418 TP!E PHYSICAL GEOGRAPHY OF THE SEA. 



nal rotation, a greater velocity than its polar edge. There seems, 

 however, to be less difficulty with regard to their turning than 

 with regard to their course; the former is now regarded as the 

 resultant of diurnal rotation and of those forces of translation 

 which propel the winds along the surface of our planet. This 

 composition of the forces of the revolving storm, and this resolu- 

 tion of them, are precisely such (§ 215) as to produce opposite ro- 

 tation on opposite sides of the equator. 



787. Many of the phenomena connected with these storms still 

 Espy's theory, remain to be explained ; even the facts with regard 

 to them are 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 

 toward 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 con- 

 vex side toward 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 hav- 

 ing 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, Newfoundland ; 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 surrounding regions, it is specifically lighter, and will as- 

 cend; in ascending, it comes under less pressure, and expands; 

 in expanding from diminished pressure, it grows colder about a 

 degree and a quarter for every hundred yards of ascent ; in cool- 

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

 many hundred 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 vapor into cloud ; in condensing its vapor into wa- 

 ter or cloud, it will evolve its latent caloric ; this evolution of la- 

 tent 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 



