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“To measure the current, a leaden pipe (the material most readily at hand), 1.25 inches in diameter and 53 feet in length, is placed near and a few inches below the mouth of the blowing machine. This pipe is coiled, as it leaves the manufactory, into a circle of about 2.5 feet in diameter, of which it makes eight turns. In the mouth of the trunk, before described as attached to the blowing-machine, is a tube of tinned iron, of the same diameter as the pipe, and bent at a right angle ; the upright branch, about six inches long, reaching to the middle of the mouth, while the horizontal portion, about five inches in length, reaches within 2.5 inches of the end of the leaden pipe. Each ventilator, when examined and tested, is placed upon the upright portion of this tube. For this purpose the ventilator has through it, or attached to its side, a corresponding tube of the same diameter. The connection between these two tubes is completed by a glass tube 4 inches long and 2 inches in diameter, and the fitting made close by means of cotton-wool fastened loosely around the extremities of the two metallic pipes. In this compound pipe the current is induced, and its velocity noted. To effect this last object, advantage is taken of the well-known action of iodine upon starch.* Iodide of potassium is dissolved in a strong solution of starch in hot water, in the proportion of three grains or more of the iodide to an ounce of the solution. A piece of paper wetted, or rather smeared, with the prepared starch is suspended within the glass tube, which can be readily removed for this purpose, by means of a wire hook attached to the metallic pipe. A current is now induced by the action of the blast upon the ventilator, and chlorine gas allowed to enter the opposite end of the pipe, which is kept carefully removed from the influence of the blast. The chlorine is carried along with the current until it reaches the starched paper, which it instantly dyes a deep blue; the chlorine, by its superior affinity for the potassium, seizing upon it, and leaving the iodine free to act upon the starch.
“Chlorine is conveniently obtained for this purpose from Labarraque's solution of chloride of soda, and its liberation quickened, if need be, by adding a few drops of sulphuric acid. When the vial containing the chlorine is closed by the finger, and held a few seconds in the hand, its warmth expels the gas more freely, and when the finger is removed it escapes in a jet, which makes the experiment more decisive.
* The action of hydrosulphuric acid upon moist carbonate of the oxide of lead was first suggested for this purpose, but the chlorine and iodide were judged most convenient.
“In making the following experiments three persons were usually employed; one to keep up a uniform blast, counting the revolutions of the handle by a watch ; a second to throw the chlorine into the pipe, and also to observe and declare the moment when the blue color appears upon the starched paper; the third to note upon a watch the interval between these two events.
“Results OF EXPERIMENTS. “1. Air in motion communicates motion to those portions of air at rest in its immediate vicinity. To this phenomenon Venturi, who discovered and explained it, has given the name of the lateral communication of motion in fluids.
"2. A jet of air falling upon any surface is never reflected, but spreads itself out, and forms a thin layer in immediate contact with that surface. It may be admitted as a principle, that fluids do not, under any velocity or any angle of incidence, possess the property of reflection, like solids, and it is, doubtless, owing to the absence of this property that they adhere to bodies against which they strike. In virtue of this adhesion, a jet of Auid striking a sphere perpendicularly to its surface spreads itself uniformly over both the superior and inferior hemispheres ; a similar jet striking a horizontal cylinder perpendicu. larly to its surface completely surrounds it, and does not leave it until the two parts of the jet meet on its inferior border and form one common sheet. (Savart, Annales de Chimie et de Physique, Tom. LIV.)
“When a jet of water strikes a truncated cone perpendicularly to its axis, and just above its lower base, it spreads out, covering more than half its surface, and, rising upward, leaves its upper base in a continuous sheet, vertically in a plane nearly coinciding in direction with that of the sides of the cone, and horizontally nearly in the direction of tangents to the surface of the cone, while a small portion only of the fluid forms two small streams, which drop down from those two points of the lower base of the cone which are at right angles with the orig. inal direction of the jet.
“When a jet meets a circular plane at its centre and perpendicularly, it forms a thin continuous sheet over the whole surface. Both the direction and continuity of this sheet are preserved far beyond the
borders of the circular plane, where its edge is thin, but it follows more or less the direction of the curve of the edge, if it is thick and rounded.* (Savart, Ann. de Chim. et de Physique, Tom. LIV. p. 119.)
“3. When a jet of air impinges upon a surface of limited extent, the atmospheric pressure upon the opposite side of the surface, in consequence of the lateral communication of motion, is diminished, and a current will be established through a tube, one of the extremities of which is placed in the point of diminished pressure, and the other beyond the borders of the surface. This is the important principle upon which the efficiency of ventilators and chimney-tops depends ; it is also important in its bearing on the position of the mouths of air-trunks for hot-air furnaces; if the mouth be placed in a point of diminished pressure, on the leeward side of a building, air may pass outward, especially from apartments on the windward side of the house.
