THE CLIMATE OF THE LAKE REGION. 205 



i 



becoming vertical at the equator about the 21st of Sep- 

 tember, and reaching the tropic of Capricorn about the 

 21st of December; pouring his vertical rays upon that 

 tropic at about the time when, from our increased prox- 

 imity to the sun, they possess the greatest inherent in- 

 tensit}^ The equator, being the half-way station in the 

 annual journey of the sun from tropic to tropic and back 

 again, receives a greater average verticality of the solar 

 rays than any other parallel. The mean heat produced 

 at the equator by the sun's influence has been ascertained 

 to be about 82. The mean temperature at any parallel 

 of latitude north or south of the equator is proportional 

 to the diameter of that parallel; or, in the language of 

 science, it is proportional to the co-sine of the latitude. 

 From this law we calculate that the normal annual tem- 

 perature of New York is 62. 51; that of Chicago is 

 61. 5; that of Mackinac is 57. 12. 



The altitude of the sun varies also with the hour of the 

 day, and the solar intensity varies accordingly. From 

 sunrise to mid-day the intensity continually increases, and 

 from mid-day to sunset it diminishes. The total heat of 

 the day is the sum of all the intensities from instant to 

 instant between sunrise and sunset. The value of the 

 total depends both on the magnitude and, as we may ex- 

 press it, the number of the intensities during the da/. 

 In other words, the total amount of heat received during 

 a day is determined both b}^ the intensity of the solar 

 rays and the length of the day. At the equator the length 

 of the day is always twelve hours. In consequence of this, 

 the total daily heat received at the equator is less than 

 the total daily heat received at places in the northern 

 hemisphere, where, though the solar intensity is less, the 



