128 
Transactions of the Boyal Society of South Africa. 
useful. If the maps are compared with Buchan's, it will be seen how 
different the isotherms are. In our warmest month Buchan gives as the 
sea-level temperature of the western border of the Transvaal something 
like 92|-°, whereas it is actually 85°. In the absence of data, Buchan must 
have relied on the analogy with other continental areas, but the plateau 
effect and the still more potent effect of great wind movement, which is so 
marked a feature of South African Meteorology, greatly alter circum- 
stances. Again, the forms of the isotherms in the mean and in the cold 
season deviate quite widely from Buchan's. 
The table and maps which follow are so simple that they do not call 
for much explanation. The table shows the amount of the reduction of 
temperatures observed at each 500 feet of altitude. Below 2,500 and 
above 6,000 feet the figures are derived by extrapolation except for 
column two, which shows the hypothetical cooling of still dry air. It is 
seen that in our coldest and driest month there is some little approach to 
hypothetical conditions. The reduction in the warmest month, when the 
atmosphere is dampest, both actually and relatively, is always less than 
half of the still-dry-air figure. 
Three maps show the distribution of sea-level temperatures during the 
warmest and coldest months of the year and for the mean of the year. 
Eeduction of Observed Temperatures (Transvaal) to Sea-level 
Temperatures. 
Altitude in Feet. 
Still Dry Air. 
Coldest Month. 
Mean 
Temperature. 
Warmest Month. 
0 
4- 0-0 
+ 0-0 
+ o°o 
+ 0-0 
500 
2-7 
2-0 
1-8 
10 
1,000 
5-4 
4-0 ' 
3-4 
20 
1,500 
8-1 
60 
4-9 
3-0 
2,000 
10-8 
7-9 
6-3 
4-0 
2,500 
13-5 
9-8 
7-7 
50 
3,000 
16-2 
11-7 
9-0 
60 
3,500 
18-9 
13-5 
10-3 
70 
4,000 
21-6 
15-3 
11-6 
8-1 
4,500 
24-3 
17-0 
12-8 
9-3 
5,000 
27-0 
18-7 
140 
10-7 
5,500 
29-7 
20-3 
15-2 
12-2 
6,000 
32-4 
21-8 
16-4 
13-8 
6,500 
35-1 
23-3 
17-6 
15-5 
7,000 
37-8 
24-7 
18-8 
17-3 
