346 MR. S. A. HILL ON THE WINDS OF NORTHERN INDIA, AND THEIR 
ever, probably nearly correct, since the disturbing circumstances are similar ; and this 
ratio shows that the diurnal convection currents, which tend to mix together 
atmospheric strata belonging to different altitudes, must be more active during the 
month of January in North Bengal than they are in the upper parts of the great 
plain—a relation which it is necessary to prove in order that the observed deviations 
of the resultant winds from their theoretical directions, in the two regions compared, 
may be explicable by Koppen’s hypothesis. 
The following considerations will enable us to discuss this point a little more fully. 
At page 236 of his valuable work on the Meteorology of the Bombay Presidency, 
Mr. C. Chambers, F.R.S., has shown that rapid convective movements of the atmo¬ 
sphere, of the kind he calls “ topsy-turvy,” probably extend in the day time up to the 
level at which the temperature is the same as that given by the law of convective 
equilibrium, which may be proved from thermodynamical principles to be, for dry air, a 
decrease of 1° F. in 183 feet. There is no reason, however, to believe that all inter¬ 
change between successive layers ceases at this level, for the molecular theory of gases 
indicates that the ultima,te distribution of temperature would be uniform in a vertical 
column of the atmosphere free from all disturbing causes. 
It is difficult to estimate, with any exactitude, the height to which such rapid 
convective movements here extend, for we do not know the rate of temperature 
decrement in the open air over the plains of India; all our observations at high levels 
having been made at mountain stations, where the temperature, at least in the day 
time, is higher than it would be in the free atmosphere. The results of Glaisher’s 
balloon observations may help us, however, to obtain some idea of this height. 
When slightly smoothed, they may be thrown into the form of Table VI., which gives 
a rapid decrement of temperature in the first 2000 feet, especially with clear sky. 
Table YI.— Puites of Temperature Decrement from Glaisher’s Observations :— 
Height. 
Clear .sky. 
Cloudy sky. 
Sea-level to 1,000 feet. 
O 
62 
O 
4*5 
1000 „ 2,000 .. 
47 
3-8 
2000 „ 3,000 „. 
3-8 
3-5 
3000 „ 4,000 „. 
3-3 
3*3 
4000 „ 5,000 „. 
3'0 
3 1 
5000 „ 6,000 „. 
2-8 
3-0 
6000 „ 7,000 ,,. 
2-6 
2-8 
7000 „ 8,000 „. 
2-6 
2-6 
8000 „ 9,000 „. 
2-5 
2-4 
9000 „ 10,000 „. 
2-4 
o-2 
Sea-level to 10,000 ,,. 
33-9 
3L2 
This Table is represented graphically on Plate 21, which also shows the line of 
uniform decrement under the law of adiabatic convection. It will be seen that, 
