EVA 
Such is the quantity of vapour which would 
rise in different circumstances, on the sup- 
position that no vapour existed in the at- 
mosphere. But this is a supposition which 
can never be admitted, as the atmosphere 
is in no case totally free from vapour. Now, 
when we wish to ascertain the rate at which 
evaporation is going on, we have only to 
find the force of the vapour already in the 
atmosphere, and subtract it from the force 
of vapour at the given temperature ; the 
remainder gives us the actual force of eva- 
poration ; from which, by the table, we 
readily find the rate of evaporation. Thus, 
suppose we wish to know the rate of eva- 
poration at the temperature 59°. From 
the table, we see that the force of vapour 
at 59° is 0.5, or ji, its force at 212°. Sup- 
pose we find by trials, that the force of the 
vapour already existing in the atmosphere 
is 0.25, or the half of J 5 . To ascertain the 
rate of evaporation, we must subtract the 
0.25 from 0.5 ; the remainder 0.25 gives us 
the force of evaporation required ; which 
is precisely one half of what it would be if 
no vapour had previously existed in the at- 
mosphere. 5. As the force of the vapour 
actually in the atmosphere, is seldom equal 
to the force of vapour of the temperature of 
the atmosphere evaporation, with a few ex- 
ceptions, may be considered as constantly 
going on. Various attempts have been made 
to ascertain the quantity evaporated in the 
course of a year ; but the difficulty of the 
problem is so great, that we can expect only 
an approximation towards a solution. 
The most exact set of experiments on 
the evaporation from the earth, was made 
by Mr. Dalton and Mr. Hoyle, during 1796, 
and the two succeeding years. The me- 
thod which they adopted was this : having 
got a cylindrical vessel of tinned iron, ten 
inches in diameter, and three feet deep, 
there were inserted into it two pipes turned 
downwards for the water to run off into 
bottles : the one pipe was near the bottom 
of the vessel, the other was an inch from 
the top. The vessel was filled up for a 
few inches with gravel and sand, and all 
the rest with good fresh soil. It was then 
put into a hole in the ground, and the space 
around filled up with earth, except on one 
side, for the convenience of putting bottles 
to the two pipes ; then some water was 
poured on to sodden the earth, and as much 
of it as would, was suffered to run through 
without notice, by which the earth might 
be considered as saturated with water. 
For some weeks the soil v/as kept above 
VOL. III. 
EUC 
the level of the upper pipe, but latterly it 
was constantly a little below it, which pre- 
cluded any water running off' through it. 
For the first year the soil at top was bare ; 
but for the two last years it was covered 
with grass, the same as any green field. 
Things being thus circumstanced, a regular 
register was kept of the quantity of fain 
water that ran off from the surface of the 
earth through the upper pipe, (whilst that 
took place), and also of the quantity of 
that which sunk down through the three 
feet of earth, and ran out through the lower 
pipe. A rain gauge of the same diameter 
was kept close by to find the quantity of 
rain for any corresponding time. The 
weight of the water which ran through the 
pipes, being subtracted from the water in 
the rain-guage, the remainder was consi- 
dered as the weight of the water evaporated 
from the earth in the vessel. From these 
experiments it appears, that the quantity 
of vapour raised annually at Manchester is 
about 25 inches. If to this we add five 
inches for the dew with Mr. Dalton, it will 
make the annual evaporation 30 inches. 
Now, if we consider the situation of Eng- 
land, and the greater quantity of vapour 
raised from water, it will not surely be con- 
sidered as too great an allowance, if we esti- 
mate the mean annual evaporation over 
the whole surface of the globe at 35 inches. 
Now, 35 inches from every square inch, on 
the superficies of the globe, make 94,450 
cubic miles, equal to the water annually 
evaporated over the whole globe. Was this 
prodigious mass of water all to subsist in 
the atmosphere at once, it would increase 
its mass by about a twelfth, and raise the 
barometer nearly three inches : but this 
never happens ; no day passes without rain 
in some part of the earth ; so that part of 
the evaporated water is constantly precipi- 
tated again. Indeed it would be impossible 
for the whole of the evaporated water to sub- 
sist in the atmosphere at once, at least in the 
state of vapour. See Manchester Memoirs. 
EUCALYPTUS, in botany, a genus of 
the Icosandria Monogynia class and order. 
Essential character : calyx superior, perma- 
nent, truncate, before flowering time covered 
with a hemispherical, deciduous lid ; co- 
rolla none ; capsules four-celled, opening 
at the top, inclosing many seeds. There 
are two species, viz. E. obliqua; oblique 
leaved eucalyptus, and E. resinifera; red 
gum tree. • These are both very large and 
lofty trees, much exceeding the English oak 
both in height and bulk. E. resinifera, con- 
F 
