8 REMARKS ON METEOROLOGY AND BAROMETRIC RESULTS. 



at the time of observation. Experience proves that this method gives more correct results. 

 The mean temperature for June, by our own observations^ confirmed by those taken for several 

 years at Fort Yuma, is 90°. By a combination of all our observations on the Gila, at Pimas vil- 

 lages, and lower portion of San Pedro, for a part of June and of July, the mean temperature was 

 found to be the same as at Fort Yuma, 90°. From the lower San Pedro to theSauz it was found 

 to be 86°, and hence to the Rio Grande 80°, corresponding with the mean temperature of the time 

 in question with observations taken at Fort Fillmore for several years under the direction of the 

 medical department of the United States army. All the observations from Fort Yuma to Fort 

 Fillmore are referred directly to the sea level at San Diego, viz : 30.018, as deduced as before 

 explained ; this had previously been assumed at 30.050, and^ in comparing identical points of 

 a previous survey, this should be compensated for, and also an allowance made for different 

 seasons.* 



An interesting phenomenon occurred at Camp 76, San Pedro river, (July 21 and 22, 1855,) 

 Avith reference to the diurnal oscillations of the barometer, which we do not remember to have 

 seen mentioned elsewhere. Several days' hourly observations, from 7 a. m. to 7 p. m., (see Plate 

 1,fig. 6.,) and one set, July 21 and 22, for 36 consecutive hours, (see Plate l,fig. 7,) gave for 

 the maximum 7 a. m. and for the minimum 5 p. m. — hut one maximum and one minmum point 

 in 24 liours. The maximum temperature each day, respectively, was 99°. 2 and 100° at 3 p. m., 

 the minimum 57. °5 at 5 a. m. ; maximum degree of dryness at 1 p. m. 30°. 5, and at 3 p. m. 30°; 

 minimum point 4° at 5 a m. Plate 1, fig. 7, will illustrate more clearly these relations of pres- 

 sure and dryness, and, when compared with Jig. 8, being a similar series of 36 hours' observa- 

 tion, taken January 16 and 17, 1855, near San Luis Obispo, (Camp 17,) may serve to explain 

 the phenomenon. In these diagrams the vertical scale is the same as in the other diagrams, 

 and, to illustrate the relation of this difference between the wet and dry bulb thermometers to 

 the pressure, the same divisions are made to represent each 1° of temperature, beginning from 

 the base of the figure. From a glance at these diagrams it will be perceived that the respective 

 maxima and minima points of pressure and dryness bear an intimate relation to each other. 

 The explanation of this phenomena appears to be this : at San Luis Obispo, as the influence of 

 the sun's presence begins materially to be felt at 10 a. m., the tension increases almost pari jiasM 

 with the decrease of pressure to their respective culminating points, and then the contrary effect 

 is produced as the sun's influence is gradually withdrawn. The influence of radiation is then 

 felt, producing a similar inverse movement until this cause ceases, and the opposite course is 

 taken until the approaching sun again begins to exert its influence on the upper regions of the 

 atmosphere, and a repetition of these movements for the next 24 hours takes place. In the San 

 Pedro case, the effect of radiation is felt from the moment of the decline of the sun unfil his 



' In the foregoing remarks I have omitted to state that the formula used for the computations of the heights was Loomis' 

 Laplace modified, as given hy him in his Practical Astronomy, with a table, &c. For the first division of our work the cor- 

 rection for air temperature was applied to the observed temperatures. Subsequent discussions of the observations in the 

 Gila region led to a different application of this correction, as above stated, and too late to revise the immense number of 

 observations in California. It is believed, however, that had this revision been made no material change would have been 

 made in the profile, since all the important points were established in camps of long duration, where the mean temperature 

 of the time of remainiug was taken, and the most important points along the survey were determined, at hours of the day 

 ranging very little from the daily mean. 



