May 27, 1915] 
NATURE 

cast-iron boilers had a perimeter of 2 ft. 11 in., and 
were 205 ft. long. They were tested both naked and 
covered, the covering being formed of one layer of 
window-glass, butt jointed, and the best results were 
obtained when they were so covered. The best run of 
an hour gave 1,442 lb. of steam at a pressure of 
15-8 lb. sq. in. abs., equivalent to 55-5 b.h.p.=63 b.h.p. 
per acre of land occupied by the plant; while the 
359 
sa — pk( V4 — 3A4) -— (1 — 7°) Ds 
peat t— pk(1 qA4) -(1 r)Dsa_ (1) 
Dsa 
and to the overall efficiency it is :— 
ee {Dsa — ph( T+ — 3A4) — (17)Dsa}(T — 568) (2) 
Dsa 
The coefficient } appears with the A*, because the 
mirrors encircled only 3 of the perimeter of the boiler. 

Fic, 2.—Shuman-Boys absorber, Mcadi, 1913. One section of the absorber from the north. 
average power for the five hours’ run on that day 
(August 22, 1913) was 50:4 b.h.p. per acre, and the 
minimum on the same day was 52-4 b.h.p. per acre, 
a decrease of only 16-8 per cent. The maximum 
thermal efficiency of the absorber alone was 4o:1 per 
cent., or 36 per cent. better than the thermal efficiency 
of the 1911 absorber, while its steam production was 
333 per cent. better. 
The author’s experiments in Egypt show that a 
decrease of 20 per cent. in the humidity of the atmo- 
sphere caused an increase of 30 per cent. in the steam 
production. 
Nearly all the technical part of the paper is con- 
tained in three appendixes, while a fourth consists of 
the bibliography of the subject. 
In appendix i. it is shown that when the 1913 
absorber was tested with naked boilers, the solar heat 
not used, and expressed in B.T.U. per hour per sq. ft. 
of boiler surface per 1° F. difference in temperature 
between the boiler and the air, is nearly constant and 
equal to 8-68. 
In appendix ii. is derived the equation to thermal 
efficiency of a solar heat absorber and the efficiencies 
calculated by means of it are compared with the actual 
thermal efficiencies. 
In appendix iii. the equation to the thermal efficiency 
of the absorber is combined with the equation to the 
thermal efficiency of a Carnot engine, thus giving the 
overall thermal efficiency. From this it is shown that 
the theoretical maximum overall efficiency of the 1913 
Egyptian plant was 5-9 per cent., while the actual 
efficiency was 4-32 per cent. Thus 73-2 per cent. of 
the maximum possible efficiency was attained. 
The equation to the thermal efficiency of the 
absorber is :— 
NO. 2378, VOL. 95| 

Inserting the values of the known quantities for the 
Egyptian plant gives :-— 
My =0'71 — 404T-!+.9'45 x 10 1°15 — 1°664x 10-1714 (3) 
where D=the width in feet of the reflector; p=the 
perimeter in feet of the boiler; y=the efficiency of 
silvered glass as a reflector of heat; s=the solar con- 
stant in B.T.U. per square foot- per min.=7-12; 
a=the coefficient of atmospheric transmission; T=the 
absolute temperature in degrees F. of the boiler; 
A=the absolute temperature in degrees F. of the 
060 
-050| 
= 
040 } q 



1030} 

ral = 10. 
-020 0-71--0166 X10) T -404T +9°45%10'|T? 




Overall Efficiency 






n 
lo 

568 580600 620 640 660 680 700 720 740 760 780 800 809 
T=Absolute Temperature of Steam, Degrees Fahrenheit > 
Fic, 3,—Curve showing the relation of the overall efficiency of the 1913 
Shuman-Boys sun-heat absorber (with naked boiler), combined with a 
Carnot engine, to the absolute temperature of the boiler steam. 
reflectors; 568=the absolute temperature in degrees. 
F. of the condenser; k=the coefficient of radiation, 
conduction, and convection=10~-1°x0-36 B.T.U. per 
sq. ft. per min., this value having been determined by 
the author in 1899 under almost the same conditions 
as those to which it is now applied. 
The graph of equation (3) is given in Fig. 3, from 
