When in the publication of this Supplement we had arrived at the title Fluids, we were struck with the importance given, in some of the journals, to The Experimental Researches of Venturi concerning the Principle of the lateral communication of Motion in Fluids, applied to the Explanation of various Hydraulic Phenomena. Of these researches we intended to lay an abridged account before our readers under the present title; but having examined the work with some attention, we find in it hardly anything of consequence which the mechanical philosopher may not learn from our articles RESISTANCE OF FLUIDS and RIVER in the Encyclopedia. That our readers, however, may find something under a title to which we rashly referred them, we shall, in the words of Nicholson's Journal of Natural Philosophy, &c., inform them what Venturi's work contains.

"This author, who is professor of experimental philosophy at Modena, has introduced an horizontal current of water into a vessel filled with the same fluid at rest. This stream entering the vessel with a certain velocity, passes through a portion of the fluid, and is then received in an inclined channel, the bottom of which gradually rises until it passes over the border or rim of the vessel itself. The effect is found to be, not only that the stream itself passes out of the vessel through the channel, but carries along with it the fluid contained in the vessel; so that after a short time no more of the fluid remains than was originally below the aperture at which the stream enters. This fact is adopted as a principle or primitive phenomenon by the author, under the denomination of the lateral communication of motion in fluids, and to this he refers many important hydraulic facts. He does not undertake to give an explanation of this principle, but shews that the mutual attraction of the particles of water is far from being a sufficient cause to account for it.

The first phenomenon which the author proposes to explain by this established principle, is the emulsion of a fluid through different adjutages applied to the reservoir which contains it. It is known that the vein of fluid which issues from an orifice or perforation through a thin plate, becomes contracted, so as to exhibit a section equal to about 0.64 of the orifice itself, supposed to be circular; and that the place of the greatest contraction is usually at the distance of one semi-diameter of the orifice itself. If a small adjutage be adapted to the orifice, having its internal cavity of the same conoidal form as the fluid itself affects in that interval, the expenditure is the same as by the simple orifice. But if at the extremity of this adjutage a cylindric tube be affixed, of a greater diameter than that of the contracted vein, or a divergent conical tube, the expense of fluid increases, and may exceed the double of that which passes through the aperture in the thin plate, though the adjutage possess an horizontal or even ascending direction." By the interposition of a small adjutage, adapted to the form of the contracted vein, Venturi ascertained, in the first place, that there is an increase of velocity in the tubes he employed, though the velocity of emission itself be less than that of the stream which issues from a hole in a thin plate. He afterwards proves, by the fact, that the interior velocity and expenditure of fluid, which is increased through tubes, even in the horizontal or ascending direction, is owing to the pressure of the atmosphere. If the smallest hole be made in the side of the tube near the place of contraction of the vein, the increased expenditure does not take place; and when a vertical tube is inserted in such a hole, the lower end of which tube is immersed in water or mercury, it is found that aspiration takes place, and the water or mercury rises; and this aspiration in conical tubes is less in proportion, as the place of insertion of the upright tube is more remote from the section where the greatest contraction would have taken place. And, lastly, the difference between the expenditure of fluid, through an orifice made in a thin plate, and that which is observed through an additional tube, does not take place in vacuo.

The influence of the weight of the atmosphere on the horizontal or ascending flux being thus established, the author considers it as a secondary cause, referable to, and explicable by, his principle of the lateral communication of motion in fluids. In conical divergent tubes, for example, the effect of this lateral communication is, that the central cylindrical jet, having for its basis the section of the contracted vein, carries with it the lateral fluid which would have remained stagnant in the enlarged part of the cone. Hence a vacuum tends to be produced in this enlarged part which surrounds the central cylindric stream; the pressure of the atmosphere becomes active to supply the void, and is exerted on the surface of the reservoir, so as to increase the velocity of the fluid at the interior extremity of the tube.

The author proves, that the velocity or total expenditure of fluid through an aperture of given dimensions, may be increased by a proper adjutage in the proportion of 24 to 10; he applies this result to the construction of the funnels of chimneys. He determines the loss of emitted fluid, which may be sustained by finnosity in pipes. He shews by experiment, that a pipe which is enlarged in any part affords a much less quantity of fluid than if it were throughout of a diameter equal to that of its smallest section. This, as he remarks, is a circumstance to which sufficient attention has not been paid in the construction of hydraulic machines. It is not enough to avoid elbows and contractions; for it sometimes happens that, by an intermediate enlargement, the whole of the advantage arising from other judicious dispositions of the parts of the machine is lost.

