ON SOME POINTS IN THE METEOROLOGY OF BOMBAY. 81 
The humidity exhibits also a single progression ; but may perhaps be rather 
characterized as evidencing a very dry season from November to February, 
and a very humid one from June to September, the latter season being that 
of the rains. The average degree of humidity in the year is very slightly 
lower than either at Toronto or at Greenwich, but is still closely approaching 
to a value expressing the presence of three-fourths of the quantity of vapour 
required for saturation. 
The mean gaseous pressure in 1843, derived from the two-hourly obser- 
vations, appears to have been (29:023 + 0:025, an index correction which 
Dr. Buist gives as that of the barometer with which the observations were 
made =) 29:048 English inches; or, measured by the height of a mercurial 
column in the latitude of 45°, 28°988. The height above the sea is thirty-five 
feet, and the latitude 19° N. 
The mean height of the barometer in the year 1843, derived from obser- 
vations at every second hour, appears to have been (29°803 + 0:025=) 
29828, or, with the correction applied for gravity, 29°768, the elevation being 
thirty-five feet above the sea. This is less than what is generally received 
as the average height of the barometer in the same latitude. From the careful 
comparison described in Dr. Buist’s report of the standard barometer with 
several other barometers, there seems great reason to believe that the mean 
height shown by it must be a very near approximation at least to the true 
mean atmospheric pressure in the year 1843 at Bombay. 
The mean height of the barometer in the four clouded months of May, 
June, July and August, is 29°667 ; and in the four clear months of November, 
December, January and February, 29°921. The mean vapour pressure in 
the same seasons is respectively 0°904 and 0°623, and the gaseous pressure 
consequently 28°763 and 29:298. There is therefore between the two sea- 
sons a difference of 0°535 in. of gaseous pressure, and of 5°84 of tempera- 
ture; the lowest pressure corresponding to the highest temperature, and vice 
versd. If we may allow ourselves to make a rough proportion drawn from 
a single case, we may estimate a decrement of 0:1 in. of pressure to an in- 
crement of 1° F. The highest temperature and lowest pressure are accom- 
panied for nearly the whole of the period by the southern monsoon; the 
lowest temperature and the highest pressure are accompanied by the north- 
ern monsoon. 
The curves of the annual variation of the gaseous, barometric, and vapour 
pressures, which are represented in the Plate, show how much of the influ- 
ence produced on the gaseous pressure, by the alternation of the overflow 
in the high regions of the atmosphere as either side of the equator becomes 
heated in its turn, is masked in the barometric curve by the combination 
in the latter of the vapour pressure, the variations of which take place 
throughout the year in the opposite direction to those of the gaseous pres- 
sure. From this cause the range of the barometric curve during the year 
is only 0°327 inch, whilst that of the gaseous pressure is 0°650 inch. 
. . The analogy of the annual and diurnal variations, considered in respect to 
the explanation which has been attempted of the latter, is too obvious to be 
_ dwelt upon. The decreased gaseous pressure in the hot season is occasioned 
_ the winter months that the gaseous pressure descends so low as to approximate to the usual 
mean gaseous pressure of the tropical regions. 
itis much to be desired that the zealous observers at Alten should observe the wet ther- 
_ mometer at the same time as the barometer; the register would also be rendered much more 
complete by the addition of another observation-hour, about 6 a.m., which might not perhaps 
be inconvenient. The atmospheric pressure and the tension of vapour at or near the coldest 
hour of the twenty-four, are important data in meteorological discussions. 
1845. G 
