252 Mr. Colebrooke on the 
summer, approximates to one inch, which seems to be the limit 
of the range of the barometer at the Cape of Good Hope. 
The utmost diurnal variation which is noticed occurred with 
a hurricane, in January, 1821; the mercury in the barometer 
descending four-tenths of an inch shortly before the storm, and 
rising six-tenths at its termination. The range of fluctuation 
within a day rarely exceeds one-tenth of an inch, and scarcely 
ever reaches two, unless attendant on tempestuous weather, as 
in the instance above-mentioned, when it doubled that quan- 
tity. 
Ordinary variations of barometric pressure, within the limits 
mentioned, occur with alteration of diurnal temperature, and 
with change of the wind’s direction, or of hygrometric condi- 
tion of the air. The barometer commonly rises on a shift of 
westerly to easterly wind, and conversely falls with a return of 
it to the western quarter; that is, the mercury ascends with 
a dry wind, and descends with a damp one, which is but saying 
in other words, that it rises with a dry atmosphere, and falls 
with ahumid one. It does so, when a change of temperature 
does not interfere ; but depression of temperature is very gene- 
rally co-ordinate with a rise of the barometer, and increase of 
it with descent of the mercury. When both causes co-operate, 
cold with dryness, or humidity with heat, the effect may be ex- 
pected to be greatest, and least when the predominant cause is 
modified by a less efficient one in opposition to it: dryness by 
heat, or cold by damp. 
It must, however, be acknowledged, that, on a cursory exa- 
mination of the diaries referred to, the changes there registered 
are not always to be so daily accounted for. It must likewise 
be admitted, that the variation of barometric pressure is scanty 
for difference of temperature, as its assignable cause, and large 
for difference of humidity. 
At the Cape it is in the coldest, though dampest, season, that 
the barometer stands highest. 
