INFLUENCE OF FOREST ON HUMIDITY. 
103 
moisten the surface of the soil, and restores it to the atmos¬ 
phere by evaporation; while in heavier rains, the large drops 
which fall upon the leaves and branches are broken into 
smaller ones, and consequently strike the ground with less 
mechanical force, or are perhaps even dispersed into vapor 
without reaching it.* As a screen, it prevents the access of 
square miles. It extends from lat. 43° 30' to 45° 20', in very nearly a 
meridian line, has a mean width of four and a half miles, with an extreme 
breadth, excluding hays almost land-locked, of thirteen miles. Its mean 
depth is not well known. It is, however, 400 feet deep in some places, 
and from 100 to 200 in many, and has few shoals or flats. The climate 
is of such severity that it rarely fails to freeze completely over, and to he 
safely crossed upon the ice, with heavy teams, for several weeks every 
winter. Thompson ( Vermont , p. 14, and Appendix, p. 9) gives the follow¬ 
ing table of the times of the complete closing and opening of the ice, 
opposite Burlington, about the centre of the lake, and where it is ten 
miles wide. 
Year. 
Closing. 
Opening. 
Days 
closed. 
Year. 
Closing. 
Opening. 
Days 
closed. 
1816 
February 9 
1836 
January 27 
April 21 
85 
1817 
January 29 
April 16 
78 
1837 
January 15 
April 26 
101 
1818 
February 2 
April 15 
72 
1838 
February 2 
April 13 
70 
1819 
March 4 
April 17 
44 
1839 
January 25 
April 6 
71 
i con 
1 February 3 
February 
l 4 
1840 
January 25 
February 20 
26 
j March 8 
March 12 
f 4 
1S41 
February 18 
April 19 
61 
1821 
January 15 
April 21 
95 
1842 
not closed 
1 S 22 
January 24 
March 30 
75 
1843 
February 16 
April 22 
65 
1823 
February 7 
April 5 
57 
1844 
January 25 
April 11 
77 
1824 
January 22 
February 11 
20 
1845 
February 8 
March 26 
51 
1825 
February 9 
1846 
February 10 
March 26 
44 
1826 
February 1 
March 24 
51 
1847 
February 15 
April 23 
68 
1827 
January 21 
March 31 
68 
1848 
February 13 
February 26 
13 
1828 
not closed 
1849 
February 7 
March 23 
44 
1829 
January 31 
April 
1S50 
not closed 
1832 
February 6 
April 17 
70 
1851 
February 1 
March 12 
39 
1833 
February 2 
April 6 
63 
1852 
January 18 
April 19 
92 
1834 
February 13 
February 20 
7 
1QQK 
| January 10 
January 23 
13 
j February 7 
April 12 
64 
In 1847, although, at the point indicated, the ice broke up on the 23d 
of April, it remained frozen much later at the North, and steamers were 
not able to traverse the whole length of the lake until May 6th. 
* We are not, indeed, to suppose that condensation of vapor and 
evaporation of water are going on in the same stratum of air at the same 
time, or, in other words, that vapor is condensed into raindrops, and rain¬ 
drops evaporated, under the same conditions; hut rain formed in one 
stratum, may fall through another, where vapor would not be condensed. 
Two saturated strata of different temperatures may be brought into con- 
