3?6 POPULAR LECTURES AND ADDRESSES. 



as the mechanical value of the radiation per 

 square metre. Imagine, then, the engines of 

 eight ironclads applied, by ideal mechanism 

 of countless shafts, pulleys, and belts, 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 would 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 impos- 

 sible combination of engines, and a physically 

 impossible paddle and fluid and containing 

 vessel, towards 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 (697,000 

 kilometres radius) as the sun, and of density 

 (1.4) equal to the sun's mean 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 



