316 METEOROLOGY. 



In this connection, it may be observed thatNervander, Buys Ballot, 

 and Dove have developed a slight inequality of temperature dependent 

 upon the Sun's rotation around his axis, and having the same period 

 of about 27 days ; but this result is not confirmed by Lamont, Pog- 

 gendorjf" s Annaltn for 1852. 



With respect to maxima and minima, the foregoing Plate 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 Tro])ic 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 })osition at the solstice in 

 summer (s), and be farthest from it at 

 the solstice of winter (lo). Thus in the 

 torrid zone, the sun's daily intensity 

 lias 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 pre- 

 cisely the same at the autumnal equinox as at the vernal ; the dif- 

 ference, however, being small, may here be neglected. And for more 

 full illustration, a horizontal projection might be drawn of the Table 

 in Section IV, showing 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 Section IV, when applied to temperatures, and thus 

 will correspond, generally, with observations. For example, the 



