292 Professor Mitchell 07i the Proximate Causes 



which, according to these views, would produce a condensation 

 of the moisture of the atmosphere to happen occasionally, it 

 could not, hke the fall of rain or snow, be an every-day occur- 

 rence. But if the air has commonly, in storms, a vertiginous 

 motion, the difficulty vanishes at once. The warm strata at 

 the surface will be carried upwards, and the cold strata brought 

 down from above, and as perfect a mixture of air, of very diffe- 

 rent temperatures produced, as any theory can demand. 



Franklin draws his illustration of the movement of the air, 

 during our north-east storms, from that of the water in a canal, 

 when the gate by which it had previously been confined is 

 raised ; and, with his views, those of Dr Hare appear nearly to 

 coincide. Dr Hare appears to regard the warm moist air that 

 rises from the surface of the Gulf of Mexico, as the repository 

 from which the rain and snow are derived, the precipitation be- 

 ing caused partly by a diminution of capacity, undergone by it 

 in consequence of ils rarefaction as it ascends, and partly by its 

 admixture with the under current of cold air that comes in from 

 the north-east, whilst it blows in from the south-west. The ac- 

 curacy of these views may be questioned on a number of diffe- 

 rent grounds. 



1 . The precipitation arising from a change of capacity produced 

 by rarefaction, must be confined to the immediate neighbour- 

 hood of the gulf, where the ascent and rarefaction take place. 

 The rain and snow descending upon the middle and northern 

 States, must therefore be ascribed simply to the lower surface or 

 stratum of the upper current of warm air flowing towards the 

 north-east, and the upper stratum of the current of cold air 

 coming from that quarter. 2. The objection just stated to the 

 doctrines of Mr Leslie, as advanced in his illustrations of the 

 Huttonian theory of rain, applies with great force here. The 

 source of refrigeration is altogether inadequate to the produc- 

 tion of the effect ascribed to it. Dr Hare remarks, that by 

 every fall of snow, twice as much caloric is liberated as would 

 be yielded by an equal weight of red hot powdered glass. But 

 not only is the amount of rain or snow falling during a north- 

 east storm very great, but the weather itself often becomes in- 

 tensely cold. Let it now be supposed, that the north current 

 of air continues to move at the rate of thirty miles an hour, and 

 the upper south-west current at the same in the opposite di- 



