the Meteorology of Bombay. 35 



in the opposite direction to those of the gaseous pressure. From 

 this cause the range of the barometric curve during the year is only 

 ()*."27 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 by the rarefaction of the air over the 

 land whilst the sun is in the northern signs, and its consequent over- 

 flow in the higher regions, producing a return current in the lower 

 strata ; and the increased pressure in the cold season is occasioned 

 by the cooling and condensation of the air, whilst the sun is on the 

 south side of the equinoctial, and its consequent reception of the 

 overflow in the upper strata from the regions which are then more 

 powerfully warmed, and which is but partially counteracted by the 

 opposite current in the lower strata. 



In concluding this communication, I beg respectfully to submit to 

 the consideration of the eminent meteorologists here present, that it 

 is very important towards the progress of this science, that the pro- 

 priety (in such discussions as the present) of separating the effect of 

 the two elastic forces which are considered to unite in forming the 

 barometric pressure, should be speedily admitted or disproved. 

 The very remarkable fact recently brought to our notice by Sir 

 James Ross, as one of the results of his memorable voyage, that the 

 mean height of the barometer is full an inch less in the latitude of 

 75° S, than in the tropics, and that it diminishes progressively from 

 the tropics to the high latitudes, presses the consideration of this 

 point upon our notice ; for it is either explained wholly or in greater 

 part by the diminution of the vapour constituent in the higher lati- 

 tudes, which diminution appears nearly to correspond throughout 

 to the decrease of barometric pressure observed by Sir James Ross ; 

 or it is a fact unexplained, and I believe hitherto unattempted to be 

 explained, on any other hypothesis, and of so startling a character 

 as to call for immediate attention. 



If, by deducting the tension of the vapour from the barometric 

 pressure, we do indeed obtain a true measure of the pressure of the 

 gaseous portion of the atmosphere, the variations of the mean annual 

 gaseous pressure, which will thus be obtained in different parts of 

 the globe, — and the differences of pressure in different seasons at 

 individual stations, — may be expected to throw a much clearer light 

 than we have hitherto possessed on those great aerial currents, which 

 owe their origin to variations of temperature proceeding partly from 

 the different angles of inclination at which the sun's rays are re- 

 ceived, and partly from the nature and configuration of the material 

 bodies at the surface of the earth : and afield of research appears to 

 be thus opened by which our knowledge of both the persistent and 

 the periodical disturbances of the equilibrium of the atmosphere may 

 be greatly extended. 



D2 



