380 POPULAR LECTURES AND ADDRESSES. 



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 metres deep over 

 the paddles, it would be impossible, by any of 

 the appliances of solar physics, to see the differ- 

 ence between our model mechanical sun and the 

 true sun. 



To do away with the last vestige of imprac- 

 ticable 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 abso- 

 lutely impervious to heat. Warm up the matter 

 of the pyramidal rod to such a temperature 

 that its material melts and experiences as much 



