360 On a solar Calorimeter used in Egypt 



and less water than is to be found either on the equatorial or 

 on the polar side of the region. The northerly wind on the 

 Nile is the Trade Wind blowing from colder to hotter latitudes 

 and always increasing its evaporative power. Thanks to this 

 power the temperature of the Nile is lower than it would 

 otherwise be. I took its temperature frequently at all hours 

 of the day, it varied only between 74' 5 and 76 Fahr., while the 

 temperature of the air above it varied from 50 to 105 Fahr. ; 

 the variations of the wet bulb thermometer are much less, 

 namely from 49'5 to 66'2 Fahr. The temperature of the 

 water of the Nile is very nearly the mean of the maximum 

 and minimum temperatures of the air above it. These means 

 were on the 



I4th, 1 5th, i6th, 1 7th, i8th, mean 

 77'5, 74'5, 79 - o, 79'5, 8275, 78'65. 



The temperature of the Nile is a little below this mean. 

 The large range of temperature in the air is partly due to the 

 cooling effect of the evaporation from the surface of the water. 



The evaporation experiments were made on water con- 

 tained in a deep plate. It contained 400 cubic centimetres 

 and exposed a free surface of 243 sq. centimetres when full, 

 and 230 sq. centimetres when nearly empty. It was set upon 

 a tin cone which raised it about 6 inches above the ground. 

 The difference of the effect of the sun upon the water and 

 upon the sand close to it was well shown on the i6th at 2 p.m. 

 when the water in the plate had a temperature of 82'5 F. 

 while the temperature of the sand was 134 F., making a 

 difference of more than 50 F. 



The following notes of the weather were made at the time 

 and are of use when taken in connection with the observations 

 of the calorimeter. 



1 6th May. The sun rose in a cloudless sky and there was 

 a very light wind from the west. As the morning wore on, 

 it came round more and more to the north and freshened 

 slightly. At 10 a.m. the wind seemed freshening and came 

 in gusts, retarding distillation. In the afternoon it was calm. 



1 7th May. The day of the eclipse, the sun rose in a 



