26 
VILLAGE LECTURES.—NO. 3. 
OATS A NT EXHAUSTING- CROP. 
Von Thaer calculates the exhaustion of a soil 
upon an average at 25 per cent.; that is, each 
succeeding crop will be one fourth less than 
the preceding one, unless the loss is repaired by 
rest, pasture, summer fallow, or manure. 
All culmiferous crops are exhausters of the 
soil, particularly so during the formation of 
their seeds. They are fibrous-rooted, spread 
themselves near the surface, and draw their 
nourishment principally from the upper stratum 
of soil. 
It is apparent that we cannot take two or 
more such crops from the same field, in succes¬ 
sive seasons, without a manifest falling off in the 
product. The reason of this, is, nature has 
provided for each species of plant, a specific 
food, suited to its organisation and wants. 
-- 
VILLAGE LECTURES.—No. 3. 
The Soil and the Air Continued .—Let me furnish 
other proofs of the fact that most of the bulky 
part of our plants is derived from the air. I 
have already proved it by showing that there is 
no other source except the air from which a 
plant can get its combustible part, and there 
are two other ways in which I can prove it—I 
can show you that the air is heavy enough to 
render it very likely that it contains enough of 
substance to grow plants of, and I can show you 
that it actually does contain the very things on 
which plants feed. 
Why has the soil always been supposed to 
furnish the substance of plants'? Is it because 
there is enough of it—good heavy stuff, that 
you might suppose able to build up heavy sub¬ 
stantial plants and trees ? Why the air sur¬ 
rounding the earth is at least twenty times as 
heavy as all the surface soil surrounding the 
earth—even supposing it to be on the average 
twelve inches deep ? Though it is so easily 
moved through, the air is heavy enough, I can 
tell you. There are 15 pounds’ weight of it 
resting on every square inch of ground. The 
whole atmosphere of the globe weighs as much 
as a ball of lead would weigh, though it were 
sixty miles in diameter. You can judge in some 
measure of the weight of the air by the way in 
which it will stretch a piece of India rubber 
extended over an open jar ficted to the plate of 
an air pump. If it were held at the four cor¬ 
ners, and weights piled upon it, they would 
stretch it, and if heavy enough they would 
break it; but if it rested on a stool they could 
not. Just so the air above it cannot stretch it 
now, because it rests on the air beneath; but 
remove the stool and the weights will stretch it • 
remove the air beneath, as can be done by this 
pump, and the air above will stretch it and 
burst through it. 
But we can actually weigh the air. Take a 
bent tube, three feet long, filled with quicksil¬ 
ver, and invert it. Why does the liquid metal 
stand 30 inches higher in one leg than in 
the other. If the air were pressing on both 
ends alike, it would stand at a common level in 
both branches of the tube; the reason why it is 
standing so high here must be because there is 
something pressing on the one surface which is 
not pressing on the other—the reason why it is 
standing so high in the one side is because the 
air is pressing only the other. The fact must 
be that the weight of the air pressing on the 
surface of the metal in the one leg of the tube 
is just the same as the weight of quicksilver 
above that level in the other. The air is sup¬ 
posed to be forty-five miles high, and I say that 
supposing this tube extended forty-five miles 
high to the outside of the atmosphere, this leg 
would contain the same weight of stuff in it as 
that, or else the balance would not be main¬ 
tained. This is in fact a balance, weighing 
whatever is put into one leg of the tube by the 
height to which it will raise the quicksilver in 
the other; the air pressing here raises the quick¬ 
silver there to a height of 30 inches; that is, 
the weight of air pressing on every square inch 
of the earth’s surface is the same as if 30 
inches deep of quicksilver rested on every 
square inch. Now, 15 pounds’ weight of the 
quicksilver would rest on every square inch, 
if it were covered 30 inches deep with it; 
therefore, 15 pounds of air rest on every 
square inch of the earth’s surface. If I let the 
air in on this end again, you will see that the 
quicksilver, (now bearing an equal weight of 
air on both sides,) will regain the same level in 
each side; so that this is in fact a measure of 
the weight of the air. 
When made in a more portable and elegant 
form, it is called a barometer , from two Greek 
words which signify a measure of weight; and 
the height of the quicksilver in the tube indi¬ 
cates the weight of the air, which presses it up; 
and as dry air weighs heavier than damp air, 
when the quicksilver sinks in the tube we an¬ 
ticipate wet weather, and so this tube becomes 
a weather glass, and when furnished with a 
float on the surface of the quicksilver, it pulls 
round an index figure on a dial plate, and 
points to rain, fair, stormy, and so on; and this 
is the principle on which your weather glasses 
act. Well then we have just the same weight 
I of air around the globe as if the surface of it 
