70 VEGETATION OF A DESERT MOUNTAIN RANGE. 



intervals from 4,000 to 9,000 feet. All of the instruments in this series 

 were located on the summits of ridges, so as to give comparable readings 

 from similar topographic situations. In addition to the six instruments 

 in this series there were also thermometers in the bottoms of cafions at 

 5,000, 6,000, and 7,600 feet; a thermometer was exposed on the top 

 of the fire tower of the Forest Service on Mount Lemmon, the actual 

 elevation of the instrument being 9,225 feet, and thermometers were 

 buried in the topmost layer of soil at 6,000 and 8,000 feet. 



Alcoholic minimum thermometers were used in the earlier years of 

 these observations, but were replaced by mercuric Six's thermometers 

 in 1913 and 1914. Various types of thermometer have been used at 

 the station at 7,600 feet, and as many as three instruments have been 

 exposed simultaneously at that place. All thermometers have been 

 calibrated before use and have been verified in place from time to time 

 by comparison with a portable thermometer of known error. The 

 readings of the thermometers have been taken at irregular intervals, 

 as opportunity afforded, and most of the figures secured are for periods 

 of several weeks, or for the several months which elapse between the 

 last visit in the autumn and the first in the spring. Only at the 7,600- 

 and 9,000-foot stations has it been possible to expose the thermometers 

 in such a manner as to secure reliable maxima; at all other stations 

 the only data secured have been the absolute minima for the intervals 

 between visits. The placing of the thermometers in small boxes, with 

 numerous perforations, has made possible the securing of good minima, 

 but no record has been made of the maxima secured under such con- 

 ditions of exposure. The conspicuousness of adequate instrument 

 shelters would have invited human interference with the thermometers 

 which would have been productive of errors. 



A few records of temperature from the same locality for a number 

 of consecutive days have been secured by Professor J. G. Brown and by 

 Dr. H. A. Spoehr, as well as by the writer. A large number of single 

 observations of minima and of current temperatures have been made 

 by the writer at various localities, and it has been possible to use these 

 in connection with data from the regular stations in determining the 

 normal gradient of temperature decrease and in ascertaining the verti- 

 cal shortening of the frostless season. 



LENGTH OF FROSTLESS SEASON. 



It has been impossible, for the most part, to make direct observations 

 of the dates of last vernal and first autumnal occurrence of a tempera- 

 ture of 32° F. at the several stations on the Santa Catalinas. The 

 dates at which visits were made to the mountain were occasionally 

 such as to estabhsh the dates exactly for one of the stations, and in 

 several cases visits were made at such frequent intervals as to place 

 the date within a week or two. In the majority of cases, however, 



