METEOROLOGY. 335 



upon the exterior of the earth's atmosphere, like the fall of rain or 

 enow, is a primary and distinct phenomenon. While passing through 

 the atmosphere to the earth the solar rays are subject to refraction, 

 absorption, polarization and radiation ; also to the effects of evapora- 

 tion of winds, clouds, and storms. Thus the heat which finally 

 elevates the mercurial colunm of the thermometer is the resultant of 

 a variety of causes, a single thread in the network of solar and ter- 

 restrial phenomena. 



Indication of tropical calms. — Should the inquiry be made, in what 

 part of the earth the sun's intensity continues most uniform for the 

 longest period, an inspection of the flexures of the curves at once indi- 

 cates the region intermediate between the equator and the tropic of 

 Cancer on the one side, and of Capricorn on the oth^r. Thus the 

 curve for latitude 10° shows the solar intensity to be nearly stationary 

 during half the year, from March to September. During October and 

 November it falls rapidly, and after remaining nearly unchanged for 

 a few days in December it again rises rapidly in January and Feb- 

 ruary. As the sun's heat is the prime cause of winds, we might infer 

 that this region would be comparatively calm during the half year 

 mentioned, and that in the remaining months there would be greater 

 atmospheric fluctuations. 



Such were the general indications of the plate representing the 

 amounts ; and, on recurring to the table representing the rates of 

 diurnal intensity, the status is precisely similar, except that the region 

 of summer calm is removed further from the equator and n^^rer to 

 the tropic. On referring to a recent work on the physical geography 

 of the sea, with respect to this circumstance, I find that "the varia- 

 bles," or calms of Cancer and of Capricorn, occur in the very latitudes 

 thus indicated by the compound effect of the amount and rate of solar 

 intensity. And, further, the annual range of solar intensity, which is 

 least upon the equator, has its counterpart in the belt of equatorial 

 calms, or "doldrums." The same effect extends also to the ocean 

 itself, and appears in the tranquillity of the Sargosso sea. While 

 the curves of intensity for the higher latitudes are significant hiero- 

 glyphs of the serenity of summer, and the more violent winds and storms 

 of March and September. The entire deprivation of the sun's intensity 

 during a part of the year within the Arctic and Antarctic circles 

 may also produce a polar calm, at least during the depth of winter. 

 But the existence of such calm, though probable, can neither be dis- 

 proved nor verified, as the pole appears not to have been approached 

 nearer than within about five hundred miles. Parry and Barrow 

 believed that a perfect calm exists at the pole. 



SECTION V. 



THE SUN'S ANNUAL INTENSITY UPON ANY LATITUDE OF THE EARTH. 



By the method explained in the last section, the diurnal intensity, 

 in a vertical direction^ might be computed for each and every day in 

 the year, and the sum total would evidently represent the annual 

 intensity. 



