1887.] on the Suns Heat. 6 



in a fluid, not now confined to a vat, but covering the wliole surface 

 of the sun to a depth of a few metres or kilometres. Arrange the 

 viscosity of the fluid and the size of each paddle so as to let the 

 paddle turn just so fast as to allow the top end of each pointed rod 

 to descend at the rate of 35 metres per year. The whole fluid will, 

 by the work which the paddles do in it, be made incandescent, and it 

 will give out heat and light to just about the same amount as is 

 actually done by the sun. If the fluid be a few thousand kilometres 

 deep over the paddles, it would be impossible, by any of the ap- 

 pliances of solar physics, to see the difference between our model 

 mechanical sun and the true sun. 



To do away with the last vestige of impracticable mechanism in 

 which the heavinesses of all parts of each long rod are supported 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 centre 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 temper- 

 ature that its material melts and experiences as much of Sir HumjDhry 

 Davy's " repulsive motion " as suffices to keep it balanced as a fluid, 

 without either sinking 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 centre, of a 

 square metre area at the surface, 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 wheu the paddle was kept moving ; but it would quickly 

 become much cooler at its surface, and to a distance of a few metres 

 down. Currents of less hot fluid tumbling down, and hotter fluid 

 coming up from below, in irregular whirls, would carry the cooled 

 fluid down from the surface, and bring up hotter fluid from below, but 

 this mixing could not gc^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 tem- 

 perature of the substance, thin films of the upper crust would fall in, 

 and continue falling in, until, for several metres downwards, the 

 whole mass of mixed solid and fluid becomes stiff" enough (like the 

 stiffness of paste or of mortar) to prevent the frozen film from fall- 

 ing down from the surface. The surface film would then quickly 

 thicken, and in the course of a few hours or days become less than 



