Chase.] g^O [February. 



sun's altitude. In the daily distribution of temperature, this is the 

 most important element, as is evident from the tabular comparisons 

 in my communication of September 21, 1866.* Absorption and 

 radiation proceed at nearly uniform rates, therefore it may be as- 

 sumed that their effects are approximately proportioned to the time 

 during which they operate. The average general variation which is 

 due to the influence of the winds is a difficult point to determine, 

 but the present investigation has led me to believe that it niay 

 be properly measured by the difference of arc (instead of the 

 sine difference), of the sun's meridian altitude. My reasons for 

 this inference are the following: 1, the general average tempera- 

 ture of the year often appears to vary very nearly as the arc in 

 question, and it seems unreasonable to suppose that a variation of 

 this character can be attributable either to the heat communi- 

 cated by the sun, or to terrestrial absorption and radiation ; 2, the 

 tendency of the air, so f;ir as it is determined by the direct heat of 

 the sun, is at all times towards that point of the earth's surface at 

 which the sun is vertical, and we may readily believe that the ther- 

 mal effect of the current should be proportional to the distance, 

 measured on a great circle of the earth, through which the air 

 would be obliged to move in order to reach the sub-solar point. 

 This distance evidently varies as the arc of the sun's zenith-distance. 



We have, then, three natural standards for admeasurement, by 

 means of which, if we rightly eliminate special and limited pertur- 

 bations, we may perhaps be able to determine the predominating in- 

 fluence, in many cases both of local and of general thermal disturb- 

 ance. Iq order to institute as broad a comparison as possible, I have 

 adopted a method of elimination which is substantially the same as 

 the one used in my previous physical investigations, and which may 

 be illustrated by a single example. 



The average monthly temperatures of the United States, as de- 

 duced from Prof. Cofiin's reductions of the " meteorological observa- 

 tions, made under the direction of the United States Patent Office 

 and the Smithsonian Institution, from the year 1854 to 1859 inclu- 

 sive, "f appear to be as follows : 



January, 28.352; February, .30.873; March, 39.049; April, 



* Ante, pp. 261-2fi9. See especially the observations at St. Bernard, and the 

 general average of Table I, p. 267. 



t The reductions referred to, embody the results of more than a million ob- 

 servations, made by nearly a thousand observers, at about eight hundred dif- 

 ferent stations. 



