92 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Effects of heat and ^^ ^^^ ^^ ^^^ cqiiator, it is whcre these two winds 

 ^^^'^'- meet (§ 246). LieutenantWarley lias collated from 



the abstract logs the observations on the temperature of the air 

 made by 100 vessels, indiscriminately taken, during their passage 

 across the trade- wind and equatorial calm^ belts of the Atlantic. 

 The observations were noted at each edge of the calm belt, in the 

 middle of it, and 5° from each edge in the trade-winds, with the 

 following averages : In the northeast trades, 6° north of the north 

 edge of the equatorial calm belt, say in latitude 14° N., air 78°. 69. 

 I^orth edge calm belt, say 9° N., air 80°.90. Middle of calm belt, 

 say 4i° ]Sr., air 82°. South edge, say 0°, air 82°.80 ; and 5° S. (in 

 southeast trades), air 81°.14. These thermometers had not all been 

 compared with standards, but their differences are probably correct, 

 notwithstanding the means themselves may not be so. Hence 

 we infer the south edge of the calm belt is 1°.4 warmer than the 

 north. The extreme difference between the annual isotherms 

 that lie between the parallels of 80° N. and 80° S. — between which 

 the trade-wind belts are included — does not probably exceed 12°. 

 According to the experiments of Gay-Lussac and Dalton, the di- 

 latation of atmospheric air due a change of 12° in temperature is 

 2J- per cent. ; that is, a column of atmosphere 100 feet high will, 

 after its temperature has been raised 12°, be 102 J- feet high. How- 

 ever, only about one third of the direct heat of the sun is ab- 

 sorbed in its passage down through the atmosphere. The other 

 two thirds are employed in lifting vapor up from the sea, or in 

 warming the crust of the earth, thence to be radiated off again, or 

 to raise the temperature of sea and air by conduction. The air 

 at the surface of the earth receives most heat directly from the 

 sun ; as you ascend, it receives less and less, and the consequent 

 temperature becomes more and more uniform ; so that the height 

 within the tropics to which the direct rays of the sun is not, as 

 reason suggests, and as the snow-lines of Chimborazo and other 

 mountains show, very great or very variable. 



249. Moreover, daily observations show most conclusively that 

 Hurricanes not due the stroug wiuds and the great winds, the hurri- 



to direct heat of the r" ^ , • n jT t j. 



sun. canes and tornadoes, do not arise irom the direct 



heat of the sun, for they do not come in the hottest weather or in 

 the clearest skies. On the contrary, winter is the stormy period 

 in the extra-tropical regions of the north ; "* and in the south, rains 



* Gales of the Atlantic, Observatory, Washington, 1856. 



