Blackman and Paine . — A Recording Tran spirometer. 113 
its orifice like a stalagmometer tube, can be used for dropping, but it is hardly 
necessary. 
In the apparatus shown in the photograph, instead of a simple revolving 
drum, which is sufficient for ordinary purposes, there is a more elaborate 
drum (seen in the centre beneath the clock) with six pens, so that a number 
of comparative observations can be made. In this case a transpiring plant 
is being compared with the evaporation from a porous cup. A comparison 
of two such tracings enables one to eliminate the effect of such factors as 
humidity, temperature, air-currents, &c., which act directly on transpiration 
and evaporation alike. 
A record obtained by this means is shown in the text (Fig. 2). The 
top line shows the time record, made by connecting a special clock (shown in 
the centre of the photograph (Fig. 1)) with a battery and one of the six pens 
mentioned above. The thick, long lines represent hours, the thin, short ones 
half-hours ; the hours between sunset and sunrise are indicated by the 
black lines below them. The second line is the transpiration record, the 
lowest line that of the porous cup. The effect of light and darkness is 
brought out very sharply ; while the transition from day to night has no 
effect on simple evaporation from the porous cup, the rate of transpiration 
is very much greater in the light, as is shown by the greater closeness of 
the vertical lines when the plant is illuminated. It will be noticed that 
the record shows that the rate of transpiration fell off as the experiment 
proceeded. In the absence of a record of simple evaporation under the 
same conditions, such a reduction might have been put down to some 
change in the plant. A glance at the lowest record shows that the 
evaporation from the porous cup is similarly reduced. In fact, while the 
experiment was started in warm dry weather, it was concluded in cold 
damp weather . 1 
Imperial College of Science and Technology, London, S.W. 
1 By arrangement with the authors the various parts of the apparatus are supplied by Messrs. 
Baird and Tatlock, Cross Street, Hatton Garden, London, E.C. 
I 
