CLIMATE OF THE SANTA CATALINA MOUNTAINS. 63 



in comparison with the April, May, and June readings, show moistures 

 of about the same amount, indicating that the after-summer is often 

 a season of as great soil aridity as the fore-summer. 



The data for shaded and unshaded soil, both in May and September, 

 corroborate similar determinations made on Tumamoc Hill and go to 

 show that in the arid seasons the influence of shade in sustaining the 

 moisture of soil is so slight as to be negligible. The influence of shade 

 in retarding the desiccation of the soil just after a rain is not without 

 its importance, but in the Desert and Encinal regions the soil in the 

 shade of trees will soon reach as low a percentage as that in the full sun. 



EVAPORATION. 



It has been frequently pointed out, in recent botanical literature, 

 that the measurement of the evaporative power of the air affords a 

 concise expression of the combined effects of temperature, humidity, 

 and air movement in so far as these factors affect the loss of water by 

 plants. The obvious importance of these factors and consequently 

 of evaporation in the environmental complex of the Santa Catalinas 

 led to the early installation of a series of atmometers (or evaporimeters) 

 at several elevations in these mountains. In the summer of 1906 Dr. 

 B. E. Livingston secured data from three porous cup atmometers at 

 elevations of 6,000, 7,500, and 8,000 feet.* In 1908 and 1910 the 

 writer installed series of atmometers at five elevations, from which 

 readings were secured which are not sufficiently complete and reliable 

 to be worthy of publication. In 1911 a new series of atmometers was 

 installed at the six rainfall stations, from 3,000 to 8,000 feet inclusive, 

 at 1,000-foot intervals. These instruments were exposed in pairs, on 

 north and south exposures, and were operated in the most careful 

 manner, in accordance with the experience of the two preceding years. 

 The atmometers were read at fortnightly intervals, or nearly so, and 

 at each reading fresh cups were installed. The actual readings were 

 reduced to standard by the use of an average between the original and 

 the final coefficients of correction. Only good distilled water was used, 

 and it was conveyed in tin canteens (rather than galvanized iron ones) 

 from which the resin remaining from the soldering had been removed 

 with carbon bisulphide. The bottles used for the atmometers had a 

 capacity of 1 gallon at the lower stations and of 2 quarts at the higher 

 stations, such ample amounts of water providing against the possible 

 danger of the atmometers going dry. The stoppers in the mouths of 

 the bottles were made very tight, to prevent the cups from blowing 

 loose, but were provided with grooves to admit air. These grooves 

 were stopped with loose cotton, to prevent the entrance of ants, and 

 the stoppers were covered by aprons to exclude ram. The atmometers 

 were all exposed in situations such that they received full insolation 



* Livingston, B. E. Evaporation and Plant Habitats. The Plant World, 11: 1-9, 1908. 



