Page:Physical Geography of the Sea and its Meteorology.djvu/377

Rh quantity of heat daily impressed by the sun upon the tmosphere at sea between the equator and 9° south. I say it takes about a day, and so infer from these data, viz.: The mean annual direction of the south-east trade-winds between 10° south and the line is south 40° east. We suppose their average velocity to be (§ 313) about 25 miles an hour. At this rate it would take them 29h. 22m. 30s. to reach the equator. During this time they receive more heat than they radiate, and the excess is just sufficient to raise them from the normal temperature of the north-east trades as they enter the calm belt in 9° north. A series of observations on the temperature of the air in latitude 9° south at sea would, for the farther study of this subject, possess great value,

651. Equatorial calm belt never stationary.—If these views be correct, we should expect to find the equatorial calm belt changing Its position with night and day, and yielding to all those influences, whether secular, annual, diurnal, or accidental which are capable of producing changes in the thermal condition of the trade-winds. The great sun-swing of this calm belt from north to south is annual in its occurrence; it marks the seasons and divides the year (§ 296) into wet and dry for all those places that are within the arc of its majestic sweep. But there are other subordinate and minor influences which are continually taking place in the atmosphere, and which are also calculated to alter the place of this calm belt, and to produce changes in the thermal status of the air which the trade-winds move. These are unusually severe winters or hot summers, remarkable spells of weather, such as long continuous rains or droughts over areas of considerable extent, either within or near the trade-wind belts. It is tremblingly alive to all such influences, and they keep it in continual agitation; accordingly we find that such is its state that within certain boundaries it is continually chanoing-place and limits. This fact is abundantly proved by in speed of ships, for the log-books at the Observatory show that it is by no means a rare occurrence for one vessel, after she may have been dallying in the Doldrums for days in the vain effort to cross that calm belt to see another coming up to her, "hand over fist," with fair