OTH MIXED GASES. |5 



convinced of the superior expansion of moist air, will readily Examination of 

 apply the principle to certain interesting phenomena, in par- Mr. Dalton's 

 licular to the origin of Tornadoes m liot countries, and the ej^g^e°. "*^* 

 variation of the barometer in temperate climates, 



Mr. Barrow, an intelligent traveller in South Africa, ob- 

 serves, that the atmosphere in Caffraria is soraetirnes heated 

 to 102 or 104 degrees: this is succeeded by local thunder 

 storm, attended -with heavy falls of rain and hail, as well as 

 violent hurricanes. I do not pretend to assign the refrigerating 

 cause, ur tlie agent that produces precipitation in thi? case ; 

 I only have to observe, that the portion of air must lose much 

 of its elasticity, vphich is suddenly cooled to 70 or 72 de- 

 jgrees, and at the same time parts with the water it held in 

 solution. This partial diminution of spring will destroy the 

 equilibriumof the adjacent parts of the atmosphere, and may- 

 be supposed to produce the tornadoes of the tropical regions. 

 The same cause probably gives rise to the fluctuations of the 

 barometer in milder climates ; for though the changes of tem- 

 perature are less in the milder than in the hottest parts of the 

 globe, the agents that precipitate the water of the atmosphere, 

 appear to act on a more extensive scale, and through a longer 

 duration in the former situations than they do in the latter. 

 Wet weather is neither momentary nor local in Europe ; pro- 

 vinces, and even kingdoms are deluged with rain for weeks 

 together. The air, which discharges such an abundance of 

 water, will lose part of its spring, according to Mr. Schmidt^a 

 experiments, even when it siiifers no change of temperature; 

 mow it is evident that the equilibrium cannot be restored in an 

 instant'; because the diminished elasticity must be augmented 

 i-n this case by currents of air coming from remote places. The 

 diminution of spring in the atmosphere is shewn by the fall of 

 the barometer ; and the subsequent ascent of the mercury indi- 

 cates the arrival of the restorative currents. According to this 

 explanation, the barometer will rise slowly but gradually in 

 the centre of the rainy district, while the motions ©fit will be 

 more rapid and less regular towards the verge of the storm. 

 High winds will also prevail in wet seasons, whieh will blow 

 towards the parts where the elastic force of the air is least; 

 that is, where the rains are most abundant. — I know not what 

 <daiva to originality is due to the foregoing hints towards th*; 



