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 35 and 

 40 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; seep. 38. 



