SCIENCE AND INDUSTRY. 
for us can, moreover, only be converted into mechanical work at a 
vast sacrifice,- which future improvements can but little diminish. 
Even in the Diesel engine, not more than some 83 per cent. of the 
energy of the fuel is realized as brake horse-power at the crank shaft, 
about two-thirds of the total supply ‘being thus lost to the exhaust. 
No doubt posterity, if civilization is to be maintained at the level 
to which the engineer has raised it, must find some substitute for our 
rapidly diminishing coal supplies, and many considerations point to 
sunshine being the principal.source of power in ages yet distant; but 
it seems highly improbable that the conversion of radiation into 
mechanical work will be effected by the essentially crude device of first 
degrading the energy into heat. Theoretically, of course, a direct and 
efficient conversion of radiation into work is possible, but as the only 
mechanism yet conceived to this end involves the construction of: a, 
kind of turbine with a bucket speed comparable to that of light, or the 
production of absolutely perfect reflectors, this theoretical possibility 
advances matters little or nothing; and we must hope that some indirect 
method may yet be discovered which will avoid the drawbacks inherent 
in such schemes as have already been tried, whilst equally evading the 
insuperable obstacles in the path of a direct attack. 
368 
