196 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



for a stratum of 500 feet, over 2,500,000 pounds are required. If we consider that 

 the evaporation continues with increased intensity after 10 a, m., and that the vapor 

 diffuses itself, although in decreasing quantity, into the higher portions of the atmos- 

 phere, we can form an adequate estimation of the heat absorbed in that process, 

 and understand the cause of the slowness of the increase of heat during the morning. 

 The decrease of temperature is quickest toward sunset, because then the source of 

 the heat disappears, but during the night it is not as considerable as we might expect 

 it to be. Although the radiation is great, its effects are balanced, in a measure, it 

 appears, by a partial precipitation of the moisture evaporated during the day, by 

 which a large amount of the heat made latent during the day is again rendered 

 sensible. Besides, the currents of air, which during the day carry the heated air on 

 high, have subsided in the evening. Thus a more rapid cooling of the earth's surface 

 during the night is prevented. The maximum of the temperature was reached at an 

 earlier hour at Kearney than at Philadelphia, namely, at 2J p. m. instead of 3£ p. m., 

 perhaps, also, on account of the greater lightness and clearness of the atmosphere, but 

 especially on account of the greater intensity of evaporation at Kearney, which caused 

 a more rapid depression of the temperature after the source of the heat had passed the 

 point of greatest intensity: in other words, after the sun had passed the meridian. 



The diagram No. VI of the mean daily oscillations of the temperature at Camp 

 Floyd during the first third of November has a very marked shape. The temperature 

 rose rapidly in the morning, because the sky was clear and the humidity exceedingly 

 small, so that only little heat was absorbed by evaporation. As soon as the maximum 

 had been passed, at 2 p. m., it began declining rapidly, and continued thus until about 

 an hour and a half after sunset. By this time the earth's surface had lost the greatest 

 portion of its surplus heat by radiation, and nearly reached the point of mean temper- 

 ature of the season. The upward currents of air had also probably died out. From 

 this time to sunrise of next morning the temperature appeared to decline uniformly, 

 but at a much lower rate. The amplitude amounted to 28° Fahrenheit, while the 

 greatest mean amplitude of any month at Philadelphia scarcely reaches 16°, and that 

 of November is considerably less. 



Still larger amplitudes were obtained from the observations at the end of May, in 

 Woodruff Valley, and in September, at Camp Floyd. The former are represented by 

 the diagram VIII. They both give the most striking illustration of an extremely con- 

 tinental climate. In Woodruff Valley the mean temperature was then 55° Fahrenheit 

 against 56° at Philadelphia, and the mean amplitude 45° Fahrenheit against 15°.4 at 

 Philadelphia. At the time of the observations at Camp Floyd, in September, the mean 

 temperature was 64°.7, while the mean temperature of the month, according to the 

 above tables, is 58°.4 against 57° at Philadelphia, and the amplitude amounted to 

 48°.3 Fahrenheit against 13°.7 at Philadelphia. The latitude of the former is 

 45' south, that of the latter 16' north of that city, while their elevation above it is 

 about 6,000 and 4,800 feet respectively. Taking into consideration the decrease 

 of temperature due to such considerable elevations, which may be put down as 

 about 18° and 14° Fahrenheit, the great northward deflection of the lines of equal 

 temperature, is again apparent. The following remarks especially apply to the varia- 



