THE SUN'S HEAT. 23 



on the thread of an ideal screw cut on a vertical shaft of ideal matter, 

 absolutely hard and absolutely frictionless : first, go back a step to 

 our supposition of just one such rod and screw working in a single pit 

 excavated down to the center of the sun, and let us suppose all the 

 rest of the sun's mass to be rigid and absolutely impervious to heat. 

 Warm up the matter of the pyramidal rod to such a temperature that 

 its material melts and experiences enough of Sir Humphry Davy's 

 " repulsive motion " to keep it balanced as a fluid, without either sink- 

 ing or rising from the position in which it was held by the thread of 

 the screw. When the matter is thus held up without the screw, take 

 away the screw or let it melt in its place. We should thus have a pit 

 from the sun's surface to his center, of a square metre area at the sur- 

 face, full of incandescent fluid, which we may suppose to be of the 

 actual ingredients of the solar substance. This fluid, having at the 

 first instant the temperature with which the paddle left it, would at 

 the first instant continue radiating heat just as it did when the paddle 

 was kept moving ; but it would quickly become much cooler at its 

 surface, and to a distance of a few metres down. Convection-currents, 

 with their irregular whirls, would carry the cool fluid down from the 

 surface, and bring up hotter fluid from below, but this mixing could 

 not go on through a depth of very many metres to a sufficient degree 

 to keep up anything approaching to the high temperature maintained 

 by the paddle ; and after a few hours or days, solidification would 

 commence at the surface. If the solidified matter floats on the fluid 

 at the same temperature below it, the crust would simply thicken as 

 ice on a lake thickens in frosty weather ; but if, as is more probable, 

 solid matter, of such ingredients as the sun is composed of, sinks in 

 the liquid when both are at the melting temperature of the substance, 

 thin films of the upper crust would fall in, and continue falling in, 

 until, for several metres downward, the whole mass of mixed solid 

 and fluid becomes stiff enough (like the stiffness of paste or of mor- 

 tar) to prevent the frozen film from falling down from the surface. 

 The surface film would then quickly thicken, and in the course of a 

 few hours or days become less than red-hot on its upper surface. The 

 whole pit full of fluid would go on cooling with extreme slowness 

 until, after possibly about a million million million years or so, it would 

 be all at the same temperature as the space to which its upper end 

 radiates. 



Now, let precisely what we have been considering be done for every 

 one of our pyramidal rods, with, however, in the first place, thin par- 

 titions of matter impervious to heat separating every pit from its four 

 surrounding neighbors. Precisely the same series of events as we 

 have been considering will take place in every one of the pits. 



Suppose the whole complex mass to be rotating at the rate of once 

 round in twenty-five days. 



Now at the instant when the paddle stops let all the partitions be 



