Imágenes de páginas

we obtain a velocity of 124.32 inches, or 10.36 feet, per second, — 7.06 miles per hour. This, therefore, may be assumed as a near ap- . proximation to the velocity of the blast, when not otherwise mentioned.

“ The velocity of the induced current being the true measure of the practical value of different forms of ventilating apparatus, it becomes necessary to ascertain this value as accurately as possible. The inconvenience attending measurements in which time is involved as one of the elements, and also, probably, the difficulty of determining the instant when a current has passed through a certain space, have led to the adoption of other means, by which the velocity of the current is not directly measured, but inferred. The mode which has been repeatedly adopted, of measuring the efficiency of a ventilator by its power of sustaining a weighted flap or valve, or a head of water, or by some other statical effect, is decidedly objectionable. Such a measure gives the correct value of the initial force or tendency to establish a current in a chimney in which there is no actual movement; but it does not indicate the velocity of the current which will be the final result of the action of the ventilator, nor is it any measure of this final velocity when ventilators of different construction are compared together. Mechanics and engineers are familiar with the difference between the statical and dynamical effects of a force. They are aware that the former may be greatly increased by the mechanical powers, so that, through the medium of a pulley or a lever, a single pound may be niade to sustain and raise a hundred times its own weight. But the dynamical effect is not correspondingly increased, for in order to raise one hundred pounds through the height of a foot, the one pound must in all cases fall one hundred feet; so that the loss of height precisely balances the gain in weight. In the same way, the dynamical effect of different springs is not to be measured by their strength alone ; it is not simply dependent upon the amount of weight which they will sustain, but equally upon their length, or rather upon the distance through which they move in restoring themselves to equilibrium. The archer's bow is a good instance of this assertion, which any one can try for himself, and he will find, that, with a given exertion of strength, he is able to throw the arrow farthest and highest with that long bow of which he can draw the string to his full arm's length, and not with the strong bow which he can hardly move. But an example more nearly allied to the case under consideration is derived from the air-pump, in which the dynamical value of any amount of exhaustion is equal to

the power required to produce it, and is, therefore, proportioned to the magnitude of the receiver when other circumstances are the same; whereas its statical power or its power to sustain a head of water is wholly independent of the magnitude of the receiver, and proportioned solely to the tension of the air within it. In all these cases, there is a striking difference between the operations of using the statical and dynamical effects, which deserves the most careful consideration, because it is essential and characteristic. The statical effect may be used for any length of time without being impaired, and the reason is obvious ; it manifests itself in a state of rest, when there is no change of condition. The dynamical, on the contrary, can be used once and but once. The one pound can balance the hundred pounds as long as the materials of the pulley and lever will endure ; a compressed spring may sustain its weight, or the expanded air its head of water, as long as we choose, without any diminution of effect. But when work is to be done, a change to be effected, a weight to be raised, a velocity to be produced, the result can only be obtained by a corresponding change in the opposite direction, an undoing of work, a falling of a weight, a consumption of power once and for ever. In the present case, in which the object is to obstruct or divert the motion of the wind in such a way that part of its velocity may be communicated to the air in the chimney, and thus produce a current, the amount of this communication and transfer of velocity cannot be measured when it does not take place, - when, on the contrary, the mouth of the chimney is entirely stopped up, so that it is impossible to produce any current within it. It would be just as proper to weigh a water-wheel by the weight which will just reduce it to a state of rest, instead of that smaller weight which reduces it to its usual working velocity, and which is universally adopted by experienced engineers as the correct measure of the power of the wheel. It should also be borne in mind, that there are resistances offered to air in motion by the tube through which it passes. These resistances are not constant; they increase as the perimeter and length of the tube directly, and also as the square of the velocity ; these, it is obvious, cannot be measured where they do not exist.

“ The plan, therefore, which has been adopted in these experiments, is to measure directly the velocity of the current produced, and it will not be surprising, after what has preceded, if some striking differences should be observed between the results thus obtained and those derived from any statical measure.

“ 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.* lodide 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

* 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.

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.

“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 fluid striking a sphere perpendicularly to its surface spreads itself uniformly over both the superior and inferior hemispheres ; a similar jet striking a horizontal cylinder perpendicularly 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 conse. quence 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 illus. tration 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.

« AnteriorContinuar »