66 Mr Buchanan, On a solar Calorimeter used in Egypt 



By far the most important agent in altering the true rate, as 

 due to the sun alone, is the wind. During the three days we were 

 fortunate in having both calm and wind, so that an idea can be 

 formed of the cooling effect of wind. On the 16th with a calm 

 afternoon the mean rate between 2 and 3 p.m. was 1*221, and on 

 the 17th, when it was breezy, the rate was 1'087 or about 10 per 

 cent. less. 



The breezes which occur on the Nile are usually cool and from 

 the north. They did not at any time exceed force 3 of Beaufort's 

 scale. They were never steady, but came in puffs or gusts, so 

 that one 20 c.c. or even 5 c.c. interval would be affected and the 

 subsequent one not. 



On the 13th some satisfactory observations were made when 

 the sun though behind a cirrus cloud was still able to keep distil- 

 lation going. The rate was 0752 at 10 a.m. When the sun had 

 cleared the cloud the rate rose to 091 at 10.30 a.m. 



If we look over the list of figures in Table III. or their 

 graphical representation in Fig. 8, we notice that there is con- 

 siderable variability in the results whether the interval which 

 we consider be that required for the distillation of 5 c.c. or 

 20 c.c. Further, this variability from one interval to another is 

 more remarkable than the change of rate due to change of the 

 sun's altitude. Yet the sun's altitude which is 83° at noon is 

 only 48° at 9 a.m. or 3 p.m. If we express it in zenith distance, 

 the zenith distance is at noon 7° and increases to 42 c at 9 a.m. 

 and 3 p.m. We conclude that the energy of the radiation received 

 by a surface held perpendicularly to the sun's rays is, within con- 

 siderable limits, very little dependent on the suns zenith distance. 



The weather on each of the three days was very fine and each of 

 them taken by itself would have been held to be very favourable for 

 this kind of experiment. Yet amongst the three very good days 

 the forenoon of the 18th was incomparably the best ; the sun 

 shone its strongest and the air was motionless ; moreover, instru- 

 mentally everything was in best working order. Therefore to 

 ascertain the greatest amount of heat that can be obtained from 

 the sun's rays we examine the results obtained in the forenoon of 

 the 18th, and we find that at about half-past ten, 5 c.c. were distilled 

 in three minutes and twenty seconds, being at the rate of 1501 c.c. 

 per minute. Nearly an hour later the same time is registered for 

 the distillation of 5 c.c, but owing to the greater zenith distance 

 of the sun the former must be held to be the higher rate. The 

 correction to be applied to either of these rates in order to reduce 

 it to its value for a vertical sun is evidently insignificant and we 

 take 1*5 c.c. per minute as the highest rate observed. 



In attempting to form an estimate of the extent to which this 

 may fall short of the true rate under perfect conditions, we have 



