160 BULLETIN 1059, U. S. DEPABTMBNT OF AGRICULTUBE. 
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 
i i i i v 50 ° X 63 _ 
we have the standard reading ^)0 ~=*>lo c. c. 
Exposure. 
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 period-. 
but nevertheless have a high value for comparative consideration. 
Eeadings 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 i- 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 support- running out horizontally from a common vertical 
beam. 
The principal disadvantages mentioned with respect to the Piche 
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 dues 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 
