Royal Irish Academy. 397 



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. These ascending currents are loaded with va- 

 pour, which (owing to the rapid evaporation) is in a highly elec- 

 trical state, and when they reach the colder regions of the atmosphere 

 the vapour is condensed, and electrical clouds are rapidly formed. 



" 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 S.E., or under the action of the first half of the 

 gale, and nine from the N.W. Their bearings have been accurately 

 taken, and the general result is, that the main direction of the S.E. 

 gale, as indicated by that of the trees, is S. 56° E., and that of the 

 N.W. 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 therefore 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 direction seldom 

 differing more than ten degrees from the mean. This is an indirect 

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

 that the transition from the S.E. to the N.W. wind was immediate. 

 There is greater regularity in the direction of the trees fallen from 

 the N.W., 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 N.W. are 

 generally larger and in a less enclosed portion of the ground. It may 

 be mentioned also, that the trees which fell from the N.W. generally 

 lie to the southward of the others, as if there had been a shifting of 

 the whole vortex in that direction. There are, however, two large 

 trees in the garden lying side by side, but in directions diametrically 

 opposed. 



" It has been already stated, that in the College Park the shifting 

 of the wind amounted to 1 80 degrees ; and it has been inferred that 

 the centre of the vortex passed over that spot. From what has been 

 said as to the nature of the phaenomenon, it will follow that in other 

 localities, over which the vortex did not pass centrally, the wind must 

 have shifted through diiFerent points of the compass, and through 

 angles smaller in proportion to the distance from the centre. Thus, 

 on the southern or south-eastern side of the line described by the 

 centre of the vortex, the change of the wind should be from S. to W., 

 and on the northern side of the same line from E. to N. We are 

 not yet in possession of facts which bear upon this point ; but from 

 the limited dimensions of the vortex, and the consequent smallness 

 of the distance necessary to produce such a variation, it is probable 

 that evidence bearing upon it may readily be obtained. I shall only 

 observe, that in seeking and comparing such evidence care must be 



