314 ON THE STORM OF THE 18TH OF APRIL, 1850. 



of notice also, that in the northern hemisphere this is the invariable 

 direction of the cyclones, or great revolving storms, to which the 

 attention of meteorologists has been directed by Colonel Eeid and 

 Mr. Eedfleld. The late storm was, however, different from a 

 cyclone, both in the dimensions of the vortex and in the causes 

 from which it originated. The horizontal section of the cyclone 

 where it meets the earth is often 500 miles in diameter ; and the 

 vortex is supposed to be the effect of two crossing currents of air, 

 which generate a movement of rotation. In the tornado (to which 

 species the late storm belonged) the vortex is of much smaller 

 dimensions, and is produced by rapidly ascending currents of air, 

 caused by the heating of a limited portion of the earth's surface 

 under the action of the sun's rays. In the temperate zones, 

 accordingly, it is never produced in winter. 



The evidence relating to the direction of the gale, and its 

 changes, as it passed over the College Park, is very complete and 

 satisfactory. In the park, and garden adjoining, nineteen trees 

 were rooted up and prostrated, eleven of them being trees of large 

 size. Of these ten have fallen from the south-east, or under the 

 action of the first half of the gale, and nine from the north-west. 

 Their bearings have been accurately taken ; and the general result 

 is, that the mean direction of the south-east gale, as indicated by 

 that of the trees, is S. 56 E., and that of the north-west gale 

 N. 53 W. I believe that these results are even more accurate than 

 those furnished by the anemometer ; and they prove that in this 

 locality the direction of the wind was exactly reversed, and, there- 

 fore, that the centre of the vortex passed over the College. 



A remarkable circumstance connected with the direction of the 

 fallen trees is their great uniformity, the individual directions 

 seldom differing more than 10 from the mean. This is an indi- 

 rect evidence of the great violence of the gale; and it proves, 

 moreover, that the transition from the south-east to the north-west 

 wind was immediate. There is greater regularity in the direction 

 of the trees fallen from the north-west than in those which have 

 been blown down from the opposite quarter. This may have 

 arisen partly from the greater violence of the gale in the former 

 direction ; but it is partly also due to the circumstance that the 

 trees which fell from the north-west are generally larger, and in a 

 less inclosed portion of the ground. It may be mentioned also, 

 that the trees which fell from the north-west generally lie to the 



