I9I 
rays has only been solved after seven years of patient experi- 
mental work and the expenditure of very large sums of 
money. 
An experimental plant was erected at Cairo in Egypt, this 
position being chosen on account of its convenience, as it was 
desired to use the plant for demonstration purposes. As the 
fuel cost absolutely nothing, the question of mechanical 
efficiency did not require to be taken into account. It was a 
question altogether of construction, upkeep and labour; and 
if the interest on the capital added to the cost of upkeep and 
attendance was so high as to cancel the profit made by not 
requiring fuel, then there was no incentive to go further in 
the matter. 
However, the results of tests of the plant had been very 
satisfactory, and it was estimated that power could be produced 
in the tropics at the same cost as if coal were less than 10s. per 
ton, and as coal in many parts of the tropics costs £2 Ios. 
per ton and upwards, the saving to be effected by means of 
sun-power plants is quite obvious. Although sun-power plants 
cost more than coal-burning plants, the saving effected 
by not requiring any fuel is sufficient to wipe out the extra 
capital cost after two years, and to pay entirely for the plant 
in four years. 
The steam produced from the rays of the sun is exactly the 
same as that obtained from the burning of any fuel, and there- 
fore the power can be used for any purpose whatever. The 
engine is a low-pressure reciprocating condensing engine, and 
the pump an ordinary reciprocating pump. Experience has 
shown that steam at atmospheric pressure gives the greatest 
return in power per pound sterling invested, and therefore the 
engine runs on steam supplied at this pressure. The steam is 
generated in five absorbers 13 ft. wide and 200 ft. long, which 
are parabolic troughs for catching the sun’s rays on mirrors 
and throwing the rays on to a boiler swung at the focal point. 
The rays are absorbed by the black surface of the boiler, and 
the heat is sufficient to generate large quantities of steam. The 
absorbers are placed with sufficient clearance so that they do 
not shade each other in the morning and evening. They rotate 
on a system of gearing in such a manner that the mirrors 
point directly towards the sun at all times of the day, and the 
correct adjustment is effected by means of a thermostat 
operated by the sun itself. The steel framework of the 
absorbers is designed in such a manner that it can be manu- 
factured very cheaply, and as each piece is very light, the 
question of transportation is not a serious one. 
Sun-power plants in this system can be built of any size up 
to 10,000 h.p. if required, and will work for ten hours per 
