ATMOMETRY AND THE ATMOMETER 109 



Records and the Correction of Readings. Atmometer records 

 should be kept in such a way that the cup number and the original 

 coefficient are always available with the record of readings. After 

 restandardization the proper coefficient for each period of opera- 

 tion is inserted and the requisite correction is made in each case. 

 If, after operation, a cup exhibits no marked change (not over 

 0.03 or 0.04,) in its coefficient, the average of the original and 

 new coefficients (the latter from restandardization) is taken 

 as the coefficient for the correction of all readings obtained 

 from that cup between the two standardizations. If, however, 

 the coefficient has changed appreciably during operation, the 

 various readings are each corrected by means of a corresponding 

 coefficient interpolated between the original and the new one. 

 If a cup shows itself incapable of restandardization, the later 

 readings cannot be corrected and are useless, though the read- 

 ings of the first week (or thereabouts) of its operation may 

 probably be corrected by means of the original coefficient. 



An example of the interpolation of coefficients follows:— 

 Suppose the original coefficient were 0.75 and the new coefficient 

 were 0.90, and suppose that the cup operated four weeks and was 

 read weekly. Let the series of weekly readings be a, b, c, and d. 

 It may be assumed that the original coefficient held for the first 

 week and that the coefficient gradually and uniformly altered 

 during the remainder of the time. Now, 0.90 — 0.75 = 0.15, 

 the amount of change in three weeks, from which we deduce that 

 the coefficients to be used for correcting the readings a, b, c, 

 and d are approximately 0.75, 0.8t), 0.85, and 0.90, and the cor- 

 rected readings become 0.75a, 0.806, 0.85c, and 0.90d. This meth- 

 od possesses far lower probability of error, in such a case than 

 would the method by which the mean of 0.75 and 0.90 (0.825) 

 is applied throughout the series. When instruments are oper- 

 ated in duplicate or in triplicate, there is usually available more 

 or less definite evidence as to whether a change in one of the 

 cups was gradual or rapid, etc. 



The Interpretation of Atmometer Readings. From what has been 

 said in the preceding discussions it appears that the thing aimed 

 at in atmometric studies is a measure of the power of the air sur- 



