-1859] 



WRITINGS OP JOSEPH HENRY. 



41 



Mississippi, along a line passing through the city of New 

 Orleans : 



The temperature of the equator is assumed to be 82°. 

 The first column gives the latitude, the second the astro- 

 nomical mean temperature, the third the observed temper- 

 ature reduced to the level of the sea, as taken from the 

 accompanying isothermal chart,* and the fourth column the 

 difference between the last two. It will be seen that the 

 difference between the calculated and the observed temper- 

 ature in the lower latitudes is quite small; but as the lati- 

 tude increases, the deviation becomes very great. This 

 difference is due to other than astronomical causes, and by 

 eliminating the latter we narrow the field of research. 



Empirical formulas of much nearer approximation to the 

 truth in high latitudes have been proposed, which will be 

 noticed hereafter, our object at present being only to exhibit 

 the difference between the astronomical results and those 

 derived from actual observation. 



Let us next consider the changes of temperature in differ- 

 ent parts of the day and in different seasons of the year, 

 produced by the varying obliquity of the sun's rays. If we 

 assume a given length of sun-beam as the representative of 

 the force, and then resolve this into two, — one perpendicular, 

 the other parallel to the horizon, — the sum of all the perpen- 

 dicular lines, from the rising to the setting of the sun on any 

 day, will represent the whole intensity of the heat on a given 

 place during that day; and in this way may be calculated 

 the relative amount of heat received on different latitudes 

 at different seasons of the year. From this estimate we shall 

 find that the amount of heat received from the sun during 

 a given day in summer, say the 16th day of June, at dif- 

 * [See Map, at page 72.] 



