1883. 3 Trans. N. V. Ac. ScL 



A paper was then presented by Prof. William P. Trowbridge^ 

 illustrated by diagrams, on 



tornadoes. 



discussion. 



Dr. A. A. JULIEN recalled the careful observations and hypotheses 

 of Redfield and Espy, according to which the initiation of a tornado 

 depended upon the ascent of a rarified column of heated air. The 

 subsequent conflict of opposing currents, and the continuance and de- 

 velopment of the central area of diminished barometrical pressure, 

 seemed to be, however, but the secondary phases of the phenomenon. 

 In illustration he described a similar class of facts observed in the 

 birthplace of the hurricanes of the tropics — the great marine plain of the 

 Caribbean Sea — duringthe " hurricane season," from July to September 

 of every year. There, as over the Western plains of our continent, 

 the atmosphere becomes strongly heated by the fervid rays of the sun, 

 quivering with ascending currents over the broad sheets of naked and 

 white coral limestone whicli forms the plateaus or rims of many isl- 

 ands. Over the sea, heaped up and swelling cloud masses of cumuli 

 mark here and there the ascent of a strong aerial current, and at times 

 numerous waterspouts project toward or down to the sea in all stages 

 of formation. Of these he had observed one or two dozen, visible at 

 the same time, while residing on the Key of Sombrero, and also while 

 voyaging through the Mona Passage. On one occasion a waterspout 

 approached the precipitous eastern cliff of Sombrero, within a distance 

 of five hundred feet, before it suddenly broke and disappeared. 



Even on the land the same ascensional tendency is frequently shown 

 by the passage of whirling dust columns, catching up sand and light ob- 

 jects in a funnel-shaped, whirling shell, sometimes fifty to a hundred feet 

 in height. So gentle is the breeze in which these whirling columns are 

 developed that they move quite slowly along the level surface of the 

 ground. A person can manage to walk at an ordinary pace within the 

 centre of the whirling current, and can distinctly exhibit its tangential 

 velocity and the form of the whirling shell by suddenly throwing into 

 it a quantity of fragments of paper or similar light material. All these 

 phenomena are produced in a still atmosphere, or among very gentle 

 breezes from easterly points ; but it is important to note that the ris- 

 ing of the wind, especially from opposite points, occurs rarely at such 

 a season, and seems to indicate the second stage of atmospheric dis- 

 turbance — the beginning of a hurricane or cyclone. 



Mr. H. L. Warner inquired as to the cause and mode of origin of 

 the extreme ascensional velocity at the centre of the tornado, and of 

 its enormous lifting power and circular motion, since he could not un- 



