CLIMATIC CONDITIONS OF THE UNITED STATES. 305 



readings from all cups related to the Livingston cylindrical standard, 

 wherever and whenever they may have been operated. 



The cups were freely exposed to sunshine and wind in every case. 

 The center of the cup was 15 cm. above the soil surface, so that our 

 records may be considered as proportional to the evaporating powers 

 of the air (as this feature would affect the transpirational water-loss 

 from a plant of about the above-named height, growing in the open) 

 at the respective stations and for the respective time periods. The 

 mountings were of the simple absorbing type^ and some rain absorp- 

 tion is undoubtedly involved in the records. At that time the non- 

 absorbing form had not yet been devised. Weekly readings were made. 



When the coefficient of correction of any cup altered, as shown by 

 the mid-summer or final calibrations, interpolations were made to 

 give probable coefficients for each of the weeks intervening between 

 calibrations, it being borne in mind that, where two cups operate side 

 by side, their weekly readings constitute a continuous series of measure- 

 ments of the relative fluctuations of their coefficients. After the 

 derivation of the coefficients for each week of the entire series of 

 observations, all readings were corrected (by multiplying the actual 

 reading by the corresponding coefficient), and the resulting pair of 

 records (when two cups had operated simultaneously at the same 

 place — that is, excepting for the period of mid-summer calibration, 

 when but one cup was exposed) were averaged, to give the corrected 

 rate for each week and station. 



To the observers at the 38 stations involved in this study our sincere 

 thanks are due; without their kind and sustained help the study could 

 not have been carried out at all. The names of the observers who so 

 generously cooperated in this work have been given by Livingston 

 (Plant World, 1911). 



Owing to various considerations, observations were not begun at 

 the same time at all stations, and were not discontinued simultaneously. 

 Also, accidents and interruptions of various sorts occurred, so that the 

 final records are of unequal completeness in several respects. Such 

 as they are, they are presented in table 16. 



The observations will be considered in five 5-week periods and a 

 final 3-week period, and each of these periods constitutes a sub-' 

 division of table 16, which gives the weekly evaporation data and also 

 the corresponding weekly records of precipitation whenever the latter 

 are available. The units employed for evaporation are cubic centi- 

 meters (of water lost by the standard cylindrical porous-cup atmom- 

 eter), and precipitation is given in inches of depth in the customary way. 



^ The mounting as actually used is described and figured in the following papers: Livingston, 

 1907, a. — Idem, A simple atmometer. Science, n. s., 28: 319-330, 1908. The non-absorbing 

 mounting was first described by Liv-ingston (Livingston, B. E., A rain-correcting atmometer for 

 ecological instrumentation, Plant World 13: 79-82, 1910). It was improved by Shive (Shivo, 

 J. W., An improved non-absorbing porous cup atmometer, Plant World, 18: 7-10, 1915). See 

 also Livingston, 1915, a. 



