Thomas Ewbank.

A descriptive and historical account of hydraulic and other machines for ... online

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this experiment it is obvious that land on a low level, as at 0, might be
drained in this manner, and the water discharged above, as at B, where-
ever a motive current could be obtained. Venturi applied the principle
with success to some marshy land belonging to the public.

In the next experiment both air and water are moved by the current,
and the pressure of the atmosphere excited to raise water as in the pipe
I of a pump. The cylindri-
cal tube k (No. 200) was
connected to a reservoir
of water, D, the surface
as before being 32 J inches
above its onfice. The pipe
K was 18 lines in diameter
and 57 long. A glass tube
A B was connected to its
upper surface at the dis-
tance of eight lines from
its junction with the reser-
voir. The other end of
the glass tube descended
mto a vessel, T, containing a colored liquid. When water flowed through



T
Nal99. No. 900.



* Floating substances have often been thrown into the Gulf Stream to ascertain its
direction. Upwards of twenty years ago we cast overboard, near the Banks, a commoD
quart bottle carefully corked and sealed, and having a few inches of red bunting tied to
toe neck. The bottle coatained a letter addressed to a gentleman in London, and an
open note in English and French, requesting the finder to put the letter into the nearest
post-office, American or European, and also a memorandum of the circumstances, date
and place of its discovery. Precisely eleven months from the day the bottle was com-
mitted to the deep, the letter was delivered by the postman, and accompanied with an-
other from an Irish clergyman. The fragile vessel floated safely ashore near Sligo. Its
little pennon excited the attention of a peasant, who broke the bottle, and not knowing
what to make of the contents, carried the whole to his priest. This gentleman politely
forwarded the letter to its destination, and wrote another containing the particulars jusi
mentioned. Both letters, we believe, were laid before the British Admiralty by the gen-
tleman to whom they were addressed.



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Chap. l.J Vacuum produced by liquid currenit, 479

R it dragged the aii at the mouth of the glass tube with it, the remaining
air dilated, and finally the whole was carried out with the efBuent water,
and the colored liquid rose to the height of 24 inches in A B. The glass
tube was then shortened to about 22 inches, when the contents of T rose
up and were discharged from K. In another experiment K was placed
in nearly a perpendicular position, being inclined a little that the jet might
not fall back on itself, but the liquid rose through A B as before. The
end of A B where it joined K was flush with the interior surface of the
latter. Several small holes were made round K ; these diminished the
velocity of the issuing current, but no water escaped through the openings.

There is a singular fact relating to the discharge of liquids from dififerenr
shaped ajutages : for example, more water flows through a short tube than
through a simple oriflce of the same diameter. A circular opening, of the
same diameter as the bore of K in the last flgure, was made in a sheet of
tin, and the latter attached to a cistern in which the water was kept at a
constant altitude of 32} inches : now while four cubic feet of water escaped
through the opening in 41 seconds, an equal quantity passed through K in
31 seconds ; and when the length of K was only twice its diameter, the
quantity discharged was still greater.

But the quantity discharged may be still further increased if the end of
R next the reservoir be made to assume the form of the contracted vnu*
This term is used to designate that contraction which a liquid column un-
dergoes when escaping through an orifice, or when entering a tube. Sup-
pose an aperture, an inch in diameter, made in the bottom of a bucket or
a cauldron, and closed by a plag. Then fill the vessel with water, and
withdraw the plug. Upon examination the descending column will be
found contracted or tapered for a short distance below the orifice, viz.
half an inch, or half the diameter of the orifice. The area oi the section
of the smallest or most contracted part will be to the orifice as 10 to 16
according to Bossut, but when a short cylindrical tube was applied to the
orifice, he found the contraction as 10 to 12.3. (The same thing occurs
whether the opening be made in the side or bottom of a vessel.) Hence
by enlarging the end of R next the reservoir, in the proportions named,
the contraction within the cylindrical part of the tube would be avoided,
and the discharge consequently increased.



