THE WIND AND BAROMETRIC PRESSURE AT BOMBAY. 
3 
and from these values we find that the maxima and minima and the times of their 
occurrence are as follows, viz. : — 
h m 
The first minimum in the day 
„ ,, maximum „ „ 
„ second minimum „ ,, 
„ „ maximum „ ,, 
= —•0293 at 3 29 
= + •0551 „ 9 32 
= -•0484 „ 16 1 
= + •0228 „ 22 5 
Now marking these times along the course of the wind-curve (fig. 1), we see that they 
and the centre of the curve are all of them almost exactly in the same straight line — 
that the straight line being drawn through the centre so as to pass as close as possible 
to each of the marks so laid down divides the curve into four branches, two turned to 
the north of the line and two turned to the south of it — and that the direction of the 
line itself is the same as the general direction of the whole curve. We now see 
that the curve is such a one as would be formed by two distinct variations being 
superimposed the one on the other — one, the movement in the direction of the line, 
having a single period like the temperature-curve (fig. 5), and the other, the north and 
south movement, having a double period, and in this respect like the barometer-curve 
(fig. 2), but differing from it in that it passes through its mean positions at the four 
times when the latter is at its extreme positions, and vice versa. A closer comparison of 
the two curves (1 and 2) shows that when the wind-curve passes to the north of the line 
A B, the barometer begins to rise, and the more rapidly the further the curve departs from 
the line, and ceases to do so on the return of the curve back to the line ; similarly the 
barometer falls when the curve passes to the south of the line, its rate of descent being 
approximately proportional to the distance of departure of the wind-curve from the line. 
4. There can be no doubt that the greater part of the more extensive east and west 
movement is the land- and sea-breeze ; and this feature and the generally accepted theory 
which explains it have been already commented upon in Appendix I. of the Bombay 
Magnetical and Meteorological Observations for 1865 to 1870, on “The Normal Winds 
of Bombay.” The explanation of the double diurnal variation which we find is super- 
imposed at Bombay on the land- and sea-breeze is not, however, so apparent ; and before 
a complete view of the nature of this variation could be obtained, it would be necessary 
to eliminate from the whole curve that part which may be considered due to the pure 
land- and sea-breeze ; but the precise nature of this part is itself unknown, and the elimi- 
nation could only be effected by a comparison of the curve for Bombay with that of some 
other place, where, other conditions being similar, the land- and sea-breeze is either 
entirely absent or reversed in direction, as, for instance, at Madras ; but similar results for 
such a place are not yet available, and an approximate view is all that can be afforded 
at present. 
5. The principal features of the pure land- and sea-breeze will be more clearly exhi- 
bited in the following manner. Assuming that its direction is completely represented 
by the straight line A B, fig. 1, which is inclined to the east and west line at an angle 
b 2 
