350 THE PHYSICAL GEOGRAPHY OF THE SEA. 



they radiate, and the excess is just sufficient to raise them from 

 the normal temperature of the northeast trades as they enter the 

 calm belt in 9° north. A series of observations on the tempera- 

 ture of the air in latitude 9° south at sea would, for the farther 

 study of this subject, possess great value.* 



651. If these views be correct, we should expect to find the 

 Equatorial calm teit cquatorial Calm bclt changing its position with night 

 never stationary. ^^^ ^r^j^ c^^^ yielding to all thosc iufluences, wheth- 

 er secular, annual, diurnal, or accidental, which are capable of pro- 

 ducing 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 in- 

 fluences 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 changing place and limits. This fact is abundantly 

 proved by the 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 with 

 fair winds, and crossing the belt after a delay in it of only a few 

 hours instead of days. 



652. Hence we infer that the position of the equatorial calm 

 It varies with the belt is determined by the difference of strength be- 



strength of the trade- , . ■. , i • t 



winds. tween tlie northeast and southeast trade-wmds, 



which difference, in turn, depends upon difference of barometric 

 pressure (§642), and difference in temperature between them in 

 corresponding latitudes north and south. In it the air which' they 



* The mean temperature of sea water in the Atlantic is, for 9° north, 80°. 2G, 565 

 oVis. ; equator, 79°.G3, 269 obs.; and 9° south, 78°.96, 223 ohs.—Manrfs Thermal 



