160 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



end of the first, the second, and the fourth week. Upon standardi- 

 zation after the last reading (that is, after the end of the fourth 

 week), the cup, suppose, had a coefficient of 0.70 in place of the 

 original coefficient of 0.62. A variation of 0.08 is therefore to be 

 distributed uniformly over the four- week period. At the end of the 

 first week the assumed coefficient is 0.62; at the end of the second 

 week it is 0.66 ; and at the end of the fourth week, 0.70. According 

 to the above assumption, the average coefficient for the first week 

 would be 0.63; for the second week, 0.65, etc. Assuming, then, 

 that the reading at the end of the first period — that is, at the end of 

 a week — was 500 cubic centimeters, by reducing to standard units 



we have the standard reading ^t = 315 c. c. 



E-rposure. 



This instrument will operate over a long period, the time de- 

 pending upon the size of the reservoir and the exposure to the 

 evaporating power of the air. For this reason the instrument may 

 be exposed upon remote habitats where weekly, bimonthly, or even 

 monthly readings may be made. Records of this kind, of course, 

 are less conclusive than those made at more frequent periods, 

 but nevertheless have a high value for comparative consideration. 

 Readings taken only bimonthly or so are accurate because of the per- 

 manent, rain-correcting adjustment of the instrument, a point which 

 has already been brought out in detail. 



Since the instrument is compact and light in weight, it is possible 

 to make the exposure practically wherever desired. If, for example, 

 the investigator desires to determine the evaporation rate near the 

 surface of the ground amidst low-growing, herbaceous vegetation or 

 arborescent seedlings, the reservoir is sunk in the soil so that the 

 avaporating surface of the atmometer is exposed at the height de- 

 sired. In cases where the evaporation rate is desired at various 

 heights, the instruments may be placed at definite vertical elevations 

 on light supports running out horizontally from a common vertical 

 beam. 



The principal disadvantages mentioned with respect to the Riche 

 evaporimeter have been eliminated in the construction of the porous 

 cup with its nonabsorbing attachments. It is of simple construction, 

 not extremely expensive, occupies a small space, and may be equipped 

 with a reservoir which will keep it in operation for an indefinite 

 period. Wind does not disturb it, and small animals and insects 

 have no effect upon its operation. The disadvantages are that the 

 bottle, connecting tubes, and especially the delicate nonabsorbing 

 apparatus are rather easily broken by inexperienced hands, or by 

 roving animals such as domestic stock and deer. A still more serious 



