conveyed and distributed by each cubic foot of water is 1,158,000 foot-pounds, and by the whole volume of Gulf-Stream water there is transferred every day from the equatorial regions 77,479,650,000,000,000,000 foot-pounds of heat. But these figures convey no definite impression of the vastness of the results. It is equal to one-fourth of all the heat received from the sun by the whole Atlantic Ocean from the Tropic of Cancer to the Arctic Circle, raising the temperature of its waters one-fifth—it is equal to about one-half the solar heat which falls upon the entire arctic regions. Under the equator there comes from the sun on each square foot of surface about 83 foot-pounds of heat every second, when the sun is in the zenith. Twenty-two per cent, of this is absorbed by the atmosphere; the remainder falls as heating-power upon the ocean. Now, the quantity conveyed is equal to all that thus falls upon 1,560,935 square miles of surface. If this source of heat be destroyed or turned away, the area of arctic winter would rapidly extend southward, covering England, both Old and New, with ice which the heat of summer would not remove.
Nor would any conceivable movements of the atmosphere supply the deficiency of heat. For not only does heated air at the equator rise, and radiate its heat in the cold regions of the upper atmosphere, but the capacity of air for heat is much less than water. So great is the difference in this respect that the Gulf Stream conveys as much heat as a current of air would if 3,234 times as large, of the same temperature, and moving with the same velocity. The heated air, however, which rises at the equator is charged with vapor, and in the opinion of Sir John Herschel this vapor, conveyed by the upper or anti-trades, is condensed in the temperate and arctic regions, greatly modifying their temperature. To the extent that the vapors are thus conveyed, the conclusion of Sir John is correct; for, as Prof. Tyndall has shown, in the conversion of one pound of aqueous vapor into water there is given out as much heat as is sufficient to melt five pounds of cast-iron. But Mr. Croll proves that the greater part of the vapor raised in the equatorial regions falls there as rain; that the upper winds are dry, and gather moisture only when they again reach the earth and become surface-winds in the temperate and arctic zones.
If all currents of the ocean and atmosphere should cease, no heat could be transferred from the equatorial to the arctic regions, and temperatures would depend on the solar heat falling in the respective latitudes. The equatorial and arctic regions would become uninhabitable, the mean temperatures rising to 135° Fahr. in the one, and sinking to 83° below zero in the other. The present difference of 80° would be increased to 218°, and only a narrow zone of temperate climate would prevail.
- A foot pound is the amount of heat-force expended in raising one pound one foot; 772 foot-pounds is the equivalent of the heat that will raise one pound of water one degree of temperature.