigious number (476 XlO 21 ) of horse-power 

 by the number (6 # 1 X 10 18 ) of square metres in the sun's surface, and 

 we find 78,000 horse-power as the mechanical value of the radiation 

 per square metre. Imagine, then, the engines of eight ironclads ap- 

 plied to do all their available work of, say, 10,000 horse-power each, 

 in perpetuity driving one small paddle in a fluid contained in a square 

 metre vat. The same heat will be given out from the square metre 

 surface of the fluid as is given out from every square metre of the 

 sun's surface. 



But now to pass from a practically impossible combination of en- 

 gines and a physically impossible paddle and fluid and containing ves- 

 sel, toward a more practical combination of matter for producing the 

 same effect : still keep the ideal vat and paddle and fluid, but place 

 the vat on the surface of a cool, solid, homogeneous globe of the same 

 size (-697X10 9 metres radius) as the sun, and of density (1*4) equal to 

 the sun's density. Instead of using steam-power, let the paddle be 

 driven by a weight descending in a pit excavated below the vat. As 

 the simplest possible mechanism, take a long vertical shaft, with the 

 paddle mounted on the top of it so as to turn horizontally. Let the 

 weight be a nut working on a screw-thread on the vertical shaft, with 

 guides to prevent the nut from turning the screw and the guides be- 

 ing all absolutely frictionless. Let the pit be a metre square at its 

 upper end, and let it be excavated quite down to the sun's center, 

 everywhere of square horizontal section, and tapering uniformly to a 

 point at the center. Let the weight be simply the excavated matter 

 of the sun's mass, with merely a little clearance space between it and 



