A Contrihidion to the Study of the JRainfall Map of South Africa. 381 
along the lower half of the west coast. In February the same isohyet has 
shifted a little south ; in March it has edged off to the north-east, while in 
April it is shown running nearly due west to east along the parallel of 11°. 
It is not drawn on the December map, but its course can be mentally 
followed between the isohyets of 3 and 5 in., and from these we see that it is 
pretty much the same as it is in March. 
In January the 1-in. isohyet runs south from near the top left-hand 
corner of the map to latitude 25°, where it curves more to the east. In 
February it is rather nearer the Atlantic, and in March still nearer — albeit 
the 2 -in. isohyet in this month has pushed out a long tongue eastwards. 
In April it has moved considerably northwards, and embraces the whole 
of the interior. In May it has retreated to the equator. In October it 
begins to show again north of the 15th parallel ; and can be followed in 
November and December closing down upon its first position near the west 
coast. 
Of the other isohyets much the same may be said. In fact they form 
together one system which moves north and south as the sun moves, but 
with a lag of a month or more. That is, in September, when the sun is on 
the equator, going south, the zone of heaviest rainfall is in the northern 
.hemisphere, and it does not reach the equator for upwards of a month; in 
March, when the sun is again on the equator, going north, the zone of heaviest 
rainfall extends along the parallel of about 10° S. This is not to say that 
the rainfall system retains a uniform aspect as it swings north and south. 
At one time the isohyets may be closing up together ; at another they may 
be opening out. The isohyets of 1 and 2 in., e.g., are further apart in 
November than they are in January. 
The zero isohyet is interesting. In June it occupies a considerable belt of 
the country lying between latitudes 5° and 18°. In July it circuits a much 
larger area, having extended outwards north and south ; but its southward 
progress has evidently received a check in the high land near the tropic, 
facing the Atlantic — an indication that occasional though trivial rain falls 
a.t this place in July. And, curiously enough, as the area within the zero 
isohyet comes still further south we see that these reluctant rains invade it 
for a considerable distance from the south, reducing the area of no rain, in 
August, by many thousand square miles, and nearly obliterating it 
altogether in September. 
Corresponding to the general movements of the main isohyetal system 
are the movements of the subsidiary systems found on the coast — the 
winter rains of the south-west, for example, which advance as the summer 
rains retreat, and vice versa, and the rains of the south and south-east 
coastal belt. The winter rains of the south-west do, indeed, now and then^ 
and the southern coastal belt rains not infrequently, push their way inland 
as far as Kimberley and Bloemfontein. 
