UTILIZATION OF SOLAR ENERGY ACKERMANN. 145 



that the best efficiency of the sun-heat absorber was only 40.1 per cent, 

 compared with 75 per cent for the best coal-fired boiler. But it has 

 taken boilermakers many years to attain this efficiency, so that 40.1 

 per cent is not a bad result when the number of sun boilers that have 

 been made is taken into account. Thermal efficiencies of engines are 

 materially affected by the heat fall of the steam, just as the efficiencies 

 of water turbines are affected by the height of the waterfall. The 

 larger the fall in either case the better the efficiency. 



It is interesting to realize from the foregoing figures that the value 

 of 2^ acres of bright sunshine for an hour is 1 ton of coal. This fact 

 is more readily realized in Egypt in the summer. With this we may 

 compare what Mr. J. C. Plawkshaw said in his presidential address 

 to the Institution of Civil Engineers in 1902, viz, that the w^ood fuel 

 produced by an acre of land in Europe is equivalent to at least 1 

 ton of coal a year. 



With so much heat generated at the surface of the earth it might 

 be thought that the temperature of the earth would rise. So it would 

 do were it not for the fact that the earth radiates into space as much 

 heat as it receives, though some of it may be stored on earth for a time 

 in the form of vegetable growth (including coal) or water raised to 

 high levels. 



Coal has been called " bottled sunshine," but the cork of the bottle 

 must be a leaky one, for Abbot says (The Sun, p. 360) : "It appears 

 from such investigations as have been made that plants may store up 

 as chemical energy in round numbers 1 or 2 per cent of the energy 

 of solar radiation which shines upon their leaves." With regard to 

 the earth's own heat, it has been estimated that the continuous supply 

 coming from the interior to the surface is equivalent to 1,280 horse- 

 power per square mile, or only 2 horsepower per acre. 



Having now considered the nature of the source and the quantity 

 of heat available, we will give a brief description of the plants which 

 have been constructed by various experimenters for the purpose of 

 utilizing solar heat. They are given in chronological order as regards 

 their solar work so far as the author has been able to discover the 

 facts. 



At one stage the author thought he had discovered the earliest 

 worker at the subject when he came across a record of Sir John Her- 

 schel's experiments in 1836, but further research disclosed that Buf- 

 fon, the celebrated French naturalist, was at work in 1747, and on 

 April 10 of that year he succeeded in setting fire to a plank of tarred 

 wood, at a distance of 150 feet, by solar rays reflected from a combina- 

 tion of flat mirrors. He did this to show the possibility of the legend 

 that Archimedes set fire to the fleet of Marcellus at Syracuse in 

 212 B. C. 



18618°— SM 1915 10 



