246 



METEOROLOGY. 



CHAPTER IX. 



METEOROLOGY. 



Sir : The meteorol 



tlie latter part of 1850, tliougli sufficiently interesting of themselves, do not furnisli data for 



averaging 



same lorm 



adopted for tlie publication of observations taken at a fixed observatory. 



The principal and most interesting portion of the observations is the result deduced and 

 embodied in the barometrical profile of the road already prepared. 



The record of both wet and dry bulb thermometers was carefully kept. The highest degree 

 noted was on the 23d October, 1850, at 3 p. M., eighty-four degrees ; while the lowest was at 

 sunrise of the Gth of December, of the same year^ when the mercury fell to 1°,5 Fahrenheit. 

 These results are interesting only so far as noting these two extreme points. From the nature 

 of the marches made, it was impossible to note with any regularity the daily change in any 



of the instruments. 



The chief phenomena noted are those of a local character. The effect of the different ranges 

 of mountains and the long succession of arid plains upon the condition of the atmosphere is 



mark 



from 



Pedro or Devil's river, but little change takes place — the same succession of dews, the usual 

 quantity of rain, and, indeed^ all the characteristics of a climate enjoying its proper share of 

 humidity are met with. The clouds of the usual forms, cirrus and nimbus, float about, and 

 nothino' as yet gives notice that we are approaching a different country. One single day's 



march 



and this is perceptibly changed. The summit of the valley of the San Pedro reached, 

 the hy^^rometrical condition of the atmosphere is altered at once, the appearance of the vege- 

 tation is different, and the whole face of the country shows the effect of the diminution of 



moisture 



pointed out by the hygrometer. We 



IT 



with certainty of the effect of the change on any of the instruments except the hygrometer; 

 but that the barometer is seriously affected there can be no doubt, though to what extent can 



more 



perfect unhurried examinations. The observations point out the existence 

 of such a change; they show, too, the line of country at which the change commences; and 

 it only remains to fix the exact amount of correction to be used, to make the barometer as useful 

 in these re^^ions as in those countries bordering on the seacoast, or where the great lack of 

 humidity is not so sensibly felt. 



Darin"" the march, there was experienced a norther of the most perfect character ; this occurred 

 on the 4th and 6th of December, and it was at the latter date that the lowest temperature was 



noted. 



The norther commenced at 4 p. m. of the 4th December, and was preceded by no change by 



which its approach could be predicted. A calm, pleasant day, with the thermometer ranging 



