200 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



considerably from those computed from the mean of temperature and mean force of 

 vapor of the same observation, the more so the larger the amplitudes of arc. As those 

 obtained in the first-stated manner are, however, more correct, I have given them in 

 the column of means. Apparent errors may thus be explained. The same may be 

 said in regard to the computation of the weight of vapor. 



The hourly changes of the quantity of vapor in the atmosphere, represented by the 

 weight of vapor in one cubic foot of air in the lower portion of the atmosphere, has 

 not been illustrated by diagrams, because the values given in the above table were not 

 obtained by direct computation of each observation, but by an indirect computation 

 from the mean values of temperature and relative humidity. If not absolutely correct 

 on that account, still they come very near being so. In the general remarks at the 

 head of this chapter, I have stated that in Western Europe generally the minimum 

 quantity of vapor in the air is to be found about sunrise, that it attains its greatest 

 maximum about 9 a. m., then decreases till toward 4 p. m., and attains a second smaller 

 maximum toward 9 p. m., when it decreases until sunrise; that in winter, however, 

 when the action of the sun is less intense, there is generally only one minimum, about 

 sunrise, and one maximum, about 2 p. m. From our Table F we see that at Philadel- 

 phia, probably on account of its situation near the coast, the changes are not so uniform. 

 In January there is a minimum between 7 and 8 a. m., and a maximum between 3 and 

 7 p. in., with the highest point probably at 6 p. m. The average amount of vapor in 

 1 cubic foot of air is 2.01 grains, and the amplitude only 0.24 grains. In April there 

 is a minimum about the time of sunrise, from 4 to 6 a. m., a maximum from 11 a. m. 

 to 6 p. m., after which the quantity of vapor decreases until 10 p. m., when it continues 

 nearly unchanged to the time of the lowest minimum. The average amount is 3.43 

 grains; the difference between the largest and smallest weight, 0.45 grains. In June 

 a minimum takes place at 5 a, m. ; the quantity is largest, with little oscillation, from 

 9 a. m. till 7 p. m., with the highest point at 6 p. m., and then it decreases till morning. 

 The mean quantity is 5.97; the amplitude 0.65 grains. In August 5 a. m. is the time 

 of the minimum; from 11 a. m. to 6 p. m. the quantity of vapor is largest, with the 

 highest maximum at 6 p. m., after which time it decreases till morning. The mean is 

 6.86 grains; the amplitude 0.83 grains. In September the minimum falls in the hour 

 of sunrise, as in the other months, namely, between 5 and 6 a, m.. The quantity 

 then increases rapidly till 9 a. m. ; then very slowly. The maximum takes place from 

 4 to 6 p. m. The mean quantity amounts to 5.63 grains; the amplitude to 0.55 grains. 



The variation at Philadelphia, at least in the above-named months, which alone I 

 have examined, show, therefore, all one decided minimum about and soon after sun- 

 rise, and one maximum, of long duration, generally between the hours of 9 a. m. and 

 7 p. m., which has its highest, but not sharply-marked point, about 6 p. m. Instead 

 of a second maximum at 9 p. m., we find about that hour rather indications of a second 



Our values in Table E, from the central portion of the continent, are altogether 

 different; they prove more than anything else the absolute difference of climate there, 

 and its extremely arid and continental character. In Woodruff Valley, at the end of 

 May, and in Camp Floyd, in August and September, we have the strongly-marked 



