INTENSITY OF SUN'S HEAT AND LIGHT. 43 



With respect to maxima and minima, Plate I exhibits a resemblance to two 

 summers and to two winters on the Equator — the sun being vertical at the two 

 equinoxes. On receding from the equator, but still in the torrid zone, the sun will 

 be vertical at equal intervals, before and after the summer solstice, which intervals 

 diminish as the sun approaches the Tropic ; the sun being vertical to each locality, 

 when his declination is equal to the latitude of the 

 place ; as indicated in the annexed diagram. 



On arriving at the Tropic in the yearly motion, the 

 sun can be vertical but once in the year, namely at the 

 summer solstice. At all places more distant from the 

 equator the sun can never be vertical, but will approach 

 nearest this position at the solstice in summer (s), and 

 be farthest from it at the solstice of winter (w). Thus 

 in the torrid zone, the sun's daily intensity has two 

 maxima and two minima annually; in the temperate 

 zones, one maximum and one minimum ; and in the frigid zones, one maximum. 



Owing to change of the sun's distance, the intensity is not precisely the same at 

 the autumnal equinox as at the vernal ; the difference, however, being small, may 

 here be neglected. And for more full illustration, we exhibit a different projection 

 of the Table in Section IV, showing (Plate IV) the Sun's Diurnal Intensity along 

 the meridian at intervals of thirty days, from June to December, and approximately 

 for the other months. The alternate curves will of course show the sun's changes 

 of intensity in intervals of sixty days. It will be seen that the sun's least yearly 

 range of intensity is not on the Equator, but about 3° of latitude from it north and 

 south. Here the daily heat is most constant, and perpetual summer reigns through 

 the year. 



In like manner, the diverging curves show an increasing yearly range, which is 

 greatest in the Polar regions. Also the changes from one day to another are most 

 rapid in spring and autumn. The greatest intensity occurs at the summer solstice, 

 June 21, and the least, at the winter solstice, December 21; so that the yearly range 

 from minimum to maximum is a little wider than the drawn curves indicate. Near 

 the Polar Circle, a singular inflection commences in summer, and the temperature 

 rises rapidly to the Pole. 



These laws of Intensity are subject to the retardation in time, mentioned in Sec- 

 tion IV, when applied to temperatures ; and thus will correspond, generally, with 

 observations. For example, the thermometric column will, during the month of 

 May, rise faster at Quebec than in Florida, and still more rapidly at the Arctic 

 Circle. 



It was proved, in Section IV, that the Sun's intensity upon the Pole during 

 eighty-five days in summer, is greater than upon the Equator. Indeed, at the 

 summer solstice it rises to 98.6 thermal units, corresponding nearly to 98° Fahren- 

 heit, which singularly coincides with the temperature of the human body, or blood 

 heat. Though this circumstance may invest the Hyperborean region with new inte- 

 rest, still Ave cannot assume a brief tropical summer with teeming forms of vegetable 

 and animal life in the centre of the frozen zone. For the measured intensity refers 



