314 PROCEEDINGS OF THE AMERICAN ACADEMY. 



with a gradual settling down of the air in the interior to supply the out- 

 ward current beneath. This vertical circulation, as in the case of the 

 ordinary cyclone, gives rise to a cyclonic motion in the interior and an 

 anticyclonic in the exterior part of the air under consideration, but in 

 this case the gyratory velocity is greatest above and is less at lower alti- 

 tudes, diminishing down to the earth's surface, where it is least. In the 

 anticyclonic part the reverse takes place, the gyratory velocity being 

 least above and greatest down near the earth's surface. The distance 

 from the centre at which the gyratory velocity vanishes and changes 

 sign, is greatest above and gradually becomes less, with decrease of alti- 

 tude down to the earth's surface, where it is nearest the centre. . . . The 

 conditions of a cyclone with a cold centre which are the most nearly 

 perfect are those furnished by each hemisphere of the globe, as divided 

 by the equator, in which the pole is the cold centre, and the temperature 

 gradient from the pole toward the equator is somewhat symmetrical in 

 all directions from the centre. . . . The easterly motions in the higher 

 latitudes and the westerly ones in the lower latitudes, in the one case, 

 correspond to the cyclonic in the interior and the anticyclonic in the 

 exterior part, and the belt of high pressure near the tropics to that of 

 high pressure in the case of any cyclone with a cold centre. . . . The 

 centre of a cyclone with a cold centre may or may not have a minimum 

 pressure, according to circumstances. A certain amount of temperature 

 gradient, and of pressure gradient which is independent of the gyratory 

 motion, as explained in the case of the general circulation of the atmos- 

 phere, is necessary to overcome the friction in the lower strata and to 

 keep up the vertical circulation, upon which the cyclone depends ; and 

 the pressure gradient, which depends upon the temperature gradient and 

 is independent of the gyrations, may be such that the increase of pressure 

 in the central part due to this cause may be greater than the decrease of 

 pressure arising from the cyclonic gyrations, especially where surface 

 friction is great." * 



The eclipse cyclone is of especial interest from a theoretical point of 

 view, because its origin, clearly connected with the fall of air tempera- 

 ture attending the eclipse, is Ut^'d from all questions of condensation 

 of vapor or of the dynamic effects due to the meeting of air currents 

 whose possible influence complicates the question as to the origin of the 



ordinary cyclone. The eclipse may he Compared to an experiment by 



Nature in which all the causes thai complicate the origin of the ordinary 

 • A Popular Treatise on the Winds, pp. 387-389. 



