•44 



WRITINGS OP JOSEPH HENRY. 



[1855- 



later, after which the radiation during the day and night 

 together exceeds the amount received from the sun during 

 the day, when the temperature b^ins to decline. The 

 action is a little complicated, on account of the fact that the 

 radiation increases with the temperature. A similar result 

 is produced in the heating of the day, as will be seen from 

 the following table of observations taken at every hour of 

 the twenty-four, at Girard College, under the direction of 

 Professor Bache : 



MEAN DIURNAL VARIATION OF THE TEMPERATURE OF THE AIR AT 

 PHILADELPHIA. 



Computed from observations in 1842, and from, July 1, 1843, to July 1, 1845. 



+ 



Maximum,. 



The result in the above table is somewhat affected by the 

 greater humidity of the atmosphere towards morning, which 

 prevents a greater radiation and fall of temperature, even 

 after the rising. of the sun. 



II. — Results of other than Astronomical Conditions. 



The deductions that have thus far been given are from 

 established astronomical data ; and unless some error has 

 been committed in the statement, their correctness cannot 

 be doubted by any person properly educated in the line of 

 physical science. The effects produced by the air, the water, 

 and the land, are however of a much more complicated 

 character, and like the problem of the mutual action of all 

 the planets on each other, have never yet been submitted to 



