STORMS, HUllRICANES, AND TYPHOONS. 417 



Avliicli propel the winds along the sniface of our planet. This 

 composition of the forces of the revolving storm, and the resolution 

 of them, are pre cii^el}" such (§ 215) as to produce opposite rotation 

 on opposite sides of the equator. 



787. Esiiys theory. — Man}- of the phenomena connected with 

 these storms still remain to be explained ; even the facts with 

 regard to them arc disputed by some. The late Professor Espy, 

 after having discussed for many years numerous observations 

 that have been made chiefly on shore, maintained that the wind 

 does not blow around the vortex or place of low barometer, but 

 directly towards it. He held that the place of low barometer, 

 instead of being a disc, is generally an oblong, in the shape of a 

 long trough, between two atmospherical waves ; that it is curved 

 with its convex side towards the east ; that it is sometimes nearly 

 straight, and generally of great length from north to south, 

 reaching in America, from the Gulf of Mexico to the great lakes 

 and beyond, and having but little breadth in proportion to its 

 length ; that it travels east, moving side foremost, requiring 

 about two days to go from the Mississippi to St. John's, Kew- 

 foundland ; that on either side of it, but many miles distant, 

 there is a ridge of high barometer ; that the wind on either side 

 of the line of low barometer, in which there is little or no wind, 

 blows toward it, etc., and, in support of these positions, he 

 advanced this theory : "When the air in any locality acquires a 

 higher temperature or a higher dew-point than that of the sur- 

 rounding regions, it is specifically lighter, and will ascend ; in 

 ascending, it comes under less pressure, and expands ; in ex- 

 panding from diminished pressure, it grows colder about a degree 

 and a quarter for every hundred yards of ascent ; in cooling as 

 low as the dew-point (which it will do when it rises as many hun- 

 dred yards as the dew-point at the time is below the temperature 

 of the air in degrees of Fahrenheit), it will begin to condense its 

 vapour into cloud; in condensing its vapour into water or cloud, 

 it will evolve its latent caloric ; this evolution of latent caloric 

 will prevent the air from cooling so fast in its farther ascent as it 

 did in ascending below the base of the cloud now forming ; the 

 current of the air, however, will continue to ascend, and grow 

 colder about half as much as it would do if it had no va2')0ur in it 

 to condense ; and when it has risen high enough to have con- 

 densed, b}" the cold of expansion from diminished pressure, one 

 hundredth of its weight of vapour, it will be about forty-eiglit 



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