298 WRITINGS OF JOSEPH HENRY. [1855- 



pieces of leaves, &c., these are found in the nucleus of the 

 hail stones. 



In order that a storm of this kind may be attended with 

 hail, it is necessary that it be of considerable violence, in 

 order that the drops of water may be carried up to a sufficient 

 height, and hence, as we have said before, this phenomenon 

 occurs usually in the warmest and most sultry weather. 



The writer is enabled to give the foregoing explanation of 

 the nucleus and the alternate spongy layers of large hail 

 stones from the effects he obtained by freezing water in a 

 glass bulb. The freezing commenced at the exterior surface, 

 to which the axes of the crystals were at right angles. The 

 air contained in the water was forced in before the advancing 

 crystallization, and formed at the centre of the globule a 

 spongy mass precisely similar to that which formed the 

 nucleus of the hail stone. 



When the uprising column assumes the form of a tornado, 

 it is more circumscribed, and is we think generally accom- 

 panied by a whirling motion. The power of the current 

 however is in an upward direction. The gyration is an acci- 

 dental circumstance, while the upward motion is an essential 

 one; and the whole power of the tornado to produce mechan- 

 ical effect? *s in this direction ; hence as it passes along over 

 the surfac-j of the earth, the air flows in on every side to 

 supply the up-moving column, trees are drawn in by the 

 force of the centripetal current, and thrown with their tops 

 towards the path of the tornado. The writer had an oppor- 

 tunity, on one occasion, of examining with Professor Bache 

 the effects of a tornado after it had passed through an orchard. 

 The trees were all prostrated in a strip of about four rods in 

 width, with their tops inward toward the middle of the path. 

 The whirling tends to contract the dimensions of the column, 

 and to give it the peculiar appearance of an inverted cone 

 descending from the clouds. The air which rushes into the 

 revolving cylinder, charged with moisture, is immediately 

 expanded, consequently cooled, and its vapor condensed into 

 visible clouds, which gives rise to the peculiar appearance of 

 the descending trunk. 



