Pressure and Temperature Results for the Great Plateau. 253 
in which y, A, B have the same meaning as before, though in general 
different values. Approximate values of A and B are 73°:4 and 22°°8 
respectively. 
In Table 29 will be found the successive’steps of the computation 
for the average maximum temperatures of Kimberley on or about the 
middle day of each month, the sun’s Declination and semi-diameter 
being taken from the Nautical Almanac. The values of y thus deter- 
mined are marked in ring dots (thus C) on Fig. 1. They evidently 
average a little higher in the spring months than the observed 
temperatures, for which, in all probability, the retardation of the 
minimum, on account of the cold wave of July, is responsible. The 
last column of Table 29 gives the numerical differences between the 
values of y for the middle day of each month and the mean monthly 
maximum temperatures, calculated minus observed, assuming—not 
quite accurately—that the latter is the same as the average maximum 
temperature of the middle day of the month. 
In Table 30 is given some comparison values for Bloemfontein and 
Aliwal North, the respective conditions being :— 
1. Bloemfontein— 
So Ibe AS 
y = 55°°8 8? cos Z + 33°. 
2. Aliwal North— 
S. Lat. 30° 41’ 
y = 53° 8S? cos Z + 33°°6. 
The temperature numbers for both the stations are averaged from 
Let L = the “‘ apparent” latitude of the place, 
D = the sun’s meridian declination, 
A = the sun’s semi-diameter, 
A = the sun’s altitude, and 
H= the hour-angle from noon. 
Then, generally, A? sin A expresses the sun’s intensity at any given instant during 
the day ; and we have— 
A’ sin A = A?(sin Lsin D + cos L cos D cos H). 
Meech observes upon this result that it is ‘“‘ strictly true only at the exterior of 
the atmospheric envelope which encompasses the globe, or at the outer limit where 
matter exerts its initial change upon the incident rays.”’ But there is every reason 
to suppose that the formula is more nearly true than would have been admitted at 
the time when this opinion was written (1855), or else that the table-land of South 
Africa is near the ‘‘outer limit.” Writers generally have mistakenly applied 
formule involving the heating power of the sun to the determination of mean 
temperatures. Logically they can only be used for maxima. 
