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Thomas and Ferguson . — On the 
and subsequently he has calibrated the instruments by comparison with 
a standard atmometer. 3 ’ 2 If comparable results are desired by investigators 
in widely separated laboratories, this entails the possession of secondary 
standards which have been calibrated by comparison with the standard 
atmometer — a condition which is evidently inconvenient and far from ideal. 
Especially when one is aiming at an accuracy of one or one-half of one per 
cent., it seems preferable to attempt to realize a standard which shall be, as 
we have already pointed out, invariable under given conditions and capable 
of easy reproduction at any place or time. The free surface of water con- 
tained in a cylindrical vessel and protected by a rim of 3 cm. depth fulfils 
these conditions. Moreover, the use of a free water surface for purposes of 
calibration avoids the assumption that the porous properties of the porcelain 
of the standard atmometers remain unchanged with time. 
It may be that other factors — such as convection — may still introduce 
such errors as to preclude the use of the open pan as a standard. But we 
think that these sources of error have been somewhat too hurriedly invoked 
to explain observational discrepancies, while the greater sinner — n — has 
been allowed to pass unrebuked. 
We do not think that the errors introduced by convection are likely to 
be as large as is commonly assumed ; were this the case, the logarithmic 
curves obtained for a series of basins of widely differing radii, tested under 
all sorts of external conditions, would not be so accurately linear as they 
actually are. But this point can only be settled by experiment, and it seems 
to us that it is well worth while to give the perhaps too hastily discarded 
water surface a further trial. 
This is all the more desirable, since the only other alternative — that of 
standardization by comparison with a standard atmometer — is at best a pis 
alter . The impossibility of standardizing one’s instrument oneself is an 
inconvenience, and the fact that the standard alters as time goes on points to 
the possibility of the introduction of errors which, small in themselves, may 
in time accumulate to something very serious. 
There are several other points of interest to the physiologist which are, 
perhaps, outside the scope of this paper ; in particular, the evaporation from 
small apertures in currents of air of various velocities demands accurate 
experimental investigation. We have seen that, ceteris paribus , the change 
from a quiet room to a slightly disturbed atmosphere produces a small but 
definite alteration in the value of n, and this change is primarily due to the 
distortion of the lines of flow of the vapour in the neighbourhood of the 
evaporating surface caused by the motion of the air. If, therefore, quan- 
titative experiments be made on the evaporation from small apertures in 
streams of air moving with various velocities, the variation in the values 
1 Livingston : A Rotating Table for standardizing Porous-cup Atmometers. Plant World, 15, 
1 912. 2 Ibid. : Atmometry and the Atmometer. Plant World, 18, 1915. 
