in a thin plate of metal, avoiding all conical terminations, or ajutages.

163. Friction is found to take place between solids and liquids, and even between the particles of fluids themselves. A stream of water is always more rapid in the centre than at the sides, as, being deeper there, the current flows on the surface of lower strata of fluid; whilst, in the shallower portions of the river, the water is exposed to the friction of the rough and unequal bottom. In the centre, also, the stream is somewhat more elevated than at the sides; as in its rapid course, it draws the water from the sides of the river, by the friction of their particles, rapidly after it.

A

In the ajutage, or escape-pipe, of a fountain, a similar fact is observed; for if it be bent abruptly, and not with a regular and gradual curve, the passage of fluid becomes much obstructed. Thus, fluids escaping under equal pressures, will rise much higher if passing through the tube ▲ than

B

164. The friction of particles is illustrated by an experiment of Bernouilli: he found that water, in passing rapidly from the narrow to the wide end of a conical tube, AB, would empty the vessel c, filled with water and communicating with AB, by a small lateral tube. Dr. Barry found that a similar effect was produced by a descending current; for when water was allowed to flow rapidly from A to B, a vessel, c, communicating with AB, by the tube D, became rapidly emptied.

T

In the circulating system of animals, an arrangement of the blood-vessels is frequently observed in accordance with these principles, so that a current of

PROPERTIES OF GASEOUS CURRENTS.

117

blood, passing along one vessel, may assist in emptying a lateral branch; or two currents entering a larger trunk at the same point, may thus exhaust the contents of a small vessel entering between them. In the human body, the termination of the left spermatic veins in the renal vein, and that of the thoracic duct in the angle formed by the internal jugular and subclavian veins, afford remarkable examples of such hydraulic arrangements in animal structures.

165. Elastic fluids, or gases, offer no important exceptions to the above laws; in escaping from lateral orifices, they produce a similar reaction against the opposite side, and corresponding tendency to motion, as in the case of denser fluids (160). In the opinions of most philosophers, they also appear to obey the conditions of the theorem of Torricelli, when escaping under the influence of pressure from orifices, unless the difference between the external and internal pressures be very considerable, in which case they offer some exception to this law (157).

It is also extremely probable that, like denser fluids, gases undergo, when escaping from apertures, a contraction in the diameter of the current; the area of the section of this contraction appears to be equal to that of the orifice through which the gas is escaping, multiplied by the decimal 0.61 or 0.62.

166. One very remarkable phenomenon, connected with the escape of a current of air under considerable pressure, must not be passed over silently. M. Clement Desormes+

* The following formula is Bernouilli's expression for the velocity of an escaping current of gas:

velocity of the gas; p=internal, and p'external, pressure; and 2k a coefficient equal to 155610 for gases at the temperature of 32° Fahrenheit.

† Annales de Phys. et Chim. xxxvi. p. 69.

has observed, that when an opening, about an inch in diameter, is made in the side of a reservoir of compressed air, the latter rushes out violently; and if a plate of metal or wood, 7 inches in diameter, be pressed towards the opening, it will, after the first repulsive action of the current of air is overcome, be apparently attracted, rapidly oscillating within a short distance of the opening, out of which the air continues to emit with considerable force. This curious circumstance is explained on the supposition, that the current of air, on escaping through the opening, expands itself into a thin disk, to escape between the plate of wood or metal, and side of the reservoir; and, on reaching the circumference of the plate, draws after it a current of atmospheric air from the opposite side, in a manner, probably, analogous to the case of friction between particles of liquids already described (164). The plate thus balanced between these currents remains near the aperture, and apparently attracted by the current of air to which it is opposed.

B

167. A similar phenomenon, and apparently explicable on similar principles, is exhibited by means of a glass tube, в, fixed at one end into a card-board disk, A. Let a piece of card-board be placed over the end of the tube, and rest on the disk A. Then, on placing the end в in the mouth, and blowing forcibly through it, the piece of card resting on A will be slightly agitated, but no force person who blows through B can exert, will enable him to displace it from the aperture A. It is, in fact, virtually attracted, in the same manner as the wooden or metallic disk, in the experiments of Desormes (166).

the

168. The application of the physical properties of fluids to the purposes of domestic economy, and the wants of civilized life, are extremely important, and afford some impor

tant objects of study to the mechanic and engineer. An outline of the mode of action of a very few of these valuable presents of science to art, will not be misplaced in this chapter, as they will afford an opportunity to the student, of explaining their mode of action on the principles already laid down.

B

E

169. Among the various instruments used to elevate fluids above their former level, those termed pumps are the most important. Their theoretical construction is extremely simple: they may be divided into two chief sections; the first including the sucking and lifting; the other, the forcing pumps.

The sucking or suction pump, as it is incorrectly termed, consists essentially of a hollow cylinder, AB, having a valve, E, opening upwards, fixed in its lower extremity. A piston, c, furnished with a valve also opening upwards, moves in the interior of the cylinder. If the lower end of the pump be immersed in water, and the piston be depressed to E, the air between CE will escape by the valve in c, and on elevating the piston, a partial vacuum is formed below c; which the water rushes in, through E, to supply. On once more depressing c, this water elevates the valve in the piston, and passes through it, so that on again elevating c, a column of water is raised with it, which eventually escapes through the side tube, or spout, D. On thus continuing alternately to raise and depress the piston, water may be raised from the reservoir in which the lower end of the pump is placed. The action of the lifting pump is so similar, that a distinct account of it is unnecessary; as usually constructed, it differs chiefly, from the pump just described, in the piston entering the cylinder from below, instead of from above.

170. The forcing pump differs from the last in the posi

B

D

E

tion of its valves: the piston в moving air-tight in the cylinder FG, as in the sucking pump, but has no valve. A valve opening upwards is fixed in the lower part of the cylinder; and at G, a lateral tube, GE, is fixed, having a valve, D, opening upwards. The rationale of the action of this apparatus is very obvious on в being depressed to c, the air is forced through the valve D; and if the pump has its lower end plunged in water, on raising в, the fluid will rush in through c, to supply the partial vacuum thus formed. And on depressing the piston, this portion of water will be forced through the valve D out of the side tube GE, as, in consequence of the valve c opening upwards, it cannot escape downwards.

171. That most valuable acquisition to modern medicine, the well-known stomach-pump,is an instrument of this description; the tube introduced into the stomach being alternately connected to the lower end, or the side tube E, according as it is required to inject fluid into, or to empty the contents of the stomach. A glance at the construction of these pumps will be sufficient to point out their similarity to the air-pump (152): in the ordinary pump, on raising the piston, water instead of air rushes in, and, on that account, the valves do not require that excessive care in their construction, which is necessary for the proper action of a good air-pump.*

In all kinds and modifications of pumps, or other instruments by which water is raised above its former level by means of atmospheric pressure, it must be recollected that

• A beautifully-constructed syringe, on the principle of the forcing pump, has lately been contrived by Mr. Read, the well-known inventor of the stomach-pump; in which the valves (which are of metal) are so well arranged, that the apparatus answers both as a pump for liquids and gases. This apparatus holds out the prospect of affording as important aid in certain cases of asphyxia, by removing the noxious gas from the airpassages, as it has already effected, in removing poisons from the stomachs of those who had inadvertently, or intentionally, swallowed them.