262 Transactions of the South African Philosophical Society. 
observing hours are different. Before the two may be compared, 
then, it is necessary first to determine the Durban means for the 
same eight years, and next to correct the readings of either for the 
diurnal oscillation. We are able, fortunately, in the case of Kim- 
berley, to do this with some precision from the known hourly varia- 
tion at Kenilworth. The result will be found in Table 38, the 
monthly numbers denoting the mean sea-level pressures at Durban 
from readings at IX. to XV. for the eight years 1890-1897, and the 
Kimberley means for the same years reduced to the mean of the 
same hours. 
Although the period used is not so great as might be wished, some 
very remarkable facts stand out very plainly. First that the highest 
and lowest monthly pressures, occurring respectively in July and 
January, differ by 0°245 inch at Durban, and by 0-242 inch at 
Kimberley, which practically means an identical range. Now in 
free air, where there is a normal decrease of temperature with 
height, a point whose altitude is equal to that of Kimberley 
(4,042 feet) above sea-level at Durban would have a barometric 
range not much greater than one-half the observed range. We 
should have, in fact, counting both temperatures and pressures 
upwards from Durban, at the given altitude— 
Tita villsysi ov clyouriir Soaihath elie a ene egs 26°14 inches 
Ine Jiamatiaiey Getto ene bok Arg eee ZG:0is” aay, 
OY bine Vie aneh sete iene ergs teats 26:08." 4; 
At the same time it is not improbable that the Durban pressures 
would be rather different if there were no table-land behind it. 
The range of monthly mean pressures at Mount Abu is but three- 
fifths of that at Karachi for a difference of elevation of 3,896 feet. 
Their monthly numbers are given in Table 38 for comparison with 
those of Kimberley. They illustrate very well how the indications 
of a mountain barometer differ from those of a table-land barometer. 
The orderly sequence of differences for the Indian stations is worth 
notice, these being least in summer and greatest In winter. Now 
all places whose altitudes are not very great have their pressures 
invariably greater in winter than in summer. But the difference of 
pressure between a sea-level station and an adjacent point at a 
moderate elevation is least in summer because the access of heat 
expands the air, thereby raising the surfaces of given pressure ; and 
in the same way the cold of winter contracts the air, thereby lowering 
the surfaces. Evidently Mount Abu conforms to this law. In South 
Africa, on the contrary, the least differences come towards the end of 