No.90i. lf«wfl



By substituting for R a compound tube of the form and proportions
figured at No. 201, the quantity discharged has been ascertamed to be
more than doubled, being to that delivered by the orifice in the tin plate
as 24 to 10 ! A, the cistern ; B, a short conical tube connecting the cylin



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480 DUchargt f^IAqnidi through Qmical Jt^utaget. [Book V.

drical one to the conical fruatrum C D. Supposing the diameter of C
to be unity or 1, that of D should be 1^, and the distance between them
9. The increased discharge ceases when the cylindrical part of the tube
B is of considerable length, and of the same bore as the snoaller end
of C D.

A tube of the form represented at No. 202 was applied by Venturi to
the same reservoir, the depth of water in wh.ch was also kept at 32^
inches. Three glass tubes, ABO, were connected to the under side of
the pipe, and their lower ends inserted into a vessel containing mercury.
When water flowed through the pipe the mercury rose 5'^ lines in A, 20
in B, and 7 in C. These quantities correspond with 62 mches of water
in A, 24 inches in B, and 8 in C. The lesj^ of the pipe should not ex-
ceed four times the diameter of its smaller end, and its sides should not
diverge from each 'other more than what is required to form an ane^le of
from three to four decrees. By this principle it will be perceived, that
water naav also be raised from a lower level and discharged at an upper
one, and m many situations it might doubtless be adopted with advantage.
See Nicholson's Journal, vol. ii, and Hachette's Trait^ £lementaire des
Machines. »

Different causes have been assi^iied for the increased discharge of
liqaids through conical tubes. One is certainly to be found in the material
of which they are made ; for when formed of or lined with any substance
that repels or refuses to coalesce or b® wetted with the effluent water, as
wax, tallow, &o. the effect ceases. The phenomenon therefore depends
upon the attractioa and adlkesion of the liqaid to the sides of the tubes*
whicih sides exert a capillary fiovce in drawing the particles of the liquid
towards them, so as not only to prevent its assuming the fi^;are of the con-
tracted vein when entering the tube No. 202, but also drawing the particles
to the diverging sides of the disehareing ajutage.

A knowledge of the increased discharge of liquids from conical tubes
has led some persons to take advantage of the fact, to the seiious injury
of others. We have heard of the purchaser of a water powec, who ac-
cording to the covenant was to connect his mill-race with the dam by a
trunk of a certain specified bore dU tkeiunciian. This he did, but making
the sides of the trunk diverge as in the last figure, the proprietor of
the dam was astounded to find the water* as if moved by instinct, giving
the new channel the preference, and unaccountably persisting in rushing
through it witll a velocity that threatened to drain the well supplied reser-
voir, and leave his own mill to take its rest This increased discharge is
not confined to tubes of a cylindrical or conical form. The walls of the
channel may be straight, and its section may be a square, a triangle, &c.
as well as a circle.

There is some reason for believing that overreaching in this way is not
wholly a modern discovery. No city, ancient or modern, was perhaps
ever supplied with water in greater profusion than old Rome ; yet the con-
tents of her aqueducts were meted 'out with economy, and, as in modern
times, a revenue was derived from the sale of the water. The superin-
tendence of the aqueducts and of the distribution of the liquid through the
streets and houses were always intrusted to a citizen of rank and talents.
The celebrated Frontinus held the office under Nerva, by whose directions
he wrote two. books on the water- works of Rome, the times of their erec-
tion, districts of the city supplied bv each, the number of public and pri-
vate fountains, quantities of^ water discharged from different sized orifices*
&c. From him we learn that numerous frauds were practiced in obtaining
more than the assigned quantity of the liquid, one of the means for pre*



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Chap. 2.] WaUr raised hy Currents of Air. 48]

veating which was this : when a pipe for the supply of a house was to be
connected to the castellum or reservoir, (which received the water from
one of the aqueducts) a brass caJ-ix, or short bent tube, (probably the same
as the modern ones which connect the lateral pipes to the mains) was de-
livered by the oiHcer in charee to the workmen, to insert into the castellum;
and it was enacted that the bore of the cyHndrical leaden pipe should be
the same as that of the calix for at least ,^y^e^ from the castellum. It
is therefore pretty clear that Roman engineers were aware, that the in-
creased discharge through enlarged orifices ceases when a considerable
length of pipe of the same bore as the calix intervened.



i



HAPTER II,



Water nhmA by carroBtt of air— Fall of the barometer during etoniM— Horricanes commence at the
leeward — ^Damage done hy •toraui not alwaye by the ImpulM of the wind — ^Vacnum prodaced by atorna
of wind— Draft of chimaeya— Currents of wind in bon a ee F ire fratos and parabolie jamba^Experi-
mentf with a sheet of paper— Experiments with curreiita of air dirough tnbes Tarlouily connected—
Effect of conical lyntafes to blowing tube*— Application of thaaa tubes to inor ea se the draA of chimneys
and to ventUate wells, mines and ships.

