QUERIES RELATIVE TO THE TENSION OF VAPOR. 7 



Michler's station, gave for the elevation of tliat place above the sea 260.7 feet. The height of 

 Lieut. Michler's station above the water is estimated, and it is probably a few feet higher.* 



These results are believed to be very nearly correct, yet there appears to be some effect upon 

 the mercurial column, which is difficult to explain without a long and careful series of observa- 

 tions. May not this effect be due to the tension of aqueous vapor? The period at which the 

 atmosphere on the Pacific coast and the lower region of the Colorado river is most heavily 

 charged with moisture is during the winter months — the mean temperature of winter is not 

 low ; the tension of vapor is, therefore, comparatively great. Does it not, then, have the effect 

 of increasing the pressure upon the cistern, causing the barometer to rise too high? The 

 months of May and June being excessively hot and dry, there would be but little or no effect 

 due to the tension of vapor, the pressure indicated being that of dry and highly rarefied air, 

 would it not be too low ? The observations of Lieut. Michler opposite Fort Yuma, above 

 referred to, uncorrected for abnormal error, give as a mean 29.865, (58° mean air temperature.) 

 Ours in June, for the same place, give 29.691, (90° mean air temperature.) The question is, 

 which of these results is nearest the truth ? If the tension of vapor has any effect, is its 

 increased effect upon barometric pressure in winter in greater ratio than its absence entirely, or 

 the decreased pressure due to rarefaction in summer? It appears to be in a sensibly greater 

 ratio at Fort Yuma in winter than in summer. 



After the normal corrections, as it were, are applied to any series of observations, does not 

 the residuum include any effect of aqueous tension, if there be such ? and if there is no effect 

 due to the tension of vapor, is not the pressure during the winter months in the Colorado 

 basin, &c., where the mean temperature is about 60°, and the atmosphere nearer its normal 

 state, more nearly correct than in summer when the mean temperature is 90° and the reflection 

 from the arid plain rarefies the air to so great an extent ? These queries are thrown out by 

 way of suggestion, as this question of the effect of aqueous vapor is still open to investigation 

 and practice. 



There are a complete set of observations with Mason's hygrometer, taken in connexion with 

 the barometric observations to be found in the general table, and it is hoped that, in the hands of 

 an experienced meteorologist, they may throw some weight on this subject. 



To the observations at the various points between the Colorado and Rio Grande, the abnor- 

 mal error was not applied ; for, as above stated, suitable observations could not be obtained 

 with which to compare. It appears, however, from the direction of the curve at San Diego and 

 Fort Yuma, as late as June, and from the curve of the various camps, that this correction for 

 the months of July and August, 1855, did not, probably, exceed plus 0.050. 



(t -i- f 64°\ 

 ~ j the mean monthly temperature, as 



nearly as could be ascertained, was taken for t' instead, as is usual, of taking the temperature 



• The result of 80 observations of Lieut. Michler, in March and April of the same year, at the initial point, 27 miles below 

 Fort Yuma, discussed and corrected in the above manner, give the height of the river at this point 156.8 feet above tide. The 

 rapid current of the Colorado river, nearly six miles per hour during moderate freshets, can be accounted for by this slope 

 of the plain through which it flows. There seems to be some reason in the supposition that this stream flows down in shutes 

 from one plateau to another. This appears to be the genera] rule of all small streams of the Colorado and Gila basins and in 

 the Great Basin where I have traveled — the communication from one steppe to another being through a shute or canon caused 

 by some great revolution of nature — a cracking due to an upheaving force, or, which is most probaple, to the shrinking of the 

 earth's crust during^the cooling process. 



