ON THE SUN'S HEAT. 



379 



done per metre of descent of the top end of 

 the weight will be just four- fifths of what it 

 was when the thread of the screw was uniform. 

 Thus, to do the 78,000 horse-power of work, 

 the top end of the weight must descend at 

 the rate of 35 metres per year : or 70 kilometres 

 per 2,000 years. 



Now let the whole surface of our cool solid 

 sun be divided into squares, for example as 

 nearly as may be of one square metre area 

 each, and let the whole mass of the sun be 

 divided into long inverted pyramids or pointed 

 rods, each 697,000 kilometres long, with their 

 points meeting at the centre. Let each be 

 mounted on a screw, as already described for 

 the long tapering weight which we first considered ; 

 and let the paddle at the top end of each screw- 

 shaft revolve in a fluid, not now confined to a 

 vat, but covering the whole 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 



