378 POPULAR LECTURES AND ADDRESSES. 



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 im- 

 practicable 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 

 condensation of the material of the weight ; 

 but a condensation so exceedingly 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 



