RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
359 
be expected. The difference is not very great in the summer months, but amounts to 
I 0° or more in January. At this season, however, the decrement in Bengal is, as we 
have seen, much more rapid, and such appears also to be the case in Southern India, 
when we compare the temperature of Dodabetta with that of Coimbatore and the 
nearest stations on the west coast. 
For the whole country, therefore, after due consideration, I have decided to adopt 
the rates of decrement found by Glaisher, and given above in Table YI. These 
include an allowance for diminishing diurnal range, since the observations were 
made in tire day time only. Though considerably more rapid for some months than 
those in the last Table, they are, perhaps, too slow on the whole, the decrement in 
India being probably greater near the ground than in England, where insolation is 
not so powerful. The last four columns of Table XII. give the probable temperatures 
at 10,000 feet, deduced by means of Table YI., the rate for cloudy skies being used 
when the average cloud proportion for the month amounts to half the expanse or 
more, and the rate for clear skies in the other months. 
The formula adopted for the barometric reductions is the simple one :—• 
log p = log P — 
h 
60,360 ( 1 + 
T + t — 64 
086 
With the values of T and t in Table XII., and the mean value of p, about 21 inches, 
it can be shown, by differentiating, that for a difference of elevation of 10,000 feet, an 
error of 1° in estimating t would give an error in the resulting value of p lying 
between ’008 and '010 inch. As the margin of possible error in many of the values 
of t may amount to several degrees, there is evidently nothing to be gained by using 
a more complicated formula, in which the variations of gravity and of density, owing 
to the presence of more or less water vapour, are taken into account. 
The computed results, which have been reduced to the standard value of gravity, 
are given in Table XIV. Considering the uncertainty of the adopted rates of 
decrease of temperature, and the widely different altitudes of the base stations, these 
results are remarkably consistent. Even apparent exceptions to this consistency, 
such as the low pressure over Dodabetta compared with the neighbouring station 
Coimbatore, or of Chikalda compared to Poona, or Mount Abu to Neemuch, serve to 
confirm the probable correctness of the rates of temperature decrement adopted ; for 
these are instances of the mid-day distribution of pressure which gives rise to the 
diurnal mountain winds, observed in all the warmer regions of the world. 
