Phenomena and Causes of Hall Storms. 249 



thousand feet, at the latitude of 30" twelve thousand feet, and 

 at the latitude of 50° six thousand * ; that beyond this line of 

 perpetual congelation, the reduction of temperature still proceeds 

 until it shortly reaches a degree of cold the most intense that 

 can be imagined. If we now contemplate a current of air, that 

 is, a wind blowing horizontally first at the surface of the earth 

 and afterwards at different elevations, we shall find that it will 

 be subject to the following modifications. We will suppose it to 

 blow first from the polar towards the equatorial regions. When 

 it moves at the surface of the earth, it will rapidly imbibe the 

 heat of the eartli as it traverses the warmer latitudes ; at the 

 height of one thousand feet it will feel the influence of the earth 

 much less, and grow warm much slower than before ; and at 

 the height of ten thousand feet, it will, for the most part, sweep 

 quite clear of the mountains, and be a current of air blowing 

 through the atmosphere alone. And since, as in the case of the 

 Gulf Stream, a fluid does not readily change its temperature 

 merely by flowing through a body of the same fluid of a diffe- 

 rent temperature, and especially air by flowing through air, a 

 wind blowing from north to south at an elevation of ten thou- 

 sand feet above the earth, will pass to a great distance without 

 materially altering its temperature. What we have here sup- 

 posed respecting the heating of a northerly wind as it blows 

 southerly, will obviously apply to the cooling of a southerly 

 wind as it blows northerly ; and since a high wind frequently 

 moves at the rate of sixty miles or about one degree an hour, 

 especially where it passes without obstruction in the upper re- 

 gions of the atmosphere, it would consequently pass over ten 

 degrees in the short space of ten hours -|-. 



These things being clearly understood, we assign as the cause 

 of hail-storms, the congelation of the watery vapour of a 



BODY of warm and HUMID AIR, BY ITS SUDDENLY MIXING 

 WITH AN EXCEEDINGLY COLD WIND, IN THE HIGHER REGIONS 



OK THE ATMOSPHERE. Let US examine the effects which would 

 result from the meeting of two opposite winds, at the height of 

 ten thousand feet, during the heat of summer, the one blowing 



• Ed. Encyc. ' Phys. Geography.' See figure, page 9. 

 f Daniel's Meteor. Ess. 113. 

 JULY — SEFTEMBEU 1830. R 



