r.AKOMKTRiC 



of sunset, but continued with considerable tore.- throughout the night At 7 p. in. the 

 thermometer read already '2<>° below the maximum, and had attained the mean tem- 

 perature of the day. Some of the causes which co-operated to produce these results are 

 the following: The radiation was very intense, the more so because the atmosphere 

 was beautifully clear, and the cloudiness of tin- sky amounted only to 0.7 against 

 -,.-> at Philadelphia as the mean tor September. The bare parched soil, on the other 

 hand, gave off its heat comparatively slowly, like a heated brick, and thus prevented 

 the temperature from sinking still more rapidly, as we might expect from a comparison 

 with some of the other diagrams ; and it thus cooled gradually until it was heated again 

 by the rising sun. Another cause for the continued strong decrease of temperature 

 during the night was the continued evaporation, which was not interrupted during the 

 night. On account of the scarcity of moisture, its effects were not so intense as they 

 would have been otherwise. If the soil would have been less dry the temperature 

 would have decreased more rapidly toward sunset, but much less during the latter 

 part of the night, when, on the contrary, precipitation would again have taken place, 

 and latent heat thus have become sensible. As it was, only so much could evaporate 

 as rose gradually to the surface from the badly-supplied substrata, and while the 

 refrigerating effects of the evaporation were not intense at any hour, they were con- 

 tinually felt throughout the whole of them. 



The temperature curve of Genoa for June shows a remarkable feature. The 

 maximum of temperature took place there at 1.30 p. m., while at Philadelphia it recurs 

 at 4 p. m. I do not suppose that this could have been produced by the same influ- 

 ences which cause the earlv maximum at San Francisco, although they are scarcely 

 160 miles apart, but I consider the peculiar situation of our camp the main cause. 

 It was pitched on the rocky slope at the very foot of the main range of the Sierra 

 Nevada, facing a little south of east. During the forenoon the sun burned intensely 

 on this slope and on the sides of the mountains, but early in the afternoon its rays fell 

 obliquely upon the ground, and it soon disappeared altogether behind the mountains. 

 Thus an early maximum was caused, and a rapid decrease of the temperature between 

 5 and 6 p. m., corresponding to one or the other diagrams at the hour of sunset, which 

 then took place only between 7 and 8 p. m. 



The variations of the relative humidity are not less interesting than those of the 

 temperature. Turning at once to the diagrams I will point out some of their most 

 remarkable features, and compare them with the results obtained at Philadelphia, con- 



