172 AMOUNT OF RAIN FALL DEPOSITED IN THE SUBSTRATA. 



between the two extremes, Forts Yuma and Fillmore, is "Zj inches, which is spread over 650 

 miles, or y\ of an inch of rain for every hiindred miles, supposing the fall were evenly distri- 

 buted ; but it is scarcely to be expected that the distribution of rain is equal, or that it lessens 

 equally over equal areas ; for the elevated land is all toward the east, and receives first the 

 saturated air, and would necessarily condense a larger proiDortion than an equal extent of 

 elevated land toward the west; much more does it condense since the western district is so much 

 lower ; so that within the eastern half, or 325 miles, near the Kio Bravo, perhaps two-thirds of 

 the total fall of rain is concentrated ; in other words, it might be hypothecated that — 



Within the first 350 miles, the fall would be from 12 to 5^. 



While within the second 300 miles, the fall would be from 5^ to 2|. 



We can never reckon on the total fall of rain over a district as the quantity available for a 

 subterranean supply. Arago tells us that not more than one-third of the rain which falls sinks 

 into the ground below one foot ; and in latitude 52°, in Great Britain, it has been found that 

 it is only the rain which falls in December, January, and February, which can be counted on as 

 feeders to springs — then, only in the coldest months of the year, can the rain sink seven feet 

 into the clay, and be thus removed from the immediate efiect of solar evaporation ; in latitudes 

 more south, a further depth must be reached to avoid evaporation, and, in parallel 32°, the 

 depth cannot be less than 12 feet; it is not, therefore, one-third the total annual fall which 

 forms the subterranean reservoir, but one-third of the quantity which falls in the three months 

 when evaporation is least ; the total fall, during this period, is 18 inches, and six inches is the 

 available amount for wells and springs, or one-sixth of the total fall. 



An amount approximating this obtains over our north temperate zone, where the fall of rain is 

 moderate (36 to 40 inches) and evaporation not excessive. But under parallel 32° it is widely 

 different; there, while the winter months are cool, so as to freeze water, little, if any, rain falls, 

 and that only above the jjlain level ; but when the mean temperature is from 75° to 90° the 

 rain falls at the time when evaporation is the greatest ; so that the chance of much water 

 sinking down 12 feet is greatly lessened. 



According to the exjjeriments of Dal ton, and the tables of Daniel, it appears that at the tem- 

 perature of 70° Fah., and during a calm, three times as much moisture is evaporated as at 40° 

 Fah. These figures being the average temperatures of Virginia and the Gadsden purchase, 

 during the period of available rain fall, it would appear that while evaporation is three times 

 greater, the fall of rain is considerably less than one-half, almost one-third, in the latter 

 mentioned place. This would give at the eastern portion of the Gadsden district 2 inches of 

 rain available for wells and springs, and at Fort Yuma one-third of an inch. 



This ratio is a very small figure to depend upon at the western limit, and it is fortunate that 

 there the travel lies along the river Gila, and thus is rendered independent of that scanty supply. 

 Chiefly for the reason given, (the great evaporation with the small rain fall,) and partly because 

 in the eastern limits there are no strata of small inclination, (except the basin districts, which 

 are not broken up or much disturbed by plutonic forces,) I have come to the conclusion that 

 sinking artesian wells is unadvisahle hetiueen the Bio Grande and Fort Yuma. 



The experiments made in artesian borings in Europe and this country have been made (with 

 a single exception) in north temperate latitudes, and none south of parallel 36°, Inferences 



