[32 J 
^2 
The rain and evaporation in summer belong more especially to our 
computation. The following results, for the several months of the 
year, $ire from long observation in England, 
Inches evaporation Inches from 
from Ground. Water. 
In January, the rain 
2.46 
1.01 
1.50 
February 
do 
1.80 
.53 
2.00 
ISlarch 
do 
.90 
.62 
3.50 
April 
do 
1.72^ 
1.49^ 
4-50^ 
May 
do 
4.18 
2.69 
4. 96 
June 
do 
2.48 
2-18 
" - 4.06 
. 6 49 I 
5. 63 f 
July 
do 
4 15 
August 
do 
5.55 
3.38 
6.06 
Scptcnibcr 
do 
3 28^ 
2.95^ 
8.90^ 
October 
do 
2. 90 
2.67 
2.35 
November 
do 
2.93 
2.08 
2.04 
December 
do 
3.20 
1.48 
l.oO 
53.55 
25.14 
44.4S 
The six summer months, 19.36 16.75 31.54 
The difference, 12.18, or at least one foot more evaporation thau 
jain. Bit ^iiese experiments must necessarily have been tried on a 
gmall and great sui-facc; but the sui'face of a reservoir is exposed to 
the wind: This exposui'e to the cause of evaporation is increased by 
the agitation of the surface of the water in waves, and even by its 
mechanical force to take up and carry off the broken, air-commin- 
gled particles, facilitating the chemical union of the water witli air, 
according to the received theory: of its solution by means of caloi'ic; 
the one lluid combining with the other through its agency. But 
against tins excessive evaporation by the wind, v^^e may set the re- 
duced temperature of this lofty region. The water being held in the 
air by means of caloric, the presence of compai'atively cooler strata 
of the air is often evinced in tlic sudden ]»roduction of clouds and 
rain. Moisture, copiously exhaled in tlie warm latitudes of the Gulf 
of Mexico, and thi-oiighout the vast valleys of the Mississippi, the 
Ohio, the Cumberland, the Tennes.see, the Alabama, is borne by the 
prevailing winds (jf spring and stnnmer in contact with the Appalla- 
chian range, and there meeting with the winds of t'ae north and the 
east, unite and precipitate those copious and genial sliowers \^'hich 
suiiply every river and fertilize every state. 
While natural philosophy and experience permit no doubt of tlie 
existence of those regular causes of an ample supply of rain in the 
spring of the year, to fill great reservoirs, we must not omit an inte- 
resting experiment in Scotland, which establishes another law, cor- 
roborative of those, but applicable to the summer season, to prove 
that it rains more, even on moderate eminences than on the plains. 
Two rain gauges were employed, one upon a hill 600 feet ahove the 
