30 



INTENSITY OP SUN'S HEAT AND LIGHT. 



With the aid of Legendre's elliptical tables, and formulas (27), (30), (31), (29), 

 the computation of annual intensities is entirely practicable. The results con- 

 verted into units, with differences for every five degrees of latitude, have been 

 carefully verified and tabulated as follows : — 



The Sail's Annual Intensity. 



Latitude. 



Thermal 

 units. 



Thermal 

 months. 



Thermal 

 days. 



Diff. 



days. 



Latitude. 



Thermal 

 units. 



Thermal 

 months. 



Thermal 

 days. 



Diff. 

 days. 



0° 



81.50 



12.00 



365.24 



1.27 



50° 



55.73 



8.21 



249.74 



20.92 



5 



81.22 



11.96 



363.97 



3.78 



55 



51.06 



7.52 



228.82 



21.06 



10 



80.38 



11.83 



360.19 



6.28 



60 



46.36 



6.83 



207.76 



19.91 



15 



78.97 



11.63 



353.91 



8.70 



65 



41.92 



6.17 



187.85 



14.81 



20 



77.03 



11.34 



345.21 



11.01 



70 



38.61 



5.69 



173.04 



9.82 



25 



74.57 



10.98 



334.20 



13.20 



75 



36.42 



5.36 



163.22 



6.59 



30 



71.63 



10.55 



321.00 



15.30 



80 



34.95 



5.15 



156.63 



3.80 



35 



68.21 



10.04 



305.70 



17.15 



85 



34.10 



5.02 



152.83 



1.24 



40 



64.39 



9.48 



288.55 



18.76 



90 



33.83 



4.98 



151.59 



0.00 



45 



60.20 



8.86 



269.79 



20.05 













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 terminates at the South or North Pole, 

 where the annual intensity is but five thermal months. 



Again, it will be interesting to note the analogy which the differences 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 snow. 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 five 

 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 — import- 

 ant meteorological phenomena depend on this fact." (Olmsted's Natural Philosophy.) 

 The differences of computed annual intensity in the table vary in a manner precisely 

 similar. While in the Temperate Zone, the decrease for every five degrees of lati- 

 tude 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 falls in winter, between certain limits. 



With reference to the connection 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 ; 

 thus, d _= (u — i)y-\-x. Since the values of the multiplier y, and constants i, x are 

 not precisely known, a graphical construction will be employed ; and it is plain that 



