196 
however, it mattered little that the output of steam was about 
17 per cent. greater at mid-day than in the morning and 
evening. 
The maximum thermal efficiency of the Egyptian sun-heated 
boiler was 40°7 per cent., which compared very favourably with 
the thermal efficiency of an ordinary steam boiler of about 
70 per cent., when the relative state of development of the two 
types of boilers was taken into account. 
During the discussion of a paper on ‘‘ The Utilization of 
Solar Energy,’’ which I read before The Society of Engineers 
(Incorporated) last April, a speaker pointed out that glass 
mirrors cause a loss on account of the absorption of the radiant 
energy by the glass of the mirrors. This is true, and possibly 
the loss may amount to 15 per cent., but it is important to 
remember that when solar radiation had been passed through 
one sheet of glass, the rays which emerge are more capable 
of passing through a second sheet of glass. In other words, 
practically all those which could not pass through glass were 
stopped by the first sheet, so that when the radiation was 
reflected from the silvered surface of the glass back through 
the latter, a second 15 per cent. was not lost. Metal reflectors 
would much reduce the loss by absorption compared with the 
silvered glass reflectors and would no doubt be more efficient, 
but unfortunately no practical metal reflector could be used 
which would have a sufficiently hard and untarnishable surface. 
If the surface were not hard it would get scratched by repeated 
removal of dust, and if it were not untarnishable it would also 
lose its efficiency. 
The Presipenr: Mr. Chairman and Gentlemen—I am afraid 
I can only make a small contribution to the discussion. I have 
followed the results of this scheme of Mr. Shuman’s with 
much interest, and a short time ago I went to Meadi, in Egypt, 
in order to see the plant at work. I regret, however, to say 
that on this occasion the sun had not been seen for a week, and 
the Nile had fallen far below the intake for the water. I was 
therefore obliged to come away without seeing the plant in 
operation. At the same time, I feel that the attempts Mr. 
Shuman is making are not only most interesting and important, 
but are certainly attempts which ought to be systematically 
continued, because it looks, especially from what Mr. Acker- 
mann has just told us of the thermal efficiency, that provided all 
mechanical difficulties can be got over, we shall be in possession 
of a source of power for the tropics which will be extremely 
efficient and extremely economical. I hope that before the 
Congress meets again we shall have a further report showing 
that success has been met with in more than one place. 
One difficulty which occurs to me is that the actual amount of 