Currents of air and other elastic fluids may be employed to raise water
in a manner different from any yet noticed ; i. e. not by any modification
of the lateral communication of motion, nor by breaking the liquid into
minute particles by the motive fluid mixing witii them, but by the removal
or diminution of atmospheric pressure. The pnnciple to which we allude
is to be found more or less active in nature, and illustrations of it are not
infrequent in common life, although for want of reflection they are seldom
noticed and are not always understood.

Meteorologists have long observed that storms of wind are accompanied
with a diminution of the air's pressure, and that the descent of the mercu-
rial column in the barometer keeps pace generally with the violence of
the tempest: thus in hurricanes the- depression is much more than during
ordinary gales, while in the vortex of a tornado or a whirlwind it is
excessive.

Some persons are apt to consider winds as proceeding directly Jrom the
power that generates them, as a stream of water proceeds from a fire-
engine or one of air from a hello ws» whereas they as often rush towards
the source that gives them birth ; and hence it is that hurricanes, some-
times if not always, commence at the leeward. Should any mystery ap-
pear in this it is easily explained : — ^if a person blow through a tube, the
blast proceeds from him ; if he suck air through it, the current is directed
to him : when we close a pair of bellows, wind issues from the nozzle ; if
they are opened while the valve in the lower board is shut, it rushes back
through the same channel : so it is with currents in the atmosphere. A
partial void is formed in the upper regions, perhaps by electricity, by
changes of temperature or humidity, by rarefaction or other causes, ana

61



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482 Removal of Atmospheric Pressure hy Currents of Air, [Book V

iiutantly oceans of the fluid matter around rush to restore the equilibrium:
then the removal of these oceans neccssavily induces others to move also
to take their place, and in this way various strata of the atmosphere, for
miles and hundreds of miles, are put in motion towards the place where
the cause of their movements is located, and in a way not unlike that by
which streams of air enter a person's mouth while he sucks an empty tube,
or a bellows during the act of opening them.

When the lowermg sky and flitting clouds announce the approach of a
violent storm, and when, like a demon broke loose, it destroys in its fury
nearly every thing in its track, we commonly suppose the mischief is done
by the direct impulse of the blast — ^that agitated and groaning forests, trees
prostrated, walls and fences leveled, buildings overturned and others un-
roofed, &c. are the results of a tempest sweeping these objects before it,
somewhat as we blow dust &c. from a table or from the cover or edge of
A, book. But this, though sometimes the case, is not always so ; for if it
were, almost every object blown down by the wind would be found lying
m the direction of the blast, whereas they are frequently discovered in the
opposite one. The effects enumerated are sometimes caused by winds
blowing over a district of country without coming in contact with the earth
or the objects upon it, but merely sweeping at some distance above them:
at other times similar results are met with at the extreme edge of a storm,
and even beyond it. In these cases a partial vacuum produced by the
%erial currents often works all the mischief, although it may be, as it fre-
quently is, but of momentary duration. Close buildmgs have been instan-
taneously destroyed by the expansion of the air within them, l^eir wafls
being thrown outwards^ and their roofs projected aloft. The tornado by
which the city of Natchez was recently destroyed furnished striking proofs
of this removal of atmospheric pressure, and of fearful damages occasioned
by the void. The doors and windows of one or two houses left standing
amid the general wreck happened to be open, and thus furnished avenues
for the dilated air to escape. In some houses the leeward gable ends
were pushed out, and the windward ones stood ; in others, the leeward
walls remained standing while those to the windward were thrown out-
wards in the face of the storm. Both gable ends were burst out in some,
and of others the sudden expansion of the air raised the roofs for a pas-
sage, and left more or less of the walls standing.*

Persons whose ideas of a vacuum are inseparably associated with air-
tight vessels, would hardly suppose that any thing approaching to one
could be formed in the open regions above and about us ; yet every breath
of wind — ^the gentle zephyr as well as the furious tempest — destroys the
equilibrium of the air's pressure, and consequently produces a partial void;
and it will be seen in this and the following chapter that a vacuum may
be produced and maintained in open tuhes. It should however be kept in
mind, that an absolute vacuity is not found in nature nor to be obtained by
art : the slightest rarefaction and the best results of the best air-pump are
•but degrees in the range of a scale, of whose limits we know but litde.

