4 Sir William Thomson [Jan. 21, 



the same effect: still keep the ideal vat and paddle and fluid, bnt 

 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 mechanism, take a long vertical shaft, 

 with the paddle mounted on the top of it so as to turn horizontally. 

 Let the weight be a nut working on a screw-thread on the vertical 

 shaft, with guides to prevent the nut from turning — the screw and 

 the guides being all absolutely frictionless. Let the pit be a metre 

 square at its upper end, and let it be excavated quite down to the 

 sun's centre, everywhere of square horizontal section, and tapering 

 uniformly to a point in the centre. Let the weight be simply the 

 excavated matter of the sun's mass, with merely a little clearance 

 space between it and the four sides of the pit, and with a kilometre or 

 so cut off the lower pointed end to allow space for its descent. The 

 mass of this weight is 326 million tons. Its heaviness, three-quarters 

 of the heaviness of an equal mass at the sun's surface, is 244 million 

 tons solar surface-heaviness. Now a horse-power is, per hour, 270 

 metre-tons, terrestrial surface-heaviness ; or 10 metre-tons, solar 

 surface-heaviness, because a ton of matter is twenty-seven times as 

 heavy at the sun's surface as at the earth's. To do 78,000 horse- 

 power, or 780,000 metre-tons solar surface-heaviness per hour, our 

 weight must therefore descend at the rate of one metre in 313 hours, 

 or about 28 metres per year. 



To advance another step, still through impracticable mechanism, 

 towards the practical method by which the sun's heat is produced, let 

 the thread of the screw be of uniformly decreasing steepness from the 

 surface downwards, so that the velocity of the weight, as it is allowed 

 to descend by the turning of the screw, shall be in simple proportion 

 to distance from the sun's centre. This will involve a uniform con- 

 densation of the material of the weight ; but a condensation so ex- 

 ceedingly small in the course even of tens of thousands of years, that, 

 whatever be the supposed material, metal or stone, of the weight, 

 the elastic resistance against the condensation will be utterly imper- 

 ceptible in comparison with the gravitational forces with which we 

 are concerned. The work 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 2000 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 con- 

 sidered ; and let the paddle at the top end of each screw-shaft revolve 



