upon the Height of the Barometer. 461 



not prevail on the surface, and consequently the heights of all 

 the columns are advancing towards a state of equality, A B 

 falling and G H rising. Hence, as they approach to this, 

 their pressure becomes as their specific gravity; and supposing 

 the strength of the falling current to be overcome at F, as 

 shown by the arrows, there will be a point to the south of 

 E F where the mean height of the atmosphere is maintained, 

 and the barometer will fall at B and D with the pressure of 

 the columns of least density, and rise at F and H where the 

 density is greatest. Thus, then, consistently with observation, 

 the barometer is depressed by a south wind and rises with a 

 north wind ; and when the latter maintains its course only to 

 a certain point, as has been supposed at F, where it is stopped, 

 the rise will be above the mean height, as at F and H. When 

 however the south wind is at last driven into the upper regions 

 of the atmosphere, the northerly current, impelled by the in- 

 creased pressure of the columns E F and G H, in addition to 

 their originally greater density, will flow with accelerated 

 velocity, and a rapid rise in the barometer will ensue. 



The barometer, however, frequently rises with a southerly 

 wind ; and it is easy to see, on the supposition that the pre- 

 ceding conclusions are correct, that this must necessarily be 

 the case ; for let the force of the south wind be overcome at 

 F as before supposed, here there will be an approach to a calm; 

 and it is most probable that our calms (or, as the air is scarcely 

 ever perfectly calm during the day, our very light winds from 

 north-west or south-west, one of which being always the di- 

 rection of the wind when the air is nearly still for any con- 

 tinued period,) are the results of the equality of the forces of 

 the two contrary currents ; a conclusion which the frequent 

 shifting of these scarcely perceptible breezes between south 

 and north of west, and without affecting the barometer, tends 

 to confirm. Now it is at these times tliat the pressure of the 

 atmosphere is unusually great. 



Let then the calm be confined to a locality of greater or less 

 extent about F, the air of the polar current continues to flow 

 from H to this place, but can proceed no further; here there- 

 fore an accumulation takes place, which can only be carried 

 off" by the increased flow of the upper current towards north, 

 occasioned by the increased height of the column ; but this 

 force can only be communicated to it after some elevation has 

 been attained; and when it does ensue, the increased pressure 

 given by it to the polar columns will urge forward yet more 

 the current of the lower strata from H to F. Now it is 

 obvious that the increased height of the atmospheric columns 

 must extend to the south as well as to the north of F ; and 



