upon the Height of the Barometer. 465 



found from these laws, it may be sufficient to remark, that any 

 general theory can only be founded upon a condition liable to 

 great variations, such as a continual decline of temperature 

 from equatorial latitudes to polar ; for it not unfrequently hap- 

 pens from local causes, that the temperature of a station is 

 considerably below that of one to the north of it, and that 

 stations on the same latitude, and not very far removed from 

 each other in longitude, have very different temperatures; but 

 such irregularities must be the causes of disturbance to the 

 general order of the phasnomena. 



It will be necessary, however, before closing this essay, to 

 apply the foregoing conclusions to the explanation of various 

 facts of a very interesting nature presented by the oscillations 

 of the barometer in connexion with storms, situation with re- 

 gard to latitude, and with seasons. 



The greatest depression of the barometer is during storms 

 from south-west or south ; those from north-east depress the 

 barometer very little, and sometimes not at all. 



As the north-east wind is attended by a counter current flow- 

 ing above, if the force of both is equally accelerated, the press- 

 ure of the atmosphere will be unaltered ; but if the lower one 

 is unequalled by the upper, a diminution of pressure in some 

 parts of its course, and an increase in others, will be the con- 

 sequence; but it is obvious it cannot be very great, because, 

 as the force of the upper current depends upon the compara- 

 tive elasticity of the upper parts of the atmospheric columns, 

 and any change in the elasticity of the lower strata must be 

 transmitted through the whole column, its acceleration will 

 very quickly follow that of the under current; but in storms 

 from south-west, which are most frequent and violent, the case 

 is widely different. 



As the south wind depresses the barometer by the flowing 

 of the air from higher columns to lower without the compen- 

 sating return, the greater the velocity of the wind the greater 

 of course will be the depression. But its degree will also de- 

 pend on the extent of the storm, because, as the height of the 

 atmosphere in its locality is diminished, the velocity of the air, 

 flowing from the upper strata of the columns, which are beyond 

 the limits of the storm both in latitude and in longitude to- 

 wards the depression, will be increased, and consequently the 

 smaller the extent of the storm, the more effectual will be the 

 supply from this source ; this, I believe, is quite consistent with 

 observation. Westerly storms, however, though beginning 

 from the south, pass before subsiding to the north of west, 

 and then, although the force of the wind is undiminished, the 

 barometer rises with a rapidity equal to that of its fall. 



Fhil. Mag. S. 3. Vol. 20. No. 1 33. Jime 1 842. 2 I 



