Reduction of Transpiration Observations. 255 
obtained for n should give important information as to the magnitude of the 
air film which is at rest in contact with the surface. Such information could 
probably be usefully applied to the analogous case of a transpiring leaf 
surface exposed to a breeze. We hope in the future to communicate the 
results of experiments dealing with this point. 
The greater part of the above work was done in the Physics Depart- 
ment of the University College of North Wales, Bangor, in the summer 
of 1916, and we desire here to express our appreciation of the assis- 
tance given to us by Professor E. Taylor Jones in generously placing 
at our disposal the resources of his department. Our thanks are also 
due to Professor V. H. Blackman, who has criticized our manuscript 
and to whose knowledge of the literature of the subject we owe several 
of our references. 
9. Summary. 
Experiments are described which show that the evaporation from 
a circular water surface is not, as commonly assumed, proportional to its 
area, unless the depth of the surface below the rim of the containing vessel 
be greater than two or three centimetres ; nor is it proportional to the linear 
dimensions of the basin, but for basins ‘ full ’ of water it is approximately 
proportional to (radius) I. 
Serious errors — sometimes of the magnitude of 40 per cent. — may 
therefore arise in determining the water surface equivalent to a given atmo- 
meter, and methods of calibration are described by which such errors may 
be avoided. 
The figures usually given to show the relation between the evaporation 
from a transpiring leaf surface and a circular water surface of the same area 
are also subject to serious errors, and, simple as the experiment is, stand in 
need of re-determination. 
