72 Mr Buchanan , On a solar Calorimeter used in Egypt 
Conclusion. 
It is usual for writers on this subject to express the heating 
effect of the sun’s rays in gramme-degrees received by one square 
centimetre exposed perpendicularly to them for one minute 
outside the limits of the earth’s atmosphere. This is termed the 
solar constant . Expressed thus our maximum rate is 0’89 gr.° C. 
per sq. centimetre per minute at the base of the earth’s atmo- 
sphere. If we add 11 per cent, for deficiencies from all sources 
we have 1 gr.° C. heat received at the sea level on a surface 
of 1 sq. centimetre exposed perpendicularly to the sun’s rays 
per minute ; and from the conditions under which the maximum 
rate was observed on the 18th May, I believe that this figure is as 
likely to be above the truth as below it. If however it is thought 
that the allowance should be more liberal, we have seen that our 
maximum rate corresponds to 0 - 84 horse-power per sq. metre ; if 
we make this one horse-power per sq. metre we have certainly got 
as much radiant energy as it is possible to collect at the level 
of the sea. Further, in speculations connected with physical 
meteorology we are not entitled to postulate a more abundant 
supply. From this supply falls to be deducted the energy of 
evaporation which however is returned on precipitation, also the 
energy of storms which to a large extent are secondary features 
attending the changing hygrometric state of the atmosphere. 
Notwithstanding the apparently perfect transparency of the 
atmosphere on the morning of the 18th we must admit that some 
of the energy was lost by absorption in the passage through the 
earth’s atmosphere ; but the small effect produced by great 
variations of the zenith distance of the sun on the rates observed 
shows that this effect cannot be great, in fact it is entirely masked 
by very slight motion of the air. Most recent writers put the 
value of the solar constant at from 3 to 5 gr.° C. per cm. 2 
per min., the greater part of which is added to the observed value 
in order to compensate for the supposed absorption by the air. 
Thus Schemer 1 in a recent work writes : — “ From what precedes 
it is apparent that the values which have been found for the 
solar constant do not differ so very much from each other. The 
older determinations have without doubt given too small values, 
the later ones point with great certainty in the direction that 
the solar constant is included between the amounts of 3*5 and 
4 0 gr. cal.” Now even the lower of these values can be true only 
1 Strahlung und Temperatur der Sonne, von Dr J. Scheiner, Leipzig, Engelmann, 
1899; see p. 38. 