A few more familiar illustrations of the removal or diminution of atmos-
pheric pressure by currents of air will not be out of place. And first, who
has not, while sitting by a winter's fire, witnessed the coals in the grate
brighten suddenly up, and heard the flames and heated air roar in the
chimney as if urged for a few moments by some invisible bellows-blower 1
— phenomena attributed, we believe, in the days of witchcraft, to elves and

• Se« an interesting account of this tornado bj Dr.TooIey, of Natchez, inthe Joanial
9f the Fraiklin Institute for June, 1840



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Chap. 2.] ExperimentM with a Sheet rf Paper. 483

fairies, those mischievous imps who, in their wayward moods, sometimes
undertook to blow the fires as well as to sweep and sand the floors of the
bouses they visited, and who, by their screams of delight on leaving their
work, were supposed to produce the hollow sounds in the flue as they
darted up to join their comrades in the tempest without ! It need hardly
be observed that it is gusts of wind, sweeping in particular directions over
the tops of chimneys, and thereby causing a partial vacuum within them,
that tkus powerfully incresuses the draft. But it is not necessary to have
fire in the grate, for the effect may be noticed in parlors during the summer
months, when those light and ornamental paper aprons with which ladies
cover the fronts of thAr grates are often thus drawn into the flues, and be-
come disfigured and spoiled.

Other examples may be derived from the movements of interior doors,
blinds and curtains of windows, &c. While we are writing, the front
door of our dwelling is opened, which affords a clear passage from the
street to a garden in the rear. The door of the room we occupy opens
into the passage, through which a flaw of wind has just passed, and in a
twinkling the blinds swing from the windows, and the door is slammed to
its frame, by the air in the room rushing to join the passing current, or to
fill the slie^ht vacuum produced by it. An open fire-place creates a draft
up the chimney, which acts as a pump to draw cold air into the room ;
hence the complaint, not at all uncommon, of being roasted in front while
facing the fire, and at the same time experiencing the unmitigated rigors
of winter behind. (In such cases the combustion should be supported by
air drawn from without by a pipe terminating beneath the j^rate — a device
patented in modem days, though it was known two centuries ago, and is
described by M. Gaugcr in his treatise on " Fires Improved," a work
translated by Desaguliers in 17 15.*) The motion of every object in nature
produces currents of air, and in every possible direction — ^the movement
of the hand in writing or sewing — the trembling of a leaf or of an earth-
quake—the flight of an eagle or of an insect — ^the ball whizzing from a
cannon's mouth, the creeping of a snail, or a wasp using her forceps.

Artificial illustrations might be quoted without end. Lay two books of
the same size, or two pieces of board, six or eight inches apart upon a
table, and place a sheet of paper over them ; then blow between the books,
and the paper, instead of being displaced by the blast, wilt be pressed
down to the table by the atmosphere above it, and with a force propor-
tioned to the intensity of the blast. Instead of the mouth next use a pair
of bellows, by inserting the nozzle under one edge of the paper, and the
effect will still be the same. The stream of wind may even be directed
partly against the under side of the paper, which notwithstanding will re-
tain Its place and be pressed down as before. Suspend the books or fix
them to the under side of a table, then hold on the paper till the blast is
applied, when the sheet will be sustained against gravity. Fold the paper
mto a tube and blow through it with the mouth, or with bellows — ^in both
cases it will be collapsed. From this* experiment we learn that the force*
which fluids exert against the sides of pipes that contain them, is greatly
diminished when they pass rapidly through. We have known a small
leak in the pipe that supplied steam to a high-pressure engine, cease to
give out vapor every time the communication was opened to the cylinder

» " Parabolical jainbe" (also patented) or backs of grates for reflecting from their.po-
lished surfaces the heat into the room, are described in the same interesting little work.
At page 140 Desaguliers speaks of bellows invented and patented by Captain Savery—
• device of his that is uo where else mentioned that we are aware of.



