PEEV ILING WINDS UPON THE COLORADO DESERT. 9 



return on the coming morning, which accounts for the absence of the night maximum and 

 morning minimum almost universally observed.* 



PHENOMENA OBSERVED. 



There are some phenomena of general interest that were casually observed during our travel 

 across the continent, which it may not be amiss to note down as a contribution to the great mass 

 of facts which are daily accumulating, illustrative of the somewhat paradoxical laws of physical 

 science. The prevailing wind upon the Colorado desert appears to be from the southeast, from 

 the indication of the movable sands and gravel, between Sackett's wells and Alamo Mocho, 

 arranging themselves in long elliptical hummocks, on the northwest side of each shrub and 

 stone which intercepted their course ; and to all appearance these hummocks are of a permanent 

 character, and not subject to each shifting wind that chances to blow. The wind, during our 

 march from Vallecito to the Colorado river, came from the southeast. For a week or more, 

 however, at Camp 60, about Y miles up the Grila, the wind blew a gentle breeze from the 

 southwest, and continued during the daytime from that quarter along the Gila, as far as the 

 Pimas villages. It is probable that this change of direction may be due to the influence of the 

 rivers and the configuration of the country, causing a portion of the southeast wind from the 

 Gulf of California to deflect up the basin of the Gila, 



*0n the morning of the 9th of June, about 5 o'clock, at Fort Yuma, a slight earthquake was 

 felt by some members of the party, but it was of short duration and seemed to excite but little 

 attention. 



On the 11th of June, at the mouth of the Gila, small cumulus and cirrus stratus clouds were 

 observed, and a few drops of rain fell for a second or two; temperature, 10*7°. After several 

 days' march up the Gila, cumulus clouds, with an occasional cirrus, appeared daily in the east, 

 at a great height ; their progress was from a few degrees south of east,t and on arriving at about 

 the meridian of 113° 30' disappeared entirely, doubtless vaporized by the ascending currents 

 of heated air from the arid plains of the Colorado and lower Gila. These clouds were watched 

 with interest daily, with fond hopes that they would precipitate rain, to relieve us from the 

 oppressive heat of 115°, and we were forcibly struck with their uniform disappearance along a 

 well defined north and south line. 



Several days after the appearance of these clouds, and they increased daily, the temperature 

 sensibly declined, and there appeared to be a general preparation of the elements for rain, and 

 frequent showers were seen depending from the clouds, like long brush marks, but no water 

 reached the earth. 



The great height from which the rain falls is remarkable, the clouds floating at an elevation 

 of about from 12,000 to 15,000 feet above the plains, far above the tops of the loftiest moun- 

 tains. No ready means suggested themselves for measuring their height, and this may be too 

 great or too small an estimate, but their unusual height was frequently remarked upon. This 



° In Kaemtz's Meteorology, page 270, a reference is made to this plienomenon, as observed by M. Dove, after having applied 

 a correction for the tension of vapor to Neuber's observations at Apenrade. In the above instance no correction of this 

 sort haa been made ; a reference to the hygrometric observations in the general table will indicate the condition of the 

 atmosphere in this respect. 



t This is the usual direction of rain clouds from the Rio Grande across the plateau. To El Paso tlioy come more from the 

 outheaet. 



2 W 



