6 Phenomena and Causes of Hail Storms. 



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 horizon- 

 tally first at the surface of the earth and afterwards at diiFerent ele- 

 vations, we shall find that it will be subject to the following modifica- 

 tions. We will suppose it to blow fi.rst from the polar towards the 

 equatorial regions. When it moves at the surface of the earth, it 

 will rapidly imbibe the heat of the earth 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 different temperature, and es- 

 pecially air by flowing through air, a wind blowing from north to south 

 at an elevation of ten thousand feet above the earth, will pass to a 

 great distance without materially altering its temperature. What we 

 have here supposed 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 5 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 regions of the atmos- 

 phere, 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 avatery vapor of a body of 



WARM AND HUMID AIR, BY ITS SUDDENLY MIXING WITH AN EXCEED- 

 INGLY COLD WIND, IN THE HIGHER REGIONS OF 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 from the latitude of 30° or from the con- 

 fines of the ton-id zone, and the other from the latitude of 50° or the 

 northern part of British America. If they had equal velocities, they 

 would meet at the parallel of 40°, that is, at our own latitude, in 

 ten hours from the time of setting out ; and according to what has 

 been premised, each current would retain nearly the original tem- 

 perature. The southerly wind blowing from a point which is still 

 two thousand feet below the line of perpetual congelation, is com- 



* Daniel's Meteor. Ess. 113. 



