(283) 
AN INVESTiaATION OF EVAPOEATION OVEE FREE 
SURFACES OF WATER IN INLAND 
SOUTH AFRICA. 
By a. H. Wallis, C.E. 
Having constructed several dams, of capacities varying from 5 millions 
to 45 million gallons, in the Bechuanaland Protectorate, and noticing the 
great losses which occurred through evaporation, the author has gone into 
this question with a view to ascertaining whether some simple method cannot 
be arrived at to guide the engineer in making provision for such losses. 
Through the courtesy of Mr. Innes, the Union Astronomer, Father 
Groetz of Bulawayo, and Dr. Sutton of Kimberley, the author was afforded 
an opportunity of inspecting the evaporation gauges at the Union Observa- 
tory, the Bulawayo Observatory, and the Kenilworth gauge near Kimberley, 
as well as, through the courtesy of Mr. Lynch, the Manager of the Kimberley 
Waterworks, Ltd., the gauge at Newton, Kimberley. The following con- 
siderations have been based on the records of the first two of these gauges. 
Some of these have already been described in scientific journals ; their 
particulars may be summarised as follows : 
(1) Union Observatory, Johannesburg; latitude 26° 11' S., longitude 
28° 45' E., altitude 5924 ft. Square tank about 6 ft. Observations 
taken by a float and a graduated arc. The gauge is on the top of a hill 
surrounded by a few trees, which probably interfere with the free movement 
of the wind. The Observatory building also casts a shadow over the gauge, 
which might lessen the observed evaporation after 5 p.m. 
(2) The Observatory, Bulawayo; latitude 20° 09' 1" S., longitude 
28° 36' 3" E., altitude 4460 ft. above sea-level. Gauge is composed of a 
circular tank 7 ft. in diameter on the top of a hill well exposed to wind 
conditions. Records taken from a float on a circular arc. 
(3) Dr. Sutton's tank, Kenilworth, Kimberley; latitude 28° 42' S., 
longitude 24° 47' E., altitude about 4000 ft. above sea-level. Observa- 
tions are taken from a 6-ft. tank built into the ground, from which a pipe 
is led off into a closed room, where a float records by clockwork. The 
tank is situated in an area which is sheltered by trees on all sides, and it is 
thought that the conditions are not ideal for evaporation purposes. 
(4) Newton, Kimberley. Observations are taken from a tank about 4 ft. 
square built into the ground on the top of a hill exposed to every condition, 
with no surrounding trees. The observations are taken by means of a hook 
gauge, which permits of readings being directly observed to one thousandth 
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