106 
state of maximum saturation, or in proportion as the leaf can tran- 
spire moisture through its cell walls. . 
Therefore any observations of evaporation that we may make for 
comparative purposes can give us only the relative evaporating 
power of the wind and not the absolute evaporation from plants and 
soils. * 
THE PICHE EVAPORIMETER. 
The simplest apparatus for observing evaporation is that known 
us the Piche evaporimeter. This consists of a glass tube closed at 
the top and hung in a free exposure; the tube is less than half an inch 
in diameter and filled with water; its lower open end is closed by a 
horizontal disk of bibulous paper about twice the diameter of the 
tube; the water evaporated from this paper is supplied from within 
the tube. The observer has simply to read the height of the water 
in the tube as it slowly descends hour by hour. The number so 
read off is easily converted into one that expresses the depth of water 
evaporated per hour from unit surface. 
The following table from Montsouris Annuaire, 1888, page 204, 
shows the average evaporation thus determined by an instrument 
placed in the shade, also the corresponding temperatures and other 
data, as observed at Montsouris during thirteen years. 
Evaporation at Montsouris. 
[Averages for 1873-1885. ] 
Temper- : sot 2 f 
Month. | Nomber ature of [Gr'vapor | hin- | velocity |evapora- 
shade. | air. | midity. |of wind.| tion. 
| Xp mm. | Percent.| Kilom. mm, 
January o22- 2) ee ee ee | 164 3.6 4.8 80.9 15.9 0. 084 
NG DRURY ree re oe ee EE eeneaiae ieee 203 6.0 5.4 77. 16.1 101 
Mar Chie scp ct ease coe ste eo eee ae 281 9.1 5.4 62.9 17.8 . 187 
prilpreaatet tos ARMA Sy. Ae tees 324 12.7 6.3 57.3 17.6 225 
Minyp mips 2 teen 20 ioe eee sene eee 341 16.2 7.4 54.0 17.5 . 257 
QUITO ea ae ese se eee eae eee 330 20.0 10.1 58.2 15.3 . 232 
SU Ye ee st ao ae eeh Baer eee ee 341 22.0 ab leal 56.5 14.7 . 254 
PANO US bere see ee eee : 341 21.6 11.4 59. 4 15.7 . 234 
September 222 ss. 22292 25-2 =etGeee sees | 330 17.6 10.2 68.0 14.4 . 154 
October S222. eae we eee eee 340 12.5 8.0 73.8 15.4 099 
INGVembers 2220, wees ss fae se seers 284 8.0 6.2 77.5 18.1 091 
Deconiber:--. 2.5 ses tn cau vee | a9 3.6 4.9 82.4 | 15.6 068 
| 
- THOMAS RUSSELL’S OBSERVATIONS. 
Prof. Thomas Russell, of the Signal Office, has published results 
of some observations on the effect of the wind on the evaporation 
from the disks of the Piche evaporimeter. (See Annual Report 
Chief Signal Officer, 1888, p. 176, or Monthly Weather Review, 1888, 
—— 
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ee a ee eee nT eee 
