78 REPORT—1845, 
therefore wholly inoperative, though the effects are comparatively feeble du- 
ring the clouded weather which accompanies the south-west monsoon*. 
If we now view together the diurnal variations of the wind and gaseous 
pressure, as shown in the Plate, we find a minimum of pressure coinciding 
with the greatest strength of the sea breeze ; a second minimum of pressure 
coinciding with the greatest strength of the land breeze; and a maximum of 
pressure at each of the periods when a change takes place in the direction of 
the aérial currents; or, otherwise stated, we find a decrease of pressure coin- 
cident with the increase of strength both of the land and sea breezes, and an 
increase of pressure coincident with their decline in strength. 
The facts thus stated appear to me to admit of the following explanation :— 
the diminution of pressure which precedes the minimum at 4" is occa- 
sioned by the rarefaction and ascent of the column during the heat of the 
day, and its consequent overflow in the higher regions of the atmosphere, 
which is but partially counterbalanced in the forenoon by the influx of the 
sea breeze at the lower part of the column. Shortly after the hottest hour 
is passed, the overflow above and the supply below become equal in amount, 
and the diminution of pressure ceases. As the temperature falls towards 
evening, the column progressively contracts, when the influx from the sea 
’ breeze more than counterbalances the overflow, and the pressure again in- 
creases until a temporary equilibrium is restored, when the sea breeze ceases 
and the pressure is stationary. 
As the night advances, the air oyer the land becomes colder than over 
the sea; the length of the column over the land contracts, and the air in its 
lower part becomes denser than in that over the sea: an interchange then 
commences of an opposite character to that which prevailed during the 
day. The outward flow is now from the lower part of the column, or 
from the land towards the sea, causing the pressure to diminish over the 
land ; it continues to do so until towards daybreak, when the strength of the 
land breeze is greatest, because the air over the land is then coldest in com- 
parison with that over the sea. As the sun gains in altitude and the tempe- 
rature of the day advances, the land heats rapidly; the density of the air 
over the land and sea returns towards an equality ; the land breeze declines 
in strength, and the drain from the lower part of the column ceases to coun- 
terbalance the overflow which the land column is at the same time receiving 
in the higher regions; the pressure consequently haying attained a second 
minimum at or near the hour of the greatest disproportion of temperature, 
again increases until the temperature and height of the column over the sea 
and land are the same, and the pressure again becomes stationary. But now 
the rarefaction of the column over the land continuing, its increase in height 
above the less heated column with which it is in juxtaposition, and its con- 
sequent overflow, occasion the pressure to decrease until the minimum at 
4. o'clock is reached. 
We have thus therefore at Bombay a double progression of the diurnal 
variation of the gaseous pressure ; the principal minimum occurring at 4 o'clock 
in the afternoon, occasioned by an overflow from the column in the higher 
regions of the atmosphere ; and the second minimum occurring at 18%, occa- 
sioned by an efflux from the lower part of the column. The first minimum 
is similar to that which has been shown to take place at Toronto, Prague and 
* There are no data in Dr. Buist’s report from which the diurnal variation in the force of 
the wind may be judged of in the days during the south-west monsoon, when no alternation 
takes place in its direction. It would seem probable that on such days the variation should 
be a single progression, weakest towards daybreak, and strongest about the hottest hour of 
the day. 
