Marsh , and its Relations to Evaporation , &c. 285 
therefore adopted. But the evaporimeter as finally used in my experiments 
was different in many respects 
from that of Livingston, and 
it may therefore be described 
here. 1 
Text-fig. 3 represents a ver- 
tical section of the evaporimeter 
in question. It consists essen- 
tially of a porous cylinder of 
earthenware (A), filled with dis- 
tilled water. 2 As the water per- 
colates through the pores of the 
earthenware, and evaporates 
from its surface, it is replaced 
by water drawn up from the 
reservoir (B) below. This re- 
servoir is filled through the 
tube (C), until the water reaches 
a certain mark on the glass 
reading-tube (D). The porous 
cylinder remains constantly 
full of water, the latter being 
supported by air pressure from 
below, as in a water barometer. 
But as evaporation proceeds 
the water-level in the reservoir 
sinks. On again filling up to 
the original level by means of 
a burette, the loss of water is 
directly measured in c.c. The 
last parts to be filled are the 
tubes (C) and (D), both of 
which have a small sectional 
area. It is thus possible to 
read the water-loss to a single 
drop, i. e. to about 0-05 c.c. 3 
1 I wish to express my warmest thanks to Dr. F. F. Blackman, not only for suggesting a number 
of the more important features adopted in the evaporimeter, but also for the helpful interest he has 
taken in the progress of the work generally. 
2 Porous battery pots of good quality were used, the closed end being sawn off. About 2*5 cm. 
at each end was then soaked in a mixture of melted paraffin wax and resin, and finally coated with 
enamel. This limited the actual evaporating surface to a definite known area, about 18*0 by 7*9 cm. 
The size of the cylinders now in use is about 13 cm. long, with a diameter of 5*8 cm., and a thick- 
ness of .5 cm. The method of calibrating the various cylinders is given in the Appendix, pp. 37 et seq. 
3 Transeau (’ 08 , p. 219), used the same porous pots as Livingston, but for a water reservoir 
