412 



REPORT OF THE COMMISSIONERS OF 



about both places is an elevated, dry region, entirely destitute of any water on the 

 surface, and with very little timber, except small fringes along the margins of streams. 

 It is perfectly clear to the author, from former experience in central and west Texas, 

 that while the mean temperature at these two places are substantially the same, the 

 natural evaporation, except accidentally, is much greater at El Paso than at Abilene, 

 although it may be pointed out that since wind movement is one of the controlling 

 factors in evaporation, the lower average velocity of wind at El Paso will tend to 

 reduce the average evaporation there. The difference in rainfall at the two places 

 must also be taken into account in considering actual evaporation results. The broad 

 proposition is that with equal rainfalls at the two places, other conditions remaining 

 as shown by the tabulation, the evaporation either from water or from land surfaces 

 will probably be greater at El Paso than at Abilene, although, possibly, the difference 

 in wind velocity might accidentally make the two not very different. Similar com- 

 parisons can be made for other stations, as, for instance, between Block Island and 

 Boston, where the mean temperatures are nearly the same, but where there are large 

 variations in the other elements controlling evaporation. A coincidence in evapora- 

 tion depths at these two places would merely mean an accidental balancing of all the 

 forces influencing evaporation. We need, therefore, not only tabulations of mean 

 monthly and yearly temperature as a necessary part of the study on this line, but 

 tabulations of all the other elements influencing evaporation before we can decide. 



CONSTANCY OF TEMPERATURE. 



Temperature is a tolerably constant meteorological element, as may be inferred 

 from the following statistics of mean annual temperatures.* 



At St. Petersburg there is a record of temperature since 1743, the mean of which 

 to 1875, inclusive, is 38. 6°, the highest annual temperature being 42. 4 in 1822, and 

 the lowest 34. i° in 1815. At Paris, records kept from 1735-1890, inclusive, show a 

 mean temperature for the whole period of 5 r .4°, the highest annual temperature of 

 5 7. 6° occurring in 1781. 



The following are the annual temperatures at Philadelphia for various groups of 

 years from 1 758-1889, inclusive: 



1758 to 1777, 



. 52.6" 



1798 to 1804, ..... 



. . . 54.2 



1829 to 1838, 



■ 5i-5° 



1825 to 1845, ..... 



53-i° 



1846 to 1867, 



54-o° 



1871 to 1889, 



53-i° 



For these long temperature records, see Russell's Meteorology, pp. 103, 104. 



