LOCAL CLIMATOLOGY. 



91 



July 20th, 1854. But the above averages show that the maximum of 

 heat is not reached on the average until about the 1st of August, when it 

 is 73.29. The maximum of cold is reached on the 3d of February ; and 

 the coldest day I have on record was February 6th, 1855, when the 

 average for the day was 14°.2 below zero. 



It may be worth observing that the greatest heat and the greatest cold 

 in the day are reached only after noon and after midnight ; and the dis- 

 tance of time after noon, for example, when the day is the hottest, is 

 about the same in proportion to the length of the day as the time of 

 greatest heat in the summer, after the solstice, is to the length of the 

 year. The hottest time in the day is generally about three o'clock p. m. ; 

 a little before in winter, and a little after in summer ; and both phe- 

 nomena are doubtless to be ascribed to the same cause — the equilibrium 

 of heating and radiation. From the 21st of December the amount of 

 heat received from the sun begins to increase, both because the days are 

 longer, and because the sun runs higher. But, as appears from the table, 

 it is not until the 3d of February that the balance comes to be in favor 

 of the heat ; and from this time on until August 1st, the earth receives 

 and absorbs more heat than it gives off by radiation and conduction, and 

 consequently is growing warmer. 



However, both processes are going on together. During the day, alike 

 in summer and in winter, the earth receives more heat than it radiates, 

 and in the night it radiates or sends off more than it receives, as is shown 

 by the fact that it is warmer in the evening than in the morning. 



As showing the ratio of heating and cooling, I have made a comparison 

 and average for the twelve years during which my records are complete. 

 I subtracted the average for the morning for each month from its aver- 

 age for the evening, and then made an average for all the twelve years, 

 with the following result : 



Month. 



Difference. 



Month. 



Difference. 



Month. 



Difference. 



January 



February 



1.56 

 1.72 

 2.05 

 3.42 



May 



2.32 

 2.50 

 3.04 

 2.23 



September 



October 



November 



December 



2.22 

 1.55 

 1.56 

 1.01 





July 





August 





The morning observation is taken at seven o'clock, and therefore in 

 winter before the air has begun to get warmer, though in summer some 

 change has doubtless taken place in that direction. If, therefore, we 



