On the Reduction of Transpiration Observations. 
BY 
NESTA THOMAS, B.Sc., 
Assistant Lecturer in Botany in the Royal Holloway College, 
AND 
ALLAN FERGUSON, M.A., D.Sc., 
Assistant Lecturer in Physics in tke University College of North Wales , Bangor. 
With one Figure in the Text. 
Contents. 
§ i. Introductory 
2. Algebraic Statement of Law of Evaporation from Circular Surfaces 
3. Experiments and Results 
4. Discussion of Results 
5. Improvements in ‘Open-pan’ Methods for Calibrating Atmometers . 
6. Example of Method 
7. Effect of Observational Errors and Deductions therefrom 
8. Concluding Remarks 
9. Summary 
1. Introductory. 
I N the attempt to correlate plant transpiration with the purely physical 
process of evaporation various methods have been followed ; in par- 
ticular, the transpiration-loss may be compared with that suffered by 
a water surface (usually circular in shape) of known area, or, as is now more 
usually the case, with the loss, under similar conditions, of a porous atmometer 
of the type introduced by Livingston. But it is eminently desirable that the 
figures obtained by different investigators, under various conditions, should 
be capable of easy and accurate comparison ; and since two atmometers, 
identical in size and shape, may differ so much in other qualities (e.g. in the 
porosity of the material of which they are constructed) as to render it 
impossible to compare the data given by them, some definite method of 
calibration has become necessary. 
The plan adopted in the past and still used by some investigators is to 
compare the evaporation-loss from the atmometer with that from a circular 
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[Annals of Botany, Vol. XXXI. No. CXXII. April, 1917.] 
