672 TRANSACTIONS OF SECTION K. 
8. The instrument may be placed in almost any conceivable position, as 
at different heights above the soil surface, &c. 
9. The porous cup may be read for much shorter time-periods than can 
any simple evaporator pan. 
Coefficient of Correction for the Porous Cup Atmometer.—This is found 
by comparing the evaporation rate, under the same conditions, from a given 
cup and from a standard cup, or from a standard water surface. The 
effective evaporating surface of every cup is apt to change with use, there- 
fore the method of standardising to standard cups is rather unsafe. In 
standardising to a free water surface the cups to be standardised are exposed 
in a room or greenhouse (in the absence of wind) alongside a series of 
several pans. The latter are of such size and form as to present approxi- 
mately the same evaporation surface as the cup, the volume of the water 
contained being automatically kept approximately the same as that of the 
cup. The entire stand, with pan and constant-level apparatus, is weighed 
at intervals of twenty-four hours, when the cups are also read. The average 
loss from the pans is taken as the standard, and is divided by the loss from 
the several cups respectively, the quotient being the coefficient. When the 
cups are in use every reading is brought back to standard by multiplying it 
by the coefficient for its particular cup. 
The alteration of the coefficient appears to be due in part to the accumu- 
lation of dust upon the clay surface, but in larger part to movements of 
soluble salts within the wall of the cup. With high evaporation rates the 
cups often become virtually glazed on the outside, owing to the outward 
diffusion of these salts and their recrystallisation on the surface. Cups 
are also often clogged by the growth of micro-organisms within and upon the 
cup. This occurs with low rates of evaporation, usually only with high 
humidity. 
It is therefore necessary to redetermine the correction coefficient from 
time to time—at intervals of a month or less. This can be done, without 
interrupting the series of observations, by simply replacing the cup by a 
new one which has been standardised, the old one being returned to the 
laboratory for retesting. The coefficient for any period is either the average 
coefficient obtained from the original and final tests, or it may be deter- 
mined by interpolation for the different partial periods as determined by the 
actual observations. The change in coefficient in a properly operated cup 
is usually practically negligible for periods not exceeding a month or six 
weeks, but safety requires that this fact be established for each instrument 
as it is operated. 
The growth of organisms in and upon the cup is prevented by an initial 
rinsing of the cup on the inside with a solution of mercuric chloride. The 
small amount of this salt necessary has no effect on the behaviour of the cup. 
The cups should be handled without touching the evaporating portion. 
It is found desirable to coat with melted sulphur, sealing-wax, or shellac 
the basal portion of the cups, thus giving a safe surface for handling. Of 
course, nothing but distilled water may be used in all operations with this 
atmometer. 
Treatment of Clogged Cups.—When cups become clogged so as not to be 
able to transmit water as rapidly as it is demanded by the evaporating 
power of the air, they may be renewed by removing a thin layer of the 
clay from the outside. They are first dried, and then scraped with freshly 
broken glass, or, better, ground on a lathe with a medium grade of sand- 
paper. Now and then it occurs that this treatment will not suffice, in 
which case it is best to discard the cup entirely. After scraping or grinding 
restandardisation is, of course, necessary. 
