That weight is a property of liquids, has been acknowledged by the
earliest observers; but the amount of that weight, its mode of acting,
and application to practice, have been left for recent times to discover.
A pint of water weighs somewhat more than a pound avoirdupois; and one
unacquainted with the facts in hydrostatics might deem it of little
consequence what shape the vessel that contained it might be, or what
the disposition and length of the column of water--for, after all, what
is it but a pound of water? No idea can be more erroneous. Under most
circumstances, it is not so much the quantity of the fluid as the manner
in which its particles are disposed, that determines its weight; and what
may appear still more extraordinary, a small quantity of fluid may be
made to balance, that is, to be of the same apparent weight as, a very
large quantity. This may be proved by taking a pair of scales, putting a
tumbler full of water into one dish, and balancing it by weights in the
other, then inverting a smaller glass and immersing it in the tumbler,
having the glass perfectly supported in the hand to prevent it touching
the sides or bottom; a portion of the water will now flow over the sides
of the tumbler--say one-half--yet the scales are still balanced; one-half
of the water is of the same weight apparently as the whole. A piece
of wood may be used instead of the glass with the same result, and it
may be of a size nearly to fill the cavity of the tumbler; yet if the
remaining water, which may amount to no more than a couple of spoonfuls,
rise to the same level as it did when full, it will exactly balance the
weights. This cannot be accounted for by saying that the wood or the
glass was equal to the water displaced, for if we use lead, which is
much heavier, or cork, and even card, which are much lighter, we shall
meet with no difference. This property belongs to the water; and as the
only constant fact was the same height of the fluid, to it must the
explanation be referred; and we thus arrive at a first principle, a law
in hydrostatics--that the pressure, or weight considered as a power, of
any fluid, is not in proportion to its quantity, but to its depth.
Aware of this principle, if we wish to use water as a power, we can
economize it wonderfully, exerting a great pressure with a small
quantity. If we take a small wooden box, water-tight, bore a hole in it,
and fill it with water, adapt a long narrow tube to the hole, and fill
it up with water, the box will now be burst, and that by the very small
quantity contained in the tube. This tube may be a yard long, and very
narrow in diameter, not holding more than two ounces of fluid, yet the
pressure, being always in proportion to its depth, is the same as if it
had been as broad as the box. This pressure amounts to nearly one pound
on the square inch for every two feet of water. In the deepest parts of
the ocean the pressure must be exceedingly great, so much so that it is
probable they are uninhabitable, the pressure being too great for the
existence of fishes. This pressure, together with the total absence of
light at great depths, renders the existence of vegetable life also a
doubtful matter. There is a certain depth beyond which divers cannot
go, owing to the pressure of water on the surface of their chests being
greater than the resistance of air inside, respiration being thereby
impeded.
A pipe a yard long, and acting on a yard square of fluid, will give a
pressure equal to the weight of fifteen cwt. if we use water. Should we
use quicksilver, the power of a ton weight may be obtained within the
space of a square foot in breadth, by a tube somewhat less than three
feet long, and not larger than a common goose quill--the pressure per
square inch in these cases depending on the height of the column of fluid.
We can now understand what extensive and sometimes irremediable injury
may arise from the collection of a small but lofty column of water,
opening into a wide but confined space below. This sometimes occurs
when water gets into a narrow chink between buildings, and, finding its
way down, opens finally into some cavity under the floor. The pressure
exerted here is immense, and there are few bodies able to resist it. It
is owing to this that the pipes for conveying water are burst, on account
of the pressure exerted on the insides of the pipes; and this occurs the
more frequently, the higher the source from which they are filled. In
practice, every vessel containing liquid should increase in strength in
proportion to its depth. We have no doubt that a process similar to this
takes place on the large scale in nature, which is capable of uprooting
trees, rending rocks, producing earthquakes; for if we suppose that some
collections of water on the surface of a hill have found their way down
through crevices into a cavity in the body of the mountain which has no
external opening, as long as this cavity remains unfilled no evil arises,
but when it and the crevices also are completely filled, the pressure
exercised here is so immense, that even the sides of the hill cannot
withstand it. Perhaps this occurrence has not been sufficiently noticed
in explaining natural phenomena. It is usual to consider earthquakes and
volcanoes as solely the result of chemical action, excluding entirely
physical agency.
The pressure of water may be rendered visible by blowing through a
tube under water into a tall glass jar. The bubble of air, small at
the bottom, as it rises, gradually enlarges from the diminution of the
pressure.
The hydrostatic bellows, formed upon this principle, consists of nothing
more than a water-tight bellows, with a long pipe fixed into the valve
aperture. If this pipe be three feet long, and hold a quarter of a pint
of fluid, it will exert a pressure sufficient to raise three cwt. laid
upon a bellows, the area of the upper side of which is equal to about a
square foot and a half. Many are the uses to which this principle might
be applied in the several arts.
Bramah’s Press is almost the only machine which has been extensively
used. By its means solid bars of iron can be cut through with ease. Hay
and cotton have been compressed by its means into a very small compass.
In the East Indies, where water-power is used, bales of cotton are
compressed into one-half the size of those from the West Indies. By its
means power may be multiplied, or rather concentrated, a thousand-fold.
As commonly made, a man working it may, by using the same force that
would raise half a cwt., apply a force amounting to twenty tons to the
work in hand; and by varying the proportions of the machine, pressure
might be brought to bear upon any body which would be perfectly
irresistible.
There is, however, in reality, be it distinctly understood, no power
absolutely gained; but the man’s force is _concentrated_, as for instance
in compressing the bale of cotton, to an extent which, if the ordinary
mechanical powers of the lever or screw were employed, would require the
aid of ponderous machinery.
Mr Bramah was therefore greatly mistaken when he published it as the
discovery of a new mechanical power: but he invented a beautiful and most
effective means of simply accumulating a prodigious force by the very
simple means of the hydrostatic pressure of fluids.
Hydraulic or Bramah presses are applied in New York and other American
ports for the purpose of raising large vessels on strong wooden platforms
out of the water, for effecting repairs, &c. They are also employed in
removing houses--some of them brick, and three stories high--from one
part of a street to another. In this case strong wooden beams, like the
ways used in ship-launching, are placed under the house, and in the
direction of the intended site, and hydraulic presses are then employed
for pushing the house along, with prodigious force, and so gradually
and gently as not even to crack the plaster of a room ceiling. By
the same means the roof of a large cotton factory near Aberdeen was
raised _entire_, and an additional story added to the building, without
displacing a single slate! In this instance the roof was lifted gradually
about four inches at a time, progressing from end to end of the building,
the height of the walls being increased by a single row of bricks at a
time.
Such are a few of the results of a single principle, a rule to which
there is no exception, which holds equally good in the organic as in the
inorganic world. Even the blood-vessels of the body are subject to this
law--the sides of all vessels below the level of the heart enduring an
additional outward pressure of half an ounce for every inch in height,
which at the toes would amount to somewhere about two pounds. When a
person stands erect in a bath, the pressure on all parts of the body is
not equal; it is greater upon the legs than upon the trunk; the former
are pressed upward, and hence in part the difficulty experienced in
standing upon the bottom in deep water.