“4. When a current strikes the extremity of a tube perpendicularly to its axis, motion is produced through the tube towards the current; and when a current already exists in the tube, if its velocity is less than that of the impinging current, that velocity will be increased.
“When two currents of air of different velocities are moving in precisely the same direction, the influence of the more rapid current in accelerating that which is less rapid is not so great as when the angle of meeting is between 20° and 40°. When two opposite currents of equal diameter and velocity meet, they form a circular sheet, perpendicular to the axis of the veins, and the resulting phenomena resemble those arising when a current strikes a circular plane. If the ve
• A simple demonstration of these propositions may be obtained by means of a card and candle. If a blast from the mouth be directed obliquely against a card, the flame of a lighted candle will be drawn towards the card, on whatever side of it the candle is held. Increasing or diminishing the velocity of the blast does not change the direction assumed by the flame, but only the velocity with which it is drawn towards the card.
If the blast be directed perpendicularly upon the centre of the card, the flame, when passed around the edge of the card, will be driven outward at all points ; and if the candle be held near the blast, and at a little distance from the plane surface, the flame will, in virtue of the lateral communication of motion, be drawn towards the surface, and yet by the current of air close to and parallel with the card it will be prevented from reaching it. A strong flame may thus be made to play, apparently with great force, upon the hand, and yet not burn it. An illustration of this principle may often be observed in the narrow pathway, so convenient for foot-passengers, found after a snow-storm, on the windward side of a high and close fence.
locities of the currents are unequal, the greater velocity diminishes the less, destroys it, or inverts it, according to the excess of velocity. The knowledge of this fact leads at once to the interposition of a plate, to prevent loss of velocity in interfering currents.
“5. A thin plate placed upon the extremity of a tube, at the proper angle, causes the impinging current to assume a certain direction, and to produce a certain velocity in the tube ; a similar plate parallel to and above this plate does not increase that velocity.
“A cone placed upon the extremity of a tube produces similar changes of direction in the impinging current, and similar movements in the tube, but another cone above the first does not increase the velocity of those movements.
“6. Beyond certain narrow limits, the velocity produced in a tube by the action of a current on its conical extremity is not increased by increasing the height or diameter of that cone. The full effect of a cone may be obtained when its lower base is not larger than one half, nor less than one third, the diameter of the flue on which it is placed.
“7. If a flat truncated cone be fitted to the extremity of a tube, and exposed to the impinging current, a velocity may be produced in the tube of 1.71 feet per second ; if a similar but much smaller hollow truncated cone be inverted and closely secured to the mouth of the first, the velocity in the same tube may by this means be increased to 2.21 feet per second. The same increase of velocity will be produced if the internal cylindrical bore of the first cone be made conical, with its larger base upward. By the addition of this secondary cone, or by the modification of the interior of the first cone, the velocity of the current is increased over that produced by the simple cone nearly in the ratio of 10 to 13, and as the effect is as the square of the velocity, its efficiency is increased nearly in the ratio of 10 to 17. This is the best form of the simple fixed cone, and the most efficient fixed ventilator, which has been examined by the Committee. Venturi has shown, that, when a conical tube is applied to a cylindrical pipe, the larger base of this conical tube being 1.8 the diameter of the pipe, and its height 9 times the diameter of this same pipe, the expenditure will, with water, be greater for the cone than for the cylindrical pipe, in the proportion of 24 to 12.1.
“8. A hollow truncated cone, with its larger base closed by a fat plate, inverted and placed above a cone similar to that last described, will increase the velocity of the current in the pipe upon which it is placed over that produced by a simple cone nearly in the ratio of 10 to 13. This is one of the most efficient fixed ventilators with a cap which have been examined by the Committee. The form described in the preceding paragraph, with Cisalpin's plate placed at a certain height above it, is to be ranked in efficiency with that last described.
“9. The velocity of the current produced in a pipe, the mouth of which is presented fairly to the blast, is nearly constant, whether the mouth be cylindrical, conical, with its larger base towards the blast, or the reverse. The diminished area exposed to the blast, in the latter case, is counterbalanced by the increased velocity consequent upon diminished atmospheric pressure within the cone.
“10. A difference of temperature between the impinging blast and the produced current does not, within the limits observed, influence the velocity of the latter.
“In the experiments, each ventilator, when examined, is placed upon a perpendicular fixed tube of tinned iron, in the centre of the mouth of the trunk of the blowing machine. This and all other tubes, when not otherwise mentioned, are 1.25 inches in diameter. The velocity of the blast is 10.36 feet per second, or, as indicated by the revolutions of the handle of the blowing.machine, one revolution per second. The time required for the chlorine to act upon the starch, from the moment it is introduced into the pipe, is given in seconds ; the velocity of the current is given in feet and decimals. The direction of the blast is indicated by the