ATMOMETRY AND THE ATMOMETER 71 



designated standards, and their coefficients were arbitrarily 

 placed at unity. Thus those which lost more water had coeffi- 

 cients below 1.00 and those losing less had coefficients above 

 1.00. It was to individuals with coefficient of unity, as thus 

 established, that the earlier 8-cm. cups were standardized, under 

 the conditions of early summer in Tucson, Arizona, and the 

 majority of this larger type gave coefficients of about 0.73. It 

 is doubtful whether this coefficient would stand, as showing the 

 relation of the 6-cm. to the 8-cm. cup, under all climatic condi- 

 tions — since the two instruments are not strictly of the same form, 

 but only similar in certain respects. However, the 6-cm. cups are 

 no longer in general use and the application of the coefficients 

 of the 8-cm. standards now used simply reduces their readings 

 to terms of an hypothetical cup, whose loss, under the same 

 surroundings, is taken as *££ of that from the average 8-cm. 

 cup. For practical purposes, as far as experience has gone, it 

 seems quite safe actually to standardize 8-cm. to 6-cm. cups, 

 and vice versa, though the procedure is obviously objectionable 

 on theoretical grounds and has not been latterly practiced by the 

 writer. 



The above explanation shows why it is, as some users of these 

 cups may have noticed, that 6-cm. cups (about half shellacked) 

 have coefficients approaching 1.00 when new, while the co- 

 efficients of 8-cm. cups, as now marketed, are in the neighbor- 

 hood of 0.73. Actually, these latter values usually lie between 

 0.67 and 0.79. 



In general it may be said that if the reading of any 8-cm. 

 cup be multiplied by its coefficient the result is the same 

 as though a large group of similar 8-cm. cups were operated 

 under the same conditions as those obtaining around the one in 

 question and the mean loss of the group were multiplied by 

 0.73. In other words, the average 8-cm. cup is now arbitrarily 

 taken as having a coefficient of 0.73 instead of 1.00. 



An important precaution in standardization arises from the 

 fact that the porous clay surface is apt to alter with continued 

 use. Such alterations become evident as change in the value 

 of the coefficient, and may be due to dust accumulation, to salts 



