Semi-Centennial Volume. 127 



tensity reaches its maximum. Unfortunately, there is no simple means 

 of determining the amount of water lost from the soil day by day as a 

 drouth proceeds. But we can measure the evaporating power of the 

 atmosphere by measuring- the actual work accomplished by the climatic 

 factors causing evaporation. 



Although the weather bureau has been in operation twenty-eight 

 years, it is only recently that it has become properly interested in the 

 evaporating power of the air as a factor in crop production and weather 

 conditions in general. While a good many observations of various kinds 

 are on record, the conditions under which the observations have been 

 taken are so diverse that comparison of one locality directly with another 

 is entirely out of question. In order to secure comparable data it was 

 necessary to standardize the methods of taking observations. This has 

 been brought about by the establishment recently of what are known as 

 Standard Class A Evaporation stations in various localities, principally 

 in the arid and semi-arid regions of the United States. 



The Standard Class A station is constructed according to specifications 

 furnished by the government, and consists of an evaporation pan four 

 feet in diameter and ten inches deep, which is filled with water to a 

 depth of seven or eight inches, and freely exposed to air and sunlight. 

 A round brass cylinder called a still well sits in this pan of water and 

 protects the water surface within it from being ruffled by the wind. This 

 makes accurate readings possible. An accurate measuring instrument 

 known as a hook gage is set on top of this well, and the level of the 

 water in the pan is measured every day at a certain hour. The gage 

 is constructed to read to the thousandth of an inch. In addition to the 

 pan there is a rain gage to correct the readings of the level in case of pre- 

 cipitation, an anemometer to measure the wind flow across the surface 

 of the pan, and a pair of thermometers to give the daily temperature 

 range. All of these instruments are arranged according to a standard 

 plan, and enclosed by a standardized net wire fence designed to offer 

 little obstruction to light and air currents. By having every feature 

 standardized every station is as near like all the rest as it can be made, 

 and the readings from diff"erent stations can be compared directly. The 

 tirst of these stations to be established was the Mitchell, Neb., station, 

 which began observations in April, 1911, under the reclamation service. 

 But in 1915, a model Class A station was established at the American 

 University, Washington, D. C, which began its observations on April 

 18, 1915. Two other stations were established in 1915, one at Gardiner, 

 Maine, and one at the Roosevelt dam in Arizona. 



During the meeting of the Botanical Society of America held at 

 Columbus, O., in December, 1915, a public announcement was made that 

 the government had decided to attack the evaporation problem in earnest, 

 and expected to establish a number of stations in advantageous locations. 

 During 1916 twenty-five stations were established, and two were added 

 in 1917. On January 1, 1918, therefore, there were thirty stations on 

 the continental United States, and one at San Juan, Porto Rico. The 

 map shown on plate I gives the locations of all active stations except 

 San Juan. 



