SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 351 



quantity of heat daily imi^rcssed 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, viz. : The mean annual direc- 

 tion of the south-east trade-winds between 10'' south and the line 

 is soutli 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 suffi- 

 (;ient to raise them from the normal temperature of the north-east 

 trades as they enter the calm belt in 9^ north. A series of obser- 

 vations on the temperature of the air in latitude 9° south at sea 

 would, for the farther study of this subject, possess ^reat 

 value, t 



651. Equatorial calm belt never stationary. —1^ these views be 

 correct, we should expect to find the equatorial calm belt chang- 

 ing 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 > its occurrence ; it marks the seasons and 

 divides the year (§ 296) into wet and dry for aU those places that 

 are withm the arc of its majestic sweep. But there are other 

 subordinate and minor influences which are continually takino- 

 place m the atmosphere, and wliich are also calculated to alte? 

 the place of this calm belt, and to produce changes in the theimal 

 status of the air which the trade-winds move. These are un- 

 usually 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 chano-ino- place 

 and limits. This fact is abundantly proved by ih^ 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 

 m 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 



* Maury's Nautical Monograph, No. 1. 

 by^56?ol.r7J'"'^T*'''.ao'^.o'^?^"**^^^"^*^^^ -^^^'^^*^« i« for 9° north, 80^.26 



