Atmospherical Phenomena. 249 



lines, I shall now turn to some facts which we have almost 

 every moment an opportunity of observing. Although the 

 density of the air becomes less at every step we ascend, 

 and to such an extent that, at a height of about ten miles, it is 

 less than we are able to make it under the receiver of a good 

 working air-pump ; yet, for the sake of simplicity, we may as- 

 sume that the upper limit of the atmosphere is just as distinct- 

 ly marked out as we find it 

 in liquid bodies. Let, then, 

 AB be a portion of the earth's 

 surface represented as level, 

 and let it have every-where 

 the same temperature ; and 

 farther let this uniformity ex- 

 tend to all points which are 

 at an equal distance from the ground ; then CD, the upper sur- 

 face of the atmosphere, would be parallel to AB, and we should 

 always find at an equal height the same density, the whole mass 

 would be still, and at whatever point on AB we should suspend 

 a barometer, it would always indicate the same pressure of the at- 

 mosphere. This state of equilibrium, however, is changed, when- 

 ever the temperature above one portion of the earth's surface be- 

 comes greater than that over another. Supposing that EF and 

 the air occurring above it were much more strongly heated than 

 AE and BF, then, owing to the expansion of the air thus pro- 

 duced, the boundary of the atmosphere is removed to G H ; 

 but, according to the laws of mechanics, an equilibrium can no 

 longer exist under these circumstances, the air flows in the up- 

 per strata in the direction of IK and ML from the warmer to 

 the colder region, until the surface of the atmosphere is again 

 parallel to the ground ; and, hence necessarily, the barometer 

 sinks where the heating has taken place, and rises where the 

 temperature remained unaltered. This altered pressure of the 

 atmosphere causes also a movement in the lower strata of the 

 air. For since over AE and FB, the vertical, and henc« 

 also the lateral pressure of the atmosphere is greater than over 

 EF, currents of air arise near the ground from the colder to 

 the warmer region. 



The consequence is the same when the temperature above 

 EF remains unchanged, while AE and FB are unusually 



VOL. XXVI. KO. LTI.— APRIL ISSQ- E 



