Intelligence and Miscellaneous Articles. 245 



other two, and especially in retarding the daily and yearly changes ; 

 the third is a subject of hourly experience, and its meteorological 

 importance is now generally recognized. 



The amount of heat which is received directly from the sun evi- 

 dently varies as the cosine of the zenith-distance, or the sine of the 

 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 maybe assumed 

 that their effects are approximately proportional 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 may be measured 

 by the difference of arc (instead of the sme-difference) of the sun's 

 meridian altitude. My reasons for this inference are the following : — 

 (1) The general average temperature of the year often appears to vary 

 very nearly as the arc in question ; (2) it seems unreasonable to 

 suppose that a variation of this character can be attributable either 

 to the heat communicated by the sun or to terrestrial absorption and 

 radiation ; (3) the tendency of the air, so far as it is determined by 

 the direct heat of the sun, is at all times toward that point of the 

 earth's surface at which the sun is vertical, and we may readily be- 

 lieve that that tendency should be proportional to the distance, mea- 

 sured on a great circle of the earth, through which the air would be 

 obliged to move in order to reach the subsolar 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 

 influence, in many cases, both of local and of general thermal dis- 

 turbance. In order to institute as broad a comparison as possible, 

 I have adopted a method of elimination which may be illustrated by 

 a single example. 



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

 duced from Professor Coffin's reductions, appear to be as follows : — 



Jan. 28-352 Feb. 30-873 Mar. 39-049 Apr. 49744 May 60-902 June 69780 

 July 75-640 Aug. 71754 Sept. 65643 Oct. 53-922 Nov. 42-350 Dec. 32132 



Averaging the temperature at equal intervals from January (taking 

 the mean temperature of Dec. and Feb., of Nov. and March, &c), 

 we get the following results : — 



Months from January. 0. 1. 2. 3. 4. 5. 6. 



Average temp. ... 28-352 31-502 40 699 51-833 63-272 70-767 75610 



Monthly diff. 3-150 12-347 23-481 34-920 42415 47288 



Ratio of do -067 -261 -497 738 -897 1-000 



Sh-Snce} '° 69 255 - 500 *« *" ™» 



The second of the above series of ratios (that of the differences in 

 the arcs of the sun's zenith-distance) is based upon the following 



* Proc. Amer. Phil. Soc. vol. x. pp. 261-269. See especially the obser- 

 vations at St. Bernard, and the general average of Table I. p. 267. 