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484



EtXperiments ttnih



[BwkV



—the pardcles of the flaid then being hurried along with a velocity too
great to allow any of them to change their direction to escape at the leak.
The following abstract of experiments made by us in 1834-5, to illus-
trate the same prinoiple, may interest some readers :— To ascertain the
extent to which atmospheric pressure was removed from under the sheet
of paper, we bent a small glass tube at right angles, and placing one end
under the paper let it rest on the table, while the other descended into a
tumbler containinff a little water. Then taking a small pair of bellows,
and directing the blast over the pipe, the water rose from one half to three
fourths of an iiich. The books upon which the sheet laid were then placed
within two inches of each other, when the effect was increased, the liquid
rising from 1^ to 2 inches. We next laid aside the paper and made use
of two tubes, one to blow through and the other to measure the ascent of
the liquid.



'^'Ih n



A C





B



No. 903. No. 901



Now 90S. No. 906. No. 907. No. 906. ^aS09. No.9ia



Two leaden or block tin tubes, straight and polished in the inside, were
united at right angles. See No. 203. A C the blowing pipe, 8 inches
long and half-inch bore. B 12 inches long and three-eighths bore. The
upper end of B was joined flush and smooth with the interior of the other
three inches from the end A. Upon applying the mouth to C and blowing
in the direction C A, indicated by the arrow, instead of the liquid rising
in B, part of the current from the lungs entered that tube and was forced
through the water in the tumbler. Various portions of the end A were
then cut off without changing the result, until half an inch only remained
in front of the joint, when the air no longer descended, but no rarefaction
was produced in B. When both tubes were made of the tame hore^ part
of the blast descended in B until the whole of A in front of the joint was
removed. In numerous trials, the water in the lower end of B was de-
pressed more or less, whether the b^ast of wind through A was weak or
strong. (From these experiments we discover the impropriety of placing
cylindrical tubes on chimney tops at right angles to the draft, and espe-
cially on locomotive carriages; as was at first proposed. In the Edinburgh
Encyclopedia, vol. xvii. p. 457, a carriage by Tredgold is described, and
a figure of it given in plate 511. The chimney is represented with a short
horizontal tube attached fore and aft to the top, as in No. 203, with a view
" to assist the draft" by the passage of the air or wind through it. The
experiments above show that the reverse would have been the case.)

As part of the air in passing through A, in No. 203, turned off into B



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Chap. 2.] Blowing Tubes. 485

the idea occurred that if the junction of B were made to form an acute
angle with the longer part of A, then the whole of the aerial current might
possibly pass out at A, since to enter B it would have very nearly to
reverse its direction. The device figured at No. 204 was made to test this.
(The part of A in front of the joint was 1 J inches long, which from several
experiments we thought produced the best eifect, when A was half an
inch in the bore — i. e. the length of this part of the blowing tube was three
times its diameter.) Upon trial part of the current passed into B and es-
caped th)*ough the liquid, as in the preceding experiment ; and even when
B was turned up in a vertical direction before entering the water, the same
effect took place.

Various modes of uniting the pipes with the view of preventing the
blast from entering the vertical one were now tried, and to ascertain the
effects produced a glass tube, three feet lone* and three-eighths of an inch
bore, was attached to the vertical or exhausting tube of each. In No. 205
a portion of B protruded into A, so as to form a partition or partial cover
to the orifice. Upon blowing through A (in the direction oi the arrow)
the water sprung up B to the height of 12 inches, and in subsequent trials
varied from 10 to 20 inches, according to the strength of the blast By
connecting the glass tube to the blowing end of A and then blowing
through B, the liquid rose from 8 to 10 inches ; the difference, no doubt
being caused by the current of air having had greater facilities in one
passage than in the other.

We next united two tubes at right angles, but instead of making the
joint flush within as No. 203, the upper end of B was cut obliquely, as



Online LibraryThomas EwbankA descriptive and historical account of hydraulic and other machines for ... → online text (page 72 of 90)