HOW THE SUN WARMS THE EARTH ABBOT 157 



and the interest and depreciation on this excess of cost would offset 

 in some measure the saving of fuel. 



In the Egyptian solar-power plant (pi. 2), fairly high tempera- 

 tures and fairly high efficiency were obtained by concentrating solar 

 rays by a battery of cylindric mirrors onto a system of connecting 

 boiler tubes. In order to follow the sun's daily march through the 

 heavens, tl)e mirrors were rotated from east to west each day at such 

 a rate as to keep the sun rays in focus upon the boiler tubes. The 

 total area of sunshine collected was 13,2G9 square feet. Each chan- 

 nel-shaped reflector and its boiler was 205 feet long, and there were 

 five such sections placed side by side. The concentration was 41/^ 

 to 1. The maximum quantity of steam produced was 12 pounds per 

 100 square feet of sunshine, equivalent to 183 square feet per brake 

 horsepower, and the maximum thermal efficiency was 40.1 percent. 

 On August 22, 1913, the average power for the 5 hour's run was 

 59.4 brake horsepower per acre, and the maximum and minimum 

 power on that day were 63 and 52.4 brake horsepower per acre, 

 respectively. 



The engineer, Mr. Ackermann, remarks: 



The problem of the utilization of solar energy is well worthy of the atten- 

 tion of engineers, for even now it is very nearly a solved problem where there 

 is jilenty of sunshine and coal costs £3 10s. a ton. It is fortunate that where 

 coal is dear, sunshine is often plentiful, and it is to be remembered that coal 

 will gi'adually get dearer while the cost of manufacture of sun-power plants 

 h;hould decrease. Sun-power plants are admirably suitable for pumping in 

 connection with irrigation, for where th'Pre is most sunshine there is need for 

 most irrigation, and the slight variation in the quantity of water pumped 

 (hroughout the day does not matter. Also, when temporarily there is no sun- 

 shine (due to clouds), probably little or no irrigation is required. 



A SOLAR COOKER 



I have myself dabbled in the enticing solar-power field. As an 

 introductory experiment I designed and constructed a solar cooker 

 on Mount "Wilson, Calif. In my experiments I desired to store the 

 heat in a suitable reservoir at such a temperature as would permit 

 all the usual cooking operations, such as stewing, preserving, boiling 

 and baking of meats, and even including the baking of bread, to be 

 carried on for some time after the cutting off of sunlight. 



As the most convenient way to combine a solar-heat collector with 

 a suitable high-temperature reservoir, it seemed best to concentrate 

 the solar rays upon a metal tube communicating to a reservoir at 

 some distance above the collector. The scheme is exactly that of a 

 household water heater, merely substituting concentrated sun rays 

 for a fire as the source of heat. In this way, the hot fluid, expanding, 

 rises by the differential action of gravity into the top of the reser- 