There are two causes of the increase of expenditure through descending pipes. The first is owing to the lateral communication of motion which takes place in descending pipes, in the same manner as in those which possess an horizontal situation; the second arises from the acceleration by gravity which takes place in the fluid while it falls through the descending tube. This second kind of augmentation was known to the ancients, though they possessed no good theory nor decisive experiments respecting it. The author endeavours to establish a theory on the principle of virtual ascension combined with the pressure of the atmosphere. His deductions are confirmed by experiment, in which he has succeeded so far as to separate the two causes of augmentation, and assign each their respective degree of influence.

Professor Venturi then proceeds to different objects of enquiry, to which his principle seemed applicable. He gives the theory of the water blowing machine (see Water Blowing Machine in this Supplement), and he determines by calculation the quantity of air which one of these machines can afford in a given time. He observes, that the natural falls of water in the mountains always produce a local wind; and he even thinks, that the falling streams in the internal parts of mountains are in some instances the cause of the winds which issue from caves. He proves, by the facts, that it is possible, in certain instances, to carry off, without any machinery, the waters from a spot of ground, though it may be situated on a lower level than that of the channel which is to receive the water.

The whirlpools, or circular eddies of water so frequent in rivers, are, according to the theory of our author, the effects of motion communicated from the parts of the current which are most rapid, to those lateral parts which are least so. In the application of this principle, he points out the circumstances adapted to produce such eddies at the surface or at the bottom of rivers. He concludes, that every movement of this kind destroys a part of the force of the current, and that in a channel through which water constantly flows, the height of this fluid will be greater than it would have been if the dimensions of the channel had been uniformly reduced to the measure of its smallest section.

There is another kind of whirling motion somewhat different in its nature from these last. It is produced in the water of a reservoir, when it is suffered to flow through an horizontal orifice. The author deduces the theory of these vortices from the doctrine of central forces. The form of the hollow funnel, which in this case opens through the fluid of the reservoir, is a curve of the 64th species of the lines of the third order, enumerated by Newton. Theory and experiment both unite here in proving, that it is not only possible, but that there really exists in nature a vortex, the convexity of which is convex towards the axis, and of which the revolutions of its different parts follow the ratio of the square of the distance from the centre. Daniel Bernoulli was in the wrong, in his Hydrodynamics, to reproach Newton for having supposed a vortex to be moved according to this law.

In the last place, the author considers that lateral communication of motion which takes place in the air as well as in the water. This is the cause of such local and partial winds as sometimes blow contrary to the direction of the general wind. It is by virtue of the same principle, that the resonant vibration, excited laterally in the extremity of an organ pipe, is communicated to the whole column of air contained in the pipe itself.

From the same principle, the author deduces the augmentation of force which found receives in conical divergent tubes, compared with those of a cylindrical form. On this occasion, he points out the remarkable diffe- differences which appear to take place between the resonant vibrations of air contained in a tube, and the sonorous pulsations propagated through the open atmosphere. See Speaking Trumpet, Encycl.

In an appendix, Venturi relates different experiments which he has made to determine the convergence and velocity of the fluid filaments which press forward to issue out of a reservoir by an orifice through a thin plate. He proves, by a very clear experiment, that the contraction of the vein is made at a greater distance from the orifice under strong than under weak pressures. He explains why, in a right lined orifice, the sides of the contracted vein correspond with the angles of the orifice and the angles with the sides. He examines the expenditure through a tube, the extremity of which is thrust into the reservoir itself, according to the method of Borda in the Memoirs of the Academy of Sciences for the year 1766.

For a full account of the author's experiments, and his deductions from them, we refer the reader either to the original work, intitled, Recherches expérimentales sur le Principe de la Communication latérale du Mouvement dans les Fluides, appliqué à l'Explication de divers Phénomènes hydrauliques. Par le Citoyen J. B. Venturi, Professeur de Physique expérimentale à Modène, Membre de la Société Italienne, &c., &c. A Paris chez Houdet et Ducros, Rue du Bacq, N° 940—Théophile Barrois, Rue Haute-chaussée, N° 23, Ann. VI. 1797—or to the 2d and 3d vols of the valuable Journal from which this abstract is taken.