820 Abstract of Meteorological Tables. [Dec. 



trie curve for the year would have been nearly symmetrical with that 

 of the barometer, except during the rainy season. 



It should be remarked, that the daily undulations of temperature for 

 Calcutta and Tirhut, are the extremes indicated by a register thermo- 

 meter exposed to night radiation and noonday sun : those for Madras 

 are only the variations of morning and afternoon heat in the shaded 

 air. They both, however, but the former more distinctly, shew to the 

 eye the influence of clouds and rain in diminishing the diurnal ex- 

 cursion ; and in this respect a direct accordance is also observable in 

 the reduced diurnal motion of the barometer ; as I long since pointed 

 out to be the case in regard to the Benares tables published in the 

 Asiatic Researches, vol. XV. 



Another material point to be noticed in the plate is the gale of the 

 3rd of August, when the Calcutta barometer dropt down to 28.8 inches 

 passing (on the plate)through the Tirhut column, which is only partially 

 affected. There is in all the lines a decided fall at the same period, but 

 only of an ordinary extent, apparently unconnected with the disturbing 

 cause of the Calcutta storm. Any who have witnessed the gathering of 

 a north-wester during the calm serenity of a sultry evening, and have 

 watched the turbulence of the clouds and commixture of upper cur- 

 rents prior to the sudden and furious generation of the whirlwind 

 below, will be prepared to consider the hurricanes and gales of longer 

 duration as equally insulated in their origin, only upon a much larger 

 scale of operation. A sudden condensation of aqueous, or perhaps of 

 gaseous matter, whether by electricity or simple cold, would, by draw- 

 ing upwards toward the vacuous space, the under air, cause a fall in 

 the barometer as certainly as if there were an absolute removal of 

 superincumbent weight, for which there would be no mode of account- 

 ing ; and this upward current could not take effect without the pro- 

 duction of a horizontal current of corresponding degree and velocity. 



The last point of instruction to be gained from the present plate,— 

 and it is a very important one, — is. the reliance that may be placed on 

 the measurement of barometrical altitudes taken by comparing the 

 observed height at places so distant as Cawnpore, or in the mountain- 

 ous regions of the Himalayas with the register of a stationary in- 

 strument at Calcutta. I confess I always had misgivings on the com- 

 parability inter se of such distant readings, until as it were my hand 

 refuted the doubts of my mind. The engraving shews that a dozen 

 contemporaneous observations (that is, observations not made at the 

 same instant, but at the same relative hour), would be ample for fixing 

 the altitude of a place within moderate limits. Moreover, it shews that 

 no reference of an observed height to a fixed unit (as 30 inches), as- 



