14 
ME. E. CHAMBERS OH THE DIURNAL VARIATIONS 
suppose, for instance, that the daily normal is a north wind with a constant velocity 
throughout the day of ten miles per hour, and that a diurnal variation like that repre- 
sented by fig. 1 is superimposed upon it, then at 21^ hours and 11 hours the vane will 
be unaltered, but the wind will be increased in velocity at 21^ hours, and diminished in 
velocity at 11 hours, whilst at 74 hours the vane will be deflected furthest to the right, 
and at 15-^ hours furthest to the left. A daily normal from the south will be oppositely 
affected both with regard to velocity and direction, supposing the observer to face the 
wind. Next, suppose that the daily normal is a wind from the west of constant velocity 
and of the same strength as before, then at 13 hours and 22 hours the vane will be 
unchanged, but the wind will have increased in velocity at the former time and diminished 
at the latter; whilst at 18^ hours the vane will be deflected furthest to the right, and 
at 74 hours furthest to the left. An east normal wind will be oppositely affected both 
as regards direction and velocity. Similarly, for daily normals from other directions, 
there will be hours (different for each different normal) when the vane will be unaffected 
by the superposition of the diurnal variation, but when the wind will be modified in velo- 
city, and other regularly recurring times of the day when each different normal will be 
deflected to its extreme right and left positions ; but these times will not be the same for 
any two different normals. Hence arise many apparently fortuitous movements of the 
vane, which, however, may be reduced to order by the foregoing method of separation. 
16. Having now acquired a knowledge of the nature of the diurnal variation of the 
wind at Bombay, and the influence which it exerts on the movements of the vane, it is 
evident from an examination of the anemograms that by far the greater part of the right- 
handed rotation of the vane at Bombay is due to the diurnal variation of the wind 
rather than to a veering round of the daily normal wind ; and it will be needful to 
make a special investigation of the wind records at Bombay, eliminating the effects of the 
diurnal variation, before it can be said that they show an excess of “ direct” over “retro- 
grade” rotation due to the cause assigned by Professor Dove in his ‘Law of Storms.’ 
17. The anemograms of the seven British observatories, published in the ‘ Quarterly 
Weather Beports of the Meteorological Office,’ show numerous instances in the summer 
months of diurnal riqht-handed rotations of the vane similar to those which occur at 
Bombay, and very many cases of like diurnal modifications of a daily normal from the 
same direction. As instances of the former kind, may be mentioned the anemograms 
for Falmouth for the 30th June, 1st and 2nd July, 1869; and of the latter kind those 
for Stonyhurst for the 30th June and 1st July, 1869. Such instances appear to show 
that a systematic diurnal variation of the wind obtained in the British Isles ; and for the 
sake of testing this point, an experimental reduction of fifty of the Falmouth anemo- 
grams for days between the 1st April and 2nd September, 1869, has been made. Those 
days on which the velocity exceeded 15 miles per hour for the whole day, or a large 
portion of it, were rejected as disturbed days. The tabulations and reductions were 
made exactly in the same manner as the Bombay observations are tabulated and reduced, 
so that the results are strictly comparable with those for Bombay. The results are con- 
tained in the following Table, and also graphically represented by fig. 12. 
