B. TERRESTRIAL PHYSICS AND METEOROLOGY. 101 



formly contrary to the movement of the hands of the watch. 

 It is to the centrifugal force due to this gyratory movement, 

 by reason of which the air may be considered as revolving 

 not only about the centre of the hurricane, but also about the 

 earth's polar axis, that Professor Ferrel attributes the greater 

 part of the depression of the barometer which is observed at 

 the centre of the storm. The amount of this depression can 

 be quite exactly calculated by the formula given by him. 

 Not only does the air revolve about, but it also preserves its 

 tendency to move inward toward the centre of lowest press- 

 ure near which it rises, and then the upper region of the 

 atmosphere continues its gyratory movement, but flows out- 

 ward, gradually descending until it meets the earth again. 

 Professor Ferrel has been successful in explaining satisfac- 

 torily, by one single formula, and in grouping together under 

 one common point of view, the phenomena of tornadoes and 

 waterspouts and extensive hurricanes. His formula, after 

 allowing for the density of water, applies also to the move- 

 ments of the ocean; and by means of it he is able to compute 

 the gradient of sea-level due to the deflecting force of the 

 earth's rotation, showing that, for instance, we obtain about 

 six tenths of a foot as the depression in sea-level due to a 

 change in position of a hundred miles on a parallel of 30, 

 the sea-level being highest in the middle of the gyration, 

 that is to say, in the central portions of the Atlantic and 

 Pacific Oceans. 4i>, November, 1874. 



THE MECHANICAL THEORY OF CYCLONES. 



Among the treatises on the mechanical theory of the 

 movements of air in revolving storms, both hurricanes and 

 tornadoes, but few have such practical value as that of 

 Colding, published in Danish in 1871, translations of which 

 have recently been prepared by Hann, of Vienna, and Abbe, 

 of Washington, by means of which the English-reading pub- 

 lic is introduced to a series of novel investigations into the 

 flow of liquids, which forms a very acceptable addition to 

 our knowledge of the subject. According to Colding, the 

 laws governing the movement of water may, with certain re- 

 strictions, be applied to the movement of the air ; and after 

 having, as an engineer, for many years successfully studied 

 the currents of rivers and oceans, Mr. Colding has applied his 



