Evaporation of Water in Inland South Africa. 
285 
The annexed tables show the evaporation in decimals of an inch 
divided by the "deficiency factor," from which it will be seen that the mean 
evaporation per annum for five years at Bulawayo is 2"081389 inches per 
1 per cent, of deficiency per annum, or '0057024 inches per 1 per cent. 
of deficiency per diem ; whilst for Johannesburg the same figures result in 
1 '46039 7 inches and'0040009 per annum and per diem respectively, whilst 
1 i J (max. + min.) ,„ t -. 
the mean annual temperature ^ were 67' 1 and 59*7 respectively. 
It will be borne in mind also that Bulawayo is 20*^ 09' 1" S. latitude, 
and Johannesburg is 26° 11' S. — a difference of 6'2° of latitude; that, 
assuming all other conditions to be equal, an allowance must be provided 
for the difference of the intensity of the sun's rays in latitude. 
The Smithsonian tables give factors for the relative intensity of 
solar radiation for the Northern Hemisphere, and the author is informed 
by Father Goetz at Bulawayo that the same factors can be applied relatively 
to the Southern Hemisphere. 
The factors for the year are as follows : For 20° latitude, 0*289 ; for 30° 
latitude, 0-268. 
But yet another correction has to be applied, and that is the " hours of 
sun above the horizon." 
Dr. Sutton, in a communication to the ' Agricultural Journal of the Union 
of South Africa,' gave relative evaporations with sunshine observations 
from a Campbell- Stokes sunshine recorder, and there appeared to be no 
actual relation between sunshine and evaporation — a result which can only 
be expected, inasmuch as a largely increased evaporation must naturally 
take place during cloudy weather, when the sun is obscured. In fact, many 
observers state that evaporation takes place to a greater extent when 
clouds are in the sky, for the simple reason that the heat rays are reflected 
by clouds, causing the actual temperature to be greater ; but it is apparent 
that the sun is primarily the cause of all heat, and therefore primarily the 
cause of all evaporation, and from the moment the sun rises above the 
horizon until it sets evaporation must be going on. Therefore the hours of 
sun above the horizon must form an important consideration. 
Most people are apt to forget that the thermometer is not an index of 
quantity of heat, since it only registers the momentary effect of heat ; hence 
the time factor has to be brought in, i. e. the hours of sun above the horizons 
and this consideration bears largely on the apparent difference between the 
evaporation of, say, January and June. At Cape Town, according to the hours 
of daylight in ' Letts' Diary,' there are in January approximately 14 hours of 
daylight per diem, or 445 hours of daylight per month, whilst in June there 
are 9-9 hours and 299 hours respectively for the same periods ; and at 
Johannesburg similar figures for January and June are 13*4 and 10*6 hoKrs 
per diem, or 418 hours as against 317 hours per month. 
