BAROMETRICAL AND M KTKOKOI.OC. 1< A I. ORSEUVATIONS. 



The hourly variation* of temperature, as they are exhibited in the *bov< 

 •rams, afford a subject of much interesting speculation. In examinii 



ee at once how much the time of the dailv maximum is variable, a 



the data obtained by our exploration, and not to enter into an elaborate discussion. I 

 will therefore confine myself merely to point out a few of the causes which co-operate 



The first din-ram presents the hourly variations of the temperature at Fort 

 Kearney, Nebr., in dune. The mean temperature was 77°.5, Fahrenheit, whfle at 



Philadelphia it is only 6H° to 69° in June, although the latter place is situated over 

 two-thirds of a degree of latitude farther south, and over 2,000 feet lower. We have 

 no reason to believe that the temperature of June, 1*58, at Fort Kearney, has been 

 so much above the average that this result should not radicate a considerable north- 

 ward bend in the lines of equal temperature for that season in the plains of the Platte 

 River region: It merely confirms prior observations, and verities the result df theo- 

 retical deductions, based upon the situation of that country, in the center of a large 

 continent, far away from any sea-coast, and open toward the north and south. It 

 should be borne in mind that the values for Philadelphia are deduced from a much 

 larger series of observations than those at Kearney, which latter might perhaps he a 

 little above the actual average. Still, these and all the other observations have not 

 been taken on days especially selected for the purpose, but indiscriminately, as the 

 execution of the surveys made it convenient. For that reason they might as well 

 present smaller values than the average. The difference between the warmest and 

 coldest hours of the day at Fort Kearney amounts to "21° Fahrenheit, while at Phila- 

 delphia, according to table F, the amplitude is only 15°.4. This, too, is a consequence 

 of the continental situation of the place. The actual amount of vapor in the atmos- 

 phere at Kearney was larger than at Philadelphia, on account of its higher tempera- 

 ture, but the relative humidity, the degree of saturation of the air, was less at Kearney. 

 For the same reason, the cloudiness was much less at Kearney, only 3.35, against 

 6.6 at Philadelphia. (See table G.) Therefore, although a little more heat was 

 absorbed or made latent by the evaporation at Kearney, it was comparatively less 

 than at Philadelphia. The greater clearness of the sky offered less obstruction to the 

 rays of the sun and to their heating influence upon the' earth's surface during the day, 

 and to the cooling by radiation during the night. Thus the mean temperature and the 

 amplitude were both increased. The increase of the temperature during the hours of 

 the morning was gradual at Fort Kearney, because the quantity of moisture which 

 was evaporated during that time, and therefore of heat made latent, was considerable. 

 It reached an amount which would justly astonish those not used to such contempla- 

 tion. From our Table E we see that the minimum amount of vapor in 1 cubic foot 

 of air was 6.38 grains at 5 a. m. ; the maximum, 7.41 grains at 10 a. m. In order to 

 supply the difference of 1.03 grains, or, rather, taking the expansion into consideration 

 which" has taken place during those hours, of 1.04 grains to a stratum of air of 1 foot 

 thick over 1 square mile, over 5,000 pounds troy of water must be evaporated, and 



