1820.] Mr. Dalton on Meteorology. 251 



the sun is north of the equator, the weight of the atmosphere 

 increases in these parts, and again diminishes when soutia of the 

 equator. 



The means by which the effect is produced, I conceive, are 

 these : the sun's action is constantly increasing the mass of 

 aqueous vapour in the atmosphere during the period from the 

 vernal to tlie autumnal equinox ; that is, the whole mass of 

 vapour existing in the atmosphere is daily increasing, notwith- 

 standing the quantity precipitated. This fact is verified by the 

 constant rise of the vapour point till the month of September, 

 after which it commonly declines pretty rapidly. Now it is 

 obvious that the addition of steam or aqueous vapour to the 

 atmosphere must add to the weight of the atmosphere ; and this 

 is, I imagine, the cause of the increase of its weight in that 



season. 



I am aware that another conclusion, the very opposite to this, 

 may be deduced from the premises. Aqueous vapour, it may 

 be said, is specifically lighter than dry air ; and, on that account, 

 the greater the quantity of aqueous vapour, the less is the 

 Aveight in any given volume of air of given elasticity. But it 

 must be remembered that the aqueous vapour at the most con- 

 stitutes but -sV^h part of the atmosphere, and any excess of this 

 which may prevail in any one place, cannot be supposed power- 

 ful enough to move the rest of the atmosphere towards any other 

 place, where the vapour is deficient. Now we have no reason to 

 believe that much intercourse takes place between the atmo- 

 spheres of the northern and southern hemispheres. The great 

 and unceasing currents of air are between the equator and the 

 polar regions ; but that any large volumes of air cross the equa- 

 tor from one hemisphere to the other, does not appear from any 

 phenomena we are acquainted with. And if the air does not 

 cross the equator, the vapour cannot, being so intimately blended 

 with the air. Thus, although there may be a constant pressure 

 or tendency of the atmosphere in the northern hemisphere to 

 invade that of the southern during our summer, and vice versa in 

 winter ; yet I conceive it never can be so effectual as to restore 

 a perfect equilibrium during the season, but will leave an excess 

 of aqueous vapour in our hemisphere, unbalanced either by air 

 or vapour of the opposite hemisphere. 



II. Of the Thermometer. 



My thermometer is situated out of a window on the second 

 floor, about 16 feet above the ground, and about six inches from 

 the wall ; it has an eastern aspect, and open, airy situation, is 

 not affc^cted by the sun, except in a summer's morning, and it is 

 then duly shaded to prevent the sun's, influence. The observa- 

 tions are taken three times a day, as witli the barometer, at 

 8 in the morning, and at 1 and 1 1 i\ the afternoon, i have 

 some reason to think the observations give a mean temperature 

 rather below than above the true mean. The temperature of 



