INTENSITY OF SUN'S HEAT AND LIGHT. 21 



A close agreement, however, could not reasonably be expected; for the Intensities 

 represent the sun's effect at the summit of the atmosphere, but the Temperatures, 

 at its base. Indeed, the sun's intensity upon the exterior of the earth's atmosphere, 

 like the fall of rain or snow, is a primary and distinct phenomenon. While passing 

 through the atmosphere to the earth, the solar rays are subject to refraction, absorp- 

 tion, polarization and radiation ; also to the effects of evaporation, of winds, clouds, 

 and storms. Thus the heat which finally elevates the mercurial column of the 

 Thermometer, is the resultant of a variety of causes, a single thread in the net- 

 work of solar and terrestrial phenomena. 



There is still a general agreement of the delineated curves of intensity with 

 actual phenomena. 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 (Plate I), at once indicates the region intermediate between 

 the Equator and the Tropic of Cancer, on the one side, and of Capricorn on the 

 other. 1 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 February. 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 Plate I, representing the amounts; and, on 

 recurring to Plate II, representing the rates of diurnal intensity, the status is pre- 

 cisely similar, except that the region of summer calm is removed further from the 

 equator, and nearer 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 variables," 

 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 hieroglyphs 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 disproved 

 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. 



1 The connection of the curves of the Sun's Intensity with the lines of Equatorial and Tropical 

 calms, was suggested by Prof. Henry. 



