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 T087 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° at 9 a.m. 
and 3 p.m. We conclude that the energy of the radiation received 
by a surface held perpendicularly to the suns 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 T501 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 
