8 
ME. E. CHAMBEES ON THE DIUENAL YAEIATIONS 
The expectation here expressed is certainly not realized by the diurnal wind-curve for 
Bombay ; but it may not therefore be inferred that such a tendency does not exist, and 
may not eventually be traceable. The inference that may be drawn is that the same 
cause, working in an unexpected manner, produces other effects of an opposite nature 
which completely overwhelm the expected tendency, of whose existence, however, in a 
much smaller degree, there can be no doubt. But the principles of Hadley’s theory 
fail also to explain the double reversal in the north and south direction which the wind 
undergoes daily ; for, in accordance with them, the wind would be reversed in direction 
but once a day, just as in the case of the land- and sea-breeze, seeing that the tempera- 
ture rises and falls but once a day. Turning to the longitudinal variation, which is 
approximately represented by fig. 7, the same difficulty is encountered in attempting to 
explain the double reversal of this curve by the same principles ; and the directions of 
the wind indicated in the morning and afternoon hours are exactly the reverse of those 
expected by Sir J. Herschel in the remark quoted above. 
8. It seems highly improbable, then, that the meridional and longitudinal variations 
of the wind at Bombay are part of a system of convection-currents, such as would satisfy 
the requirements of Hadley’s theory. The form of the curves seems rather to suggest 
the notion of a general bodily movement of the atmosphere (its lower strata as well as 
its upper) outwards in all directions from the middle of the hemisphere which is being 
heated towards the middle of that which is being cooled, succeeded by oscillatory move- 
ments consequent upon the disturbance of equilibrium thus caused. The notion of a 
general outflow of air (not an upper surface overflow merely) from the heated hemi- 
sphere has already been advanced by Sir John Herschel in his explanation of the 
diurnal variation of the barometer*; but he does not seem to have suspected that there 
would be more than one complete oscillation in the twenty-four hours, or that these 
movements of the air would be more marked than the convection-currents already 
referred to. After explaining that the heat of the sun produces on the air of one hemi- 
sphere a “ considerable elevation of the lines of equal density [which when so elevated 
cease to be statical level lines], while the nightly chill on the other side acts in a con- 
trary way on the opposite hemisphere,” he says, “ an indirect effect ” on the barometer 
“ results from the elevation of the surface of equilibrium on one side and depression 
below it on the other. To form some rough estimate of this effect, whose exact calcu- 
lation would be difficult, we must consider that an elevation of 363 feet on one side 
[taking, exempli gratia, 20° Fahr. for the difference of clay and night temperature] 
and a similar depression on the other, correspond to a slope of 3 ,,- 5 at the common 
boundary of the two hemispheres, down which the centre of gravity of each aerial 
column situate on that boundary (being unsupported laterally) tends to glide. The 
effect of this will be the production of a general movement of air setting outwards from 
the heated hemisphere , and which, though feeble (as its velocity would at its maxi- 
mum hardly exceed a mile an hour), yet acting over the whole circumference of a great 
* See Herschel’s ‘ Meteorology,’ art. 77 a. 
