ASTRONOMY AND METEOROLOGY. 401 



On our own globe the disturbing causes are so numerous that the winds 

 cannot manifest any degree of regularity, except in the extensive and vig- 

 orous circulation which prevails between the tropics. The unequal suscepti- 

 bility of land and water to the solar heat, causes the alternating breezes 

 which are felt on many sea coasts, and the high temperature which the sun 

 imparts to deserts, serves also to bring our aerial ocean into a state of activ- 

 ity. But excessive showers of rain are attended with a still more effective 

 cause of atmospheric commotions, according to a principle first pointed out 

 by Professor Espy. The heat arising from the extensive condensation of 

 vapor on these occasions, must rarefy the air so much as to give it an upward 

 movement, and cause aerial currents to flow into the place where the greatest 

 fall of rain occurs. Although this seems to be the chief source of the motive 

 power which produces storms, yet there is reason to believe that the con- 

 stant discharge of electricity along the moist air contributes, through the 

 medium of electrical repulsion, to increase the rapidity of the aerial move- 

 ment. The constant repulsion of the air from the discharge of its electricity, 

 Dr. Hare regards as the chief cause of hurricanes. 



In an article by Mr. Tracy, of Utica, published fifteen years ago, in Silli- 

 man's Journal, it has been shown that atmospheric movements of this kind, 

 whether due to the action of heat or electricity, would acquire a rotation 

 from the right hand to the left in this hemisphere ; in consequence of the 

 eastward deflection of the air rushing from the south, while that from the 

 north is deflected to the west. But it appears, from the foregoing investiga- 

 tions, that the eastern and western aerial currents tend to maintain the same 

 rotation, the former being deflected south on account of their excessive cen- 

 trifugal force, while the latter from an opposite cause incline to the north. 

 It may also be shown that, in the southern hemisphere, the rotation must 

 be in an opposite direction. Dove's celebrated law of the rotation of 

 storms is thus shown to be the necessary result of physical causes. 



The orbital movement of storms appears to be an inevitable consequence 

 of their rotary motion. As the air on the east side of the aerial vortex is 

 advancing towards the pole, it must cool by the change of latitude; while 

 on the west side the air is moving south, and has its capacity for holding 

 vapor continually augmented by an increasing temperature. Accordingly, 

 the focus of the rain and the storm would be constantly shifted to the north- 

 east if the atmosphere had been previously in a state of repose ; but be- 

 tween the tropics, the action of the trade winds must supersede the west- 

 ward motion, and the whirling mass of air will be therefore transported in a 

 north-east course in these regions. This accounts for the fact that all great 

 storms commencing within the tropics, steadily advance towards the north 

 and west, until their entrance into the temperate zone, when they soon 

 change to the north-east, and continue in this course until they reach the 

 very high latitudes. 



While the regularity of the wind is much disturbed or wholly obliterated 

 by these commotions, it is differently affected by the unevenness of the 

 land. The friction arising from this cause tends to give the air the velocity 

 of rotation of the places which it visits, and this prevents a retrograde 

 movement from an excess or a deficiency of centrifugal force. The great 

 atmospheric circulation depending on the regular action of the solar heat, 

 must accordingly occupy a wider zone than our calculations would assign to 

 it, and it is in consequence of the want of great friction from bodies of land, 



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