SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 337 



time, we should have in the band of winds an actinometer which 

 would disclose to us the average quantity of heat daily imjDressed by 

 the sun upon the atmosphere at sea between the equator and 9^ 

 south. I say it takes about a day, and so infer from these data, 

 Tiz. : The mean annual direction of the south-east trade- winds 

 between 10^ south and the line is south 40^ east.* "We suppose 

 then average velocity to be (§ 343) 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 fiom 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. If these views be correct, we should expect to find the 

 Equatorial calm belt cquatorial Calm belt changing its position with night 

 never stationary. ^^^ (jg^y^ ^;^^ yielding to all thosc uifluences, whether 

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

 ducing changes in the theiTQal condition of the trade- viads. The 

 great sun-s"^^ng of this calm belt from north to south is annual in 

 its occui'rence ; it marks the seasons and divides the year (§ 296) 

 into wet and djy 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 ak which the 

 trade-winds move. These are, unusually severe wuiters or hot 

 summers, remarkable spells of weather, such as long continuous 

 rahis or droughts over areas of considerable extent, either mthin 

 or near the trade- wind belts. It is tremblingly ahve 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 sliips, 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 cahn belt, to see another coming up to her, 



* Maury's Nautical Monograph, No. 1. 



t The mean temperature of sea water in the Atlantic is for 9^ north, 80^.26 by 

 565 obs. ; for equator, 79^.03, by 269 obs. ; and for 9^ south, 78^.96. 223 obs.— 

 Maury's Thermal Chads. 



Z 



