300 ANNUAL OF SCIENTIFIC DISCOVERY. 



present investigation has led me to believe that it may be meas- 

 ured 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 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 ter- 

 restrial 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 -verti- 

 cal, and we may readily believe that that tendency should be pro- 

 portional 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. 



From extensive comparisons, the following inferences seem to 

 be warranted, all of them being confirmed by other considera- 

 tions : 



1. Taking into view the entire land surface of the globe and the 

 entire range of the year, the direct heat of the sun and the induced 

 aerial currents appear to be about equally instrumental in deter- 

 mining fluctuations of temperature. 



2. The influence of the winds is most marked in the northern 

 and western hemispheres ; that of solar obliquity in the southern 

 and eastern hemispheres. 



3. Where the sun's rays are least intense, as in the polar re- 

 gions, and where the winds are most variable, the ratios exhibit 

 the nearest parallelism to the increments of arc ; J;>ut where the 

 winds are most uniform, in or near the region of monsoons, they 

 correspond more closely with the sinal increments. 



4. The general changes of temperature at midwinter, and at 

 the equinoctial seasons, when the sun's declination is changing 

 most rapidly, are most dependent upon the local solar heat; 

 the midsummer changes are more subject to the influence of the 

 winds. 



5. The greatest conflict of opposing forces occurs during the 

 sun's passage between the comparatively wind-governed northern 

 hemisphere and the sun-governed southern hemisphere. This 

 conflict is manifested in the spring and autumn rains. 



6. The closest and most general approximation of ratios is 

 shown in the monthly temperature change at midsummer, which 

 corresponds almost precisely with the change of arc. 



INFLUENCE OF TREES ON TEMPERATURE. 



According to M. Becquerel, as stated before the French Acad- 

 emy, " Comptes Rendus," April 29, 1867, in summer the mean 

 temperature in free air is slightly in excess of that under trees ; 

 the contrary is the case in winter. The trees, notwithstanding 

 their inferior conductibility, very slowly assume a temperature in 

 equilibrium with that of the air. The diurnal maximum takes 

 place toward midnight under the trees, but toward 3 P. M. in free 



