the Meteorology of Bombay. 33 



The humidity exhibits also a single progression ; but may perhaps 

 be rather characterized as evidencing a very dry season from No- 

 vember to February, and a very humid one from June to Septem- 



ated to the north of the great continents, the gaseous pressure should be in- 

 creased in summer, and that the curve of annual variation should become the con- 

 verse of what it is in the lower latitudes. It appears from the meteorological ob- 

 servations made in 1843 by Messrs. Grewe and Cole, and presented to the British 

 Association at the York meeting by Dr. Lee, that such is the case at Alten, near 

 the north cape of Europe. The barometer and thermometer were observed three 

 times a day, from October 1842 to December 1843 inclusive. The hours of ob- 

 servation were 9 a.m., 3 p.m. and 9 p.m. No hygrometric observations were 

 made, but we are able to infer the approximate tension of the vapour from the 

 record of the thermometer. The quarterly means of the barometer and thermo- 

 meter in 1843 are as follows ; the barometer being reduced to the level of the 

 sea, and corrected for gravity : — 



Barometer. Thermometer, 



in. 



December, January, February 29-375 24 F. 



March, April, May 29-948 277 



June, July, August 29-905 52-4 



September, October, November ... 29-716 34-2 



Mean of the year 29-736 34-6 



Assuming the humidity in each quarter of the year to be 75, or the vapour to 

 be in each case three-fourths of that required for saturation at the respective tem- 

 peratures, we should have the following gaseous pressures : — 



in. 



December, January, February 29-25 7 



March, April, May 29-804 



June, July, August 29-616 



September, October, November, December ... 29-566 



29-561 



It appears therefore that in the six summer months the mean barometric pressure 

 exceeded that of the winter months by 0-381 inch; and the mean gaseous pres- 

 sure of summer exceeded that of winter by about 0-3 inch. As in this case the 

 curve of the gaseous pressure, as well as that of the aqueous vapour, accords in 

 character with the curve of temperature, i. e. ascends with ascending temperature, 

 and descends with descending temperature, — the barometric annual range is 

 greater than the gaseous annual range, which is contrary to what takes place in 

 the temperate and equatorial zones. It is not improbable that in the Antarctic 

 Circle the phaenomenon which we have just noticed as taking place in the Arctic 

 Circle, viz. the summer increase of the gaseous pressure, — may not be found in the 

 same degree, if at all ; for the two hemispheres present a remarkable contrast in 

 their respective proportions of sea and land, and the rarefaction of the atmosphere 

 over the middle latitudes of the southern hemisphere during its summer must be 

 greatly less than in the same latitudes of the northern hemisphere in the corre- 

 sponding season. The barometrical observations made by Sir James Ross in 

 summer in the Antarctic Circle accord with this inference ; since after correcting 

 them for the shortening of the column of mercury by the increased force of gravity 

 in the high latitudes, and abstracting the small tension of vapour corresponding 

 to the temperature, the mean gaseous pressure deduced from them, though nearly 

 equal to the mean gaseous pressure of the year at Bombay, does not exceed it ; 

 whereas at Alten it is only in the winter months that the gaseous pressure descends 

 so low as to approximate to the usual mean gaseous pressure of the tropical re- 

 gions. 



It is much to be desired that the zealous observers at Alten should observe the 



Phil. Mag. S. 3. Vol. 28. No. 184. Jan. 1846. D 



