352 LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
moisture suhtr actively ^ even were the tension of vapour satisfactorily determined for 
the whole pressure. The barometer/a//^ with increasing saturation of the atmosphere 
with moisture, indicating the displacement of comparatively dry air by dilated vapour, 
the density of which is less than that of drier air, and a certain amount of pressure 
is taken off the barometer and the mercury falls on account of the diminished density 
and elasticity of humid air ; but as soon as this vapour is condensed into rain the drier 
air resumes its ordinary density and elasticity, and the mercury mounts again. If 
the vapour came simply as an addition to the drier air, the density and pressure of the 
compound should increase, and the barometer should rise ; but this is contrary to 
fact. I would therefore not apply a subtractive correction for that vapour, which 
has already acted directly upon the barometer in diminishing pressure by displacing 
denser air. The object of applying any corrections at all to the barometer, on ac- 
count of moisture in the atmosphere, is stated to be to ‘‘ obtain from them the 
pressure of the atmosphere of dry air*;” but as such a state of the air is a physical 
impossibility as long as there is a drop of water upon the earth to be vaporized, as 
evaporation goes on at all temperatures, it may be asked, what practical advantage 
can be obtained from any such determination ? and the more so, may the question 
be asked, when experimenters are not in accord with respect to the numerical values 
of the tension of vapour at different temperatures to be applied as corrections'!-. 
On the whole it appears very desirable that to direct means recourse should always 
be had, if possible, for the determination of the dew-point, to Daniell’s hygrometer, 
or to Regnault’s condensing hygrometer ; and short of this, that persevering compa- 
risons should be made for years, in extended ranges of temperature, depression and 
pressure, and at different elevations, to obtain a more trustworthy wet bulb formula, 
or an unquestionable series of factors. 
Rain. 
If it were necessary to suggest caution in generalizations from a limited number of 
local observations for the determination of the dew-point, caution is equally, if not 
more necessary in attempting to fix the normal rain-fall even in a narrow area, much 
less in a district or province. For instance, within the limits of part of the small island 
of Bombay, seven miles by two miles, the following is the result of observations with 
nine rain-gauges. The Fort and Esplanade are necessarily proximate; the Observa- 
tory at Colabah two miles distant, and the most remote gauge at the Government 
House at Parell, only five miles from the Fort, the other stations being interme- 
diate between the Fort and Parell. 
* Glaisher’s Hygrometrical Tables, p. 12, edition of 1847. 
t At 32° Fahr. Dalton’s tension of vapour, in inches of mercury, is 0'200, at 95°= 1 '59297, atl22°=3'500 in. 
The Physical Committee of the Royal Society adopted for 32° Fahr. 0'186, and for 95°=1'610, and 122° 
Fahr. 3'542 ; Regnault, at 32° Fahr., has 0'18124, and at 95° Fahr. 1'64380, and at 122°=3'619. Kaemtz, 
at 32° Fahr., has 0'17999, and at 95° Fahr, 1'57789 ; of course dilFerent results are come at by the use of 
these different values. 
