OF THE WIND AND BAROMETRIC PRESSURE AT BOMBAY. 
11 
that represented by the typical curve (fig. 10) upon the south-east trade-wind of that station, 
and they appear to put beyond doubt the existence of a double diurnal left-handed 
rotatory variation of the wind in southern low latitudes. 
“ From Table XLI. we learn that the wind is more easterly generally during the 
hours of the day than during the night, and that it is most easterly about noon. The 
direction varies little during the night, but there is a tendency to a more southerly 
direction about daybreak, and just after sunset. The force of the wind appears to have 
a decided maximum between 22 h and 23 h *, and a minimum about 4 h . From 5 h or 6 U 
to 15 1 * it remains nearly stationary, except that there is a tendency to a second maximum 
at ll h shown in both years, followed by a minimum also of inferior character. At 13 h , 
at 0\ and 16 h the pressure coincides with the mean of the twenty-four hours.” 
The variation in direction lias also two maxima and two minima with regular pro- 
gressions from one to another, and the turning-points occur at intervals of about six 
hours. 
13. Having advanced reasons for believing that a regular diurnal variation in the 
movements of the atmosphere does really exist, and that it is probably universal, and 
having shown, to the extent to which the limited nature of the observations permit, the 
probable character of that variation, it may now be shown that the diurnal variation of 
the barometer is directly deducible from the wind variation. The best way of showing 
this will perhaps be to adopt the suggestions recently made by Hr. Balfour Stewart, 
and to suppose the existence of several “ meteorological blockades.” A figure will best 
illustrate what is intended (see Plate XXXIV. fig. 15). 
Let W E represent the earth’s equator, divided into hourly intervals, and let the 
horizontal lines above and below it represent parallels of north and south latitude 
respectively. The arrows will then indicate the directions towards which the wind is 
blowing at the times at which they are placed, in accordance with the typical curves 
fig. 9 and fig. 10. They are marked only for those times when the winds are at one 
or other of the four cardinal directions ; but it is to be understood that there is a gradual 
progression from one to the other with the time, producing two right-handed rotations 
along the parallel of north latitude, and two opposite rotations along the parallel of south 
latitude ; and that at the equator the wind gradually dies away to a calm, and is then 
succeeded by a wind in the opposite direction, which gradually increases to a maximum. 
Let meridian lines be drawn through those times when the meridional variation of the 
wind on the parallels of latitude passes through zero, which are about the times when 
the longitudinal variation attains its maxima east and west positions ; these lines will 
divide the space between the parallels of latitude into four areas, a, b, c, and d, the 
boundaries of which may be taken to represent those of four separate blockades. An 
inspection of the figure will show that air is flowing out of the areas a and c in all 
directions, and flowing into the areas b and d from all directions. A barometer placed 
in the area cl or c will therefore show a diminishing pressure, while one placed in the 
area b or d will show an increasing pressure. 
* Astronomical reckoning. 
c 2 
