370 On a solar Calorimeter used in Egypt 



When the sun had cleared the cloud the rate rose to 0*91 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 

 considerable 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 perpendicidarly to the sun's 

 rays is, within considerable limits, very little dependent on the 

 sun's 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 i8th was incom- 

 parably the best ; the sun shone its strongest and the air was 

 motionless ; moreover, instrumentally 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 i8th, 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 1-501 c.c. per minute. 

 Nearly an hour later the same time is registered for the dis- 

 tillation 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 i'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 to consider the rates observed at other times during 

 the three days and the following table, which gives the amount 

 of water distilled in each hour in the different days, may be 

 used. 



