480

Discharge of Liquids through Conical Ajutages.

[Book V. drical one to the conical frustrum C D. Supposing the diameter of C to be unity or 1, that of D should be 1.8, and the distance between them The increased discharge ceases when the cylindrical part of the tube BC is of considerable length, and of the same bore as the smaller end of C D.

9.

A tube of the form represented at No. 202 was applied by Venturi to the same reservoir, the depth of water in which was also kept at 321 inches. Three glass tubes, A B C, 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 53 lines in A, 20 in B, and 7 in C. These quantities correspond with 62 inches of water in A, 24 inches in B, and 8 in C. The length of the pipe should not exceed 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 angle of from three to four degrees. By this principle it will be perceived, that water may also be raised from a lower level and discharged at an upper one, and in many situations it might doubtless be adopted with advantage. See Nicholson's Journal, vol. ii, and Hachette's Traité Élémentaire des Machines.

Different causes have been assigned for the increased discharge of liquids 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 be wetted with the effluent water, as wax, tallow, &c. the effect ceases. The phenomenon therefore depends upon the attraction and adhesion of the liquid to the sides of the tubes, which sides exert a capillary force in drawing the particles of the liquid towards them, so as not only to prevent its assuming the figure of the contracted vein when entering the tube No. 202, but also drawing the particles to the diverging sides of the discharging ajutage.

A knowledge of the increased discharge of liquids from conical tubes has led some persons to take advantage of the fact, to the serious injury of others. We have heard of the purchaser of a water power, who according to the covenant was to connect his mill-race with the dam by a trunk of a certain specified bore at the junction. 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 with a velocity that threatened to drain the well supplied reservoir, 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 contents of her aqueducts were meted out with economy, and, as in modern times, a revenue was derived from the sale of the water. The superintendence 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 erection, districts of the city supplied by each, the number of public and private 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

Chap. 2.]

Water raised by Currents of Air.

481

venting 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 calix, or short bent tube, (probably the same as the modern ones which connect the lateral pipes to the mains) was delivered by the officer in charge to the workmen, to insert into the castellum; and it was enacted that the bore of the cylindrical leaden pipe should be the same as that of the calix for at least fifty feet from the castellum. It is therefore pretty clear that Roman engineers were aware, that the increased discharge through enlarged orifices ceases when a considerable length of pipe of the same bore as the calix intervened.

CHAPTER II.

Water raised by currents of air-Fall of the barometer during storms-Hurricanes commence at the leeward-Damage done by storms not always by the impulse of the wind-Vacuum produced by storms of wind-Draft of chimneys-Currents of wind in houses-Fire grates and parabolic jambs-Experiments with a sheet of paper-Experiments with currents of air through tubes variously connected— Effect of conical ajutages to blowing tubes-Application of these tubes to increase the draft of chimneys and to ventilate 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 with them, but by the removal or diminution of atmospheric pressure. The principle 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 mercurial 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 from the power that generates them, as a stream of water proceeds from a fireengine or one of air from a bellows, whereas they as often rush towards the source that gives them birth; and hence it is that hurricanes, sometimes if not always, commence at the leeward. Should any mystery appear 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, and

482 Removal of Atmospheric Pressure by Currents of Air. [Book V.

instantly oceans of the fluid matter around rush to restore the equilibrium: then the removal of these oceans necessarily 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 lowering 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 o'erturned and others unroofed, &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 in 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 aerial currents often works all the mischief, although it may be, as it frequently is, but of momentary duration. Close buildings have been instantaneously destroyed by the expansion of the air within them, their walls 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 outwards 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 passage, and left more or less of the walls standing.a

Persons whose ideas of a vacuum are inseparably associated with airtight 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 tubes. 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 little.

A few more familiar illustrations of the removal or diminution of atmospheric 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? -phenomena attributed, we believe, in the days of witchcraft, to elves and

See an interesting account of this tornado by Dr. Tooley, of Natchez, in the Journal of the Franklin Institute for June, 1840.

Chap. 2.]

Experiments with a Sheet of 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 houses 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 thus powerfully increases 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 their grates are often thus drawn into the flues, and become 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 slight 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 grate-a device patented in modern days, though it was known two centuries ago, and is described by M. Gauger in his treatise on Fires Improved," a work translated by Desaguliers in 1715.) 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 earthquake 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, will be pressed down to the table by the atmosphere above it, and with a force proportioned 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 retain 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 into 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

a "Parabolical jambs" (also patented) or backs of grates for reflecting from their polished 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 Saverya device of his that is no where else mentioned that we are aware of.

484

Experiments with

[Book V. -the particles of the fluid 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 illustrate the same principle, 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 containing 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 inch. 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.

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 same bore, 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 depressed more or less, whether the blast 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 especially 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,