105 
Summer wind roses of evaporation at Montsouris. 
ind. Daily | 
wae Average minima | Evapo- 
daily | ofrela-| ration |_,2otal 
tempera-| 4; < rainfall. 
mirection Number} Hourly |"ture tive hu-| daily. 
; 3 of days. |velocity. * | midity. 
Sec. kilo. (6 Per cent.| mm. mm. 
HWromenorthnisee. cs seca season eens se 8 12.8 18. 24 45.9 6.35 0.0 
IMronnnorune@ast=a- 6. 2555-2 -250 = sc=. 8 14.6 19. 02 46.4 9 bil 4.4 
LOM CAStm2cres = sc. eee RE oss a eee: 5 10.1 20.54 45.4 4,72 0.0 
Mronisoutheasty: ~22-22 22-2 2e-s. oe a= 8s 4 7.2 20.08 47.8 4.15 0.0 
ITOMMSOUGN SE! = os ose = Sc esees cee cne 10 11.0 19.71 55.2 2.37 45.8 
Hromisouthwest: 22.226 2522224 222 ce se 20 15.4 18. 72 51.9 3.54 39.3 
HMTOMRWOS tit sas s2555 65s see ece ese cs= 7 20 14.2 17.21 50.8 3. 60 23.8 
Hrommotihwestissssess.--— eee eee 12 11.2 17.00 51.3 3.70 10.4 
VWEHATO | Fas 2 er eee neers 5 14.8 17.76 46.6 3.17 0.7 
Threenmonthseessos- se 22 see eso se “92 eee eee | ees raat Spade os Lad gore St So 124.4 
We see that the driest winds, or those whose relative humidity is 
small, such as the north and east winds, give a large evaporation, and 
that the velocity and temperature of the west winds, which are a 
little less than those of the southwest winds, does not compensate for 
the dryness, which latter enables them to evaporate a little less than 
the southwest winds. 
By multiplying the average daily evaporation by the number of 
days we obtain the total evaporation from the saturated paper of the 
Piche instrument. This exceeds the total rainfall, but we are not to 
infer that the evaporation from ground and leaves must also neces- 
sarily exceed the rainfall, although this is generally true for the sum- 
mer season. 
BOSTON DATA FROM E. J. FITZGERALD. 
The evaporation of the water from leaves and from the ground 
depends upon the temperature, wind, and humidity of the air. It 1s 
a rather complex result; if the above-mentioned elements remain con- 
stant for any time at the surface of the mass of water the evaporation 
from that surface will be closely represented by the following formula 
which is due to Fitzgerald, of Boston, 2 
E=0.0166 (P—p) (1+4 W), 
where W is the velocity of the wind in miles per hour; P the tension 
of vapor in inches of mercury corresponding to the temperature of the 
water ; p is the tension of vapor corresponding to the dew point in the 
free air; E is the evaporation expressed in inches of depth of water 
evaporated per hour under atmospheric pressure between 29 and 31 
inches of the barometer. 
The evaporation from leaves and soils is usually less than that from 
water about in the proportion in which the soil approximates its 
