OF THE WIND AND BAEOMETEIC PEESSEEE AT BOMBAY. 
13 
double diurnal variation and the land- and sea-breeze, and which may be called the daily 
normal wind (this wind is the most irregular in direction and variable in velocity). 
The mean resultant wind for the whole year is represented by the line C F, fig. 1 *. 
When all three winds are blowing normally there will be no rotation of the vane, but 
only a diurnal oscillation right and left about the daily normal wind. This is the most 
usual condition of the vane. The range of the oscillations will be very small if the 
daily normal is very strong, and will increase as the normal diminishes in strength. 
But whenever the daily normal wind becomes so small or varies in direction so as to lie 
wholly within the diurnal-variation curve, a single right-handed revolution of the vane 
will occur on that day, but it will not be a uniform movement all round the compass. 
Suppose, for instance, that the daily normal is a north wind of velocity one mile per 
hour; and let the thick line C D in fig. 1 represent this normal, then lines drawn 
from the point D to any part of the diurnal- variation curve will show the direction 
of the vane at the times indicated along the course of the curve and to which the lines 
are drawn. It will be observed that the vane will alter its direction very gradually 
in the afternoon hours, and rather quickly between 21 and 0 hours; that it will be 
almost steady at east from 6 to 9 hours, after which a very rapid movement will take 
place, changing its direction 180° in about three hours. Such movements of the 
vane are of very frequent occurrence at Bombay during most of the year, but especi- 
ally during the month of October, when the daily normal wind has a smaller velocity 
than at any other time of the year and is very variable in direction. Professor Dove 
pointed out the fact that right-handed rotations of the vane at Bombay were more 
numerous in October than in any other month of the year ; and he suggested that the 
operation of local causes might account for the excess, but he left the point to the 
decision of the observer. The foregoing explanation of right-handed rotations of the 
vane throughout the year affords also an explanation of the greater number of rotations 
in October, and shows it to be due to the fact that the daily normal winds of that month 
are smaller and more variable than in any other month, and therefore lie more frequently 
within the diurnal-variation curve. A separate treatment of the daily normal winds, so 
as to show their variability at different times of the year, is now in progress, and it 
brings out the fact here mentioned very prominently. Further, whenever the land- and 
sea-breeze and also the daily normal are simultaneously almost entirely suspended (which, 
however, rarely occurs, and never except during the monsoon months), there will be a 
double rotation of the vane in one day, the movement round the compass approaching 
more and more nearly to uniformity of progression as the land- and sea-breeze and the 
daily normal become less and less. 
It will be noticed that the daily normals which blow from different directions will all be 
differently modified by the superposition upon them of the diurnal variation of the wind : 
* The almost exact coincidence of the direction of the resultant wind for the whole year with that of the 
sea-breeze is remarkable ; and the fact seems to point to the same cause for the origination of one as for the 
other, viz. the higher temperature (on the average) of the air over the land compared with that of the air over 
the sea. Observations made on the eastern sea-hoard of India will show whether there is any truth in this 
conjecture. 
