OBSERVATIONS TAKEN IN INDIA. 
349 
and finally, with the coefficient 
/»=/— oii47(<-«')x^*; 
and with this formula'I' Colonel Boileau of the Beng'al Engineers, in charge of the 
Magnetic Observatory at Simla, to aid meteorologists, has calculated a series of tables 
of the tension of vapour from minus 10°Fahr. to 170 °Fahr. for 30° of depression of 
the wet bulb by tenths, and for variations of pressure from 19 inches to 31 inches. 
It was these tables I used in the reduction of the preceding wet bulb observations ; 
and it was in the progress of their use, during several months’ labour, that at certain 
temperatures doubts were raised in my mind respecting the accuracy of the formula, 
from supposing the fraction of saturation unreasonably high when the depression of 
the wet bulb was inconsiderable, and unreasonably low when the depression of the 
wet bulb was considerable. The tension of vapour at considerable depressions startled 
me, until at last I found in testing the tables that with a depression of the wet bulb 
of 19° at a temperature of 52° Fahr., and at a pressure of 29 or 30 inches, the results 
became impossible ; that is to say, the tension of vapour of the dew-point arrived at by 
the formula had a greater numerical value than the tension of vapour at the tempe- 
rature of the wet bulb, e. g. 
Fahr. Bar. 
o in- 
Dry bulb 52 30 
Wet bulb 33 20640 
Difference 19 = (a) = ‘01 147 — ^')=21794 
It is objected that a depression of the wet bulb of 19°, at a temperature of 52°, 
never can occur in nature:}:; but very considerable depressions, both of the wet 
* Apjohn’s formula is expressed in English measures : /" is the fofce of aqueous vapour at the dew-point ; 
/' the tension of vapour at the temperature of evaporation ; a specific heat of air ; e the latent heat of aqueous 
vapour; d the depression or difference between the temperature of the air and wet bulb (t—f ) ; p the pressure 
of the air in inches. 
t Differing little from August’s formula, and converting it into the measures and scale used by August ; for 
all practical purposes, the two formulae are the same. 
1 In a synopsis of nine years’ observations of the wet bulb at Mahabuleshwur, the following maximum de- 
pressions of the wet bulb occur in the respective months of the year : — 
In a preceding page it was shown that water in bottles under wet straw, exposed to the wind at Ahmednugger, 
cooled down to 65" Fahr., the temperature of the air being 98°, difference 33°, barometer 28 inches, dew-point 
by Apjohn 41°‘7, by Glaisher 48°'5. 
