METEOROLOGY. 



337 



The sun's annual intensity. 



From this table it will be seen that, at the tropic of Capricorn, or 

 of Cancer, the sun's annual intensity is but eleven thermal months, 

 being twelve on the equator. In the latitude of New Orleans the 

 annual intensity in a vertical direction is ten and a half thermal 

 months, and in the latitude of Philadelphia nine and a half. At 

 London the annual intensity is reduced to eight thermal months ; 

 and at the polar circle to six months, being just one-half the value 

 on the equator. Thus the intensity irregularly decreases till it ter- 

 minates at the South or North Pole, where the annual intensity is but 

 fire thermal months. 



Again, it will be interesting to note the analogy which the differ- 

 ences for every five degrees of latitude, in the last column of the table, 

 bear to the corresponding differences of height in the atmosphere which 

 limit the region of perpetual snoiv. It has been observed that the 

 different heights of perpetual frost "decrease very slowly as we recede 

 from the equator until we reach the limits of the torrid zone, when 

 they decrease much more rapidly The average difference for every 

 fire degrees of latitude in the temperate zone is 1.318 feet, while from 

 the equator to 30° the average is only 664 feet, and from 60° to 80° 

 it is only 891 feet — important meteorological phenomena depend on 

 this fact. — (Olmsted's Natural Philosophy.) The differences of com- 

 puted annual intensity in the table vary in a manner precisely 

 similar. While, in the temperate zone, the decrease for every five 

 degrees of latitude is from 13 to 21 thermal days, yet it averages only 

 about 6 thermal days within the tropics and beyond the polar circles. 

 The line of congelation evidently rises in summer and fiiUs in winter 

 between certain limits. 



With reference to the connexion between these annual intensities 

 and the observed annual temperatures, the analogy of the centigrade 

 scale shows that units of intensity may be converted into degrees 

 Fahrenheit by a multiplier and constants. Since the values of t!ie 

 multiplier and constants are not precisely known, a grapliical con- 

 struction will be employed ; and it is plain that if com[)uted intensi- 

 ties and observed temperatures both follow the same law of change, 

 their delineated curves will be symmetrical. 



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