which propel the winds along the surface of our planet. This composition of the forces of the revolving storm, and the resolution of them, are precisely such (§ 215) as to produce opposite rotation on opposite sides of the equator.
787. Espys theory.—Many of the phenomena connected with these storms still 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 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, 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 the surrounding regions, it is specifically lighter, and will ascend; 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 cooling 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 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 vapour in it to condense; and when it has risen high enough to have condensed, by the cold of expansion from diminished pressure, one hundredth of its weight of vapour, it will be about forty-eight
