TRANSACTIONS OF THE SECTIONS. 2fi7 



equator, would, if it came from polewards, proceed westward of the centre of rare- 

 faction, and if it came from the equatorial side, would proceed to eastward of it, and 

 that the two currents thus meeting would form a mechanical " couple " of forces, 

 and generate a rotation which, it will be easily seen, must be invariably from left to 

 right in the southern hemisphere, and from right to left in the northern, as is the 

 fact. 



He corroborated this view by the consideration that, according to it, cyclones 

 should not be formed near the equator, since the " couple " of forces would there be- 

 come nearly opposite — a view exactly in accordance with observed facts ; whereas, 

 on Mr. Redfield's hypothesis, they ought to be oftener found there, since at the 

 equator the smface and upper cuiTents of the atmosphere become interchanged, and 

 so would have a tendency to produce horizontal rotation, as assumed by him. 



• The author suggested that rarefaction might be produced by the latent heat 

 evolved by a copious precipitation of aqueous vapour in any particular region, as- 

 senting to Col. Keid's view that cyclones are not caused by' islands. 

 . With regard to the continued ascent of air in a cyclone, the author criticised Mr. 

 Redfield's views as expressed in the following extract from ' Silliman's Journal,' 

 vol. xxxiii. p. 59, ascribed in the Index to that writer, viz. : — " The condition of force 

 by which the propulsion is maintained is found in the pressure of the surrounding 

 atmosphere iipon all sides of the whirling and therefore mechanically rarefied co- 

 liunn; and if the expansive whuiing motion be sufficiently active to produce 

 nearly a vacuum at the centre, the external propelling force will be nearly 15 lbs. 

 to the square inch." The author observed that the pressure, as registered by the 

 barometer, never exceeded about 1 lb. to the square inch ; and contended that if 

 even this pressure were to operate as a motive force at all, it would cause, not con- 

 tinuance of the rotation, but total coUapse of the cyclone from the bottom upwards, 

 — a conclusion which he illustrated by the disappearance of a water-spout in this 

 manner, and also by an experiment with a bottle containing some water in a state 

 of rapid rotation, in which a similar result was shown to follow from the pressure 

 of the column of water as the centeifugal force declined. He considered that the 

 external pressiu-e of the atmosphere was the effect, and not the cause, of the continu- 

 ance of the cyclone, and suggested as a cause of this continuance the passage of a 

 current of air over the top of the cyclone, in the upper regions of the atmosphere, 

 analogous to the draught similarly caused in a chimney ; since Maiu-y has shown 

 that such a cm-rent actually exists in the upper atmosphere, blowing in a direction 

 the reverse of the trade-wind, the rotating coliunn of air composing the cyclone 

 being capable of being regarded as a chimney. The author had observed that such 

 a current, passing over the top of the fimnel of a steamer, caused the smoke to rotate 

 in two spu-al colimms, foldmg into each other below on the leeward side of the ' 

 funnel ; so that if a person stood facing the funnel with his back to the wind, the 

 left-hand column would coiTespond, ruside the fimnel, with the cyclones of the 

 southern hemisphere, and the right-hand column with those of the northern. Since 

 such an upper cm-rent, as shown by Maury, wovdd descend and become a sm-face- 

 ciirrent at the tropics, we might expect cyclones often to die out here; and the author 

 refen-ed to the opinion of Lieut. Fyers, E,.E., Secretary to the Meteorological So- 

 ciety_ of Mauritius, that such was sometimes the fact, as in the case of the Mamitius 

 hurricane of November 1854, as evidenced by the log of the ship ' Sesostris ' (see 

 ' Transactions of the Meteorological Society of Mam-itius,' vol. iii. p. 18). Mr. Ashe 

 also exhibited a chart of storms traced in the South Indian Ocean by the above- 

 mentioned Society, in which, out of twelve storms, only one was traceable as far 

 as 29° S. lat., while ten could not be traced beyond 25° S. 



_ The shape of a cyclone the author considered to be, not really circular, but ellip- 

 tical, the major axis rimning from west to east, and the vortex being on the east 

 side of the minor axis, since this figure would result from the fact that a particle 

 of air proceeding towards the vortex from polewards would go more to the west- 

 ward than a particle coming from the same distance on the side of the equator 

 would go to eastwards of the vortex, owing to the increasing ratio with which the 

 circumferences diminish in the circles formed by the parallels of latitudes as the 

 distance of these from the equator increases ; for the radii of these circles ai-e equal 

 to the cosines of angles increasing in arithmetical progi-ession from 0° to 90°. 

 Hence Mr. Ashe deduced that the westernmost portion of the cyclone would be the 



