tipon the Height of the Barometer. 463 



torial current ; and therefore that which flows to its top will 

 accumulate, and will tend to produce the same gradation of 

 heights in the atmospheric columns as though no deviation from 

 the relative mean temperature of E F existed. 



The pressure of this column, therefore, supposing this ef- 

 fected, would be in the proportion which its density bears to 

 the density of a column whose temperature is that due to the 

 latitude. In the present instance this temperature is 33°, and 

 the actual temperature of the column E F 25°. Now the re- 

 lative densities of air at these temperatures, under a pressure 

 of 30-000 inches of the barometer, are 1000 and 1023, which 

 will give 30"690 inches for the pressure of the column at 25°. 



According to this calculation, therefore, an unseasonable 

 reduction of the thermometer of 8°, under the circumstances 

 here considered, would producp a rise of the barometer of 

 0*690 inch ; but it would, in fact, be very much more than 

 this, because no allowance has been made for the continued 

 increase of density that would ensue from the increased press- 

 ure ; and it is clearly impossible to calculate to what height it 

 might rise, because the result here supposed could never be 

 perfectly attained, on account of the increased elasticity given 

 to the column, beyond that of lateral columns, where the cold 

 does not prevail, which would cause a flow to take place lon- 

 gitudinally in the upper strata that would partially carry off 

 the accumulation ; therefore the greater the extent in longi- 

 tude to which the cold prevails, the greater will be the ele- 

 vation of the barometer. 



It is also obvious, that when the height of the atmosphere 

 has been greatly reduced by a strong south wind, a decrease 

 in the force of the wind, with no change in its direction, will 

 (as occasionally happens) cause a rise of the barometer, because 

 the loss sustained will then be partially restored by the influx 

 of air from lateral columns, which have been beyond the limits 

 of the wind. 



In these various cases of increase of atmospheric pressure, 

 therefore, when the wind is southerly, we have the explanation 

 of the fact, that though the south wind in the majority of in- 

 stances depresses the barometer, its mean height during north 

 winds and south winds is nearly the same. 



Thus far I have confined myself to the consideration of the 

 mechanical effects alone, of the atmospheric currents upon the 

 height of the barometer, persuaded that its variations are 

 mainly owing to them. But the connexion long established 

 between its height and the state of the weather, a low and fall- 

 ing barometer generally attending precipitations of vapour, 

 demands an explanation. 



