By means of this instrument, it can be told with great accuracy how much water has been added to spirits, for the greater the quantity of water, the higher will the scale rise above the surface.

The adulteration of milk with water, can also be readily detected with it, for as new milk has a specific gravity of 1032, water being 1000, a very small quantity of water mixed with it would be indicated by the instrument. (See Specific Gravity in Chemistry.)

The Syphon.

Take a tube, bent like the letter U, and having filled it with water, place a finger on each end, and in this state plunge one of the ends into a vessel of water, so that the end in the water shall be a little the highest, then remove the fingers, and the liquid will flow out, and continue to do so, until the vessel is exhausted.

A tube acting in this presented by fig. 90. Fig. 90.

manner, is called a syphon, and is reThe reason why the water flows from

the end of the tube a, and consequently ascends through the other part, is, that there is a greater weight of the fluid from b to a, than from c to b, because the perpendicular height from b to a is the greatest. The weight of the water from b to a falling downwards, by its gravity, tends to form a vacuum, or void space, in that leg of the tube; but the pressure of the atmosphere on the water in the vessel, constantly forces the fluid up the other leg of the tube, to fill the void space, and thus the stream is continued as long as any water remains in the vessel.

The action of the syphon depends upon the same principle as the action of the pump, namely, the pressure of the atmosphere, and therefore its explanation properly belongs to Pneumatics. It is introduced here merely for the purpose of illustrating the phenomena of intermitting springs; a subject which properly belongs to Pneumatics.

Alcohol has a specific gravity of 809; what, in reference to this, is the specific gravity of water? In what manner is a syphon made? Explain the reason why the water ascends through one leg of the syphon, and descends through the other.

Some springs, situated on the sides of mountains, flow for a while with great violence, and then cease entirely. After a time they begin to flow again, and then suddenly stop, as before. These are called intermitting springs. Among ignorant and superstitious people, these strange appearances have been attributed to witchcraft, or the influence of some supernatural power. But an acquaintance with the laws of nature will dissipate such ill founded opinions, by showing that they owe their peculiarities to nothing more than natural syphons, existing in the mountains from whence the water flows.

Fig. 91 is the section of a mountain and spring, showing how the principle of the syphon operates to produce the effect described. Suppose there is a crevice, or hollow in the rock, from a to b, and a narrow fissure leading from it, in the form of the syphon b c. The water, from the rills f, e, filling the hollow, up to the line a d, it will then discharge itself through the syphon, and continue to run until the water is exhausted down to the leg of the syphon b, when it will cease. the water from the rills continuing to run until the hollow is again filled up to the same line, the syphon again begins to act, and again discharges the contents of the reservoir as before, and thus the spring p, at one moment, flows with great violence, and the next moment ceases entirely.

Then

The hollow, above the line a d, is supposed not to be filled with the water at all, since the syphon begins to act whenever the fluid rises up to the bend d.

What is an intermitting spring? How is the phenomenon of the intermitting spring explained? Explain fig. 91, and show the reason why such a spring will flow, and cease to flow, alternately.

During the dry seasons of the year, it is obvious, that such a spring would cease to flow entirely, and would begin again only when the water from the mountain filled the cavity through the rills.

Such springs, although not very common, exist in various parts of the world. Dr. Atwell has described one in the Philosophical Transactions, which he examined in Devonshire, in England. The people in the neighborhood, as usual, ascribed its actions to some sort of witchery, and advised the doctor, in case it did not ebb and flow readily, when he and his friend were both present, that one of them should retire, and see what the spring would do, when only the other was present.

HYDRAULICS.

It has been stated, that Hydrostatics is that branch of Natu ral Philosophy, which treats of the weight, pressure, and equilibrium of fluids, and that Hydraulics has for its object the investigation of the laws which regulate fluids in motion.

If the pupil has learned the principles on which the pressure and equilibrium of fluids depend, as explained under the former article, he will now be prepared to understand the laws which govern fluids when in motion.

The pressure of water downwards, is exactly in the same proportion to its height, as is the pressure of solids in the same direction.

Suppose a vessel of three inches in diameter has a billet of wood set up in it, so as to touch only the bottom, and suppose the piece of wood to be three feet long, and to weigh nine pounds; then the pressure on the bottom of the vessel will be nine pounds. If another billet of wood be set on this, of the same dimensions, it will press on its top with the weight of nine pounds, and the pressure at the bottom will be 18 pounds, and if another billet be set on this, the pressure at the bottom will be 27 pounds, and so on, in this ratio, to any height the column is carried.

Now the pressure of fluids is exactly in the same proportion; and when confined in pipes, may be considered as one short column set on another, each of which increases the pressure of the lowest, in proportion to their number and height.

How does the science of Hydrostatics differ from that of Hydraulics? Does the downward pressure of water differ from the downward pressure of solids, in proportion? How is the downward pressure of water illustrated?

w

9

12

g. 92.

Thus, notwithstanding the lateral pressure of fluids, their downward pressure is as their heights. This fact will be found of importance in the investigation of the principles of certain hydraulic machines, and we have therefore endeavoured to impress it on the mind of the pupil by fig. 92. where it will be seen, that if the pressure of three feet of water be equal to nine pounds on the bottom of -18 the vessel, the pressure of twelve feet will be equal to thirty-six pounds.

The quantity of water which will be discharged from an orifice of a given size, will be in propor27tion to the height of the column of water above it, for the discharge will increase in velocity, in proportion to the pressure, and the pressure, we have already seen, will be in a fixed ratio to the height.

-36

Fig. 93.

a

b

If a vessel, fig. 93, be filled with water, and three apertures be made in its side at the points a, b, and c, the fluid will be thrown out in jets, and will fall towards the earth, in the curved lines, a, b, and c. The reason why these curves differ in shape, is, that the fluid is acted on by two forces, namely, the -pressure of the water-above the jet, which produces its velocity forward, and the action of gravity, which impels it downward. It therefore obeys the same laws that solids do when projected forward, and falls down in curved lines, the shapes of which depend on their relative velocities.

C

The quantity of water. discharged, being in proportion to the pressure, that discharged from each orifice will differ in quantity according to the height of the water above it.

It is found, however, that the velocity with which a vessel

Without reference to the lateral pressure, in what proportion do fluids press downwards? What will be the proportion of a fluid discharged from an orifice of a given size? Why do the lines described by the jets from the vessel, fig. 93, differ in shape?

discharges its contents, does not depend entirely on the pressure, but in part on the kind of orifice through which the liquid flows. It might be expected, for instance, that a tin vessel of a given capacity, with an orifice of say an inch in diameter through its side, would part with its contents sooner than another of the same capacity and orifice, whose side was an inch or two thick, since the friction through the tin might be considered much less than that presented by the other orifice. But it has been found by experiment, that the tin vessel does not part with its contents so soon as another vessel, of the same height and size of orifice, from which the water flowed through a short pipe. And, on varying the length of these pipes, it is found that the most rapid discharge, other circumstances being equal, is through a pipe, whose length is twice the diameter of its orifice. Such an aperture discharged 82 quarts, in the same time that another vessel of tin without the pipe, discharged 62 quarts.

This surprising difference is accounted for, by supposing that the cross currents, made by the rushing of the water from different directions towards the orifice, mutually interfere with each other, by which the whole is broken, and thrown into confusion by the sharp edge of the tin, and hence the water issues in the form of spray, or of a screw, from such an orifice. A short pipe seems to correct this contention among opposing currents, and to smooth the passage of the whole, and hence we may observe, that from such a pipe, the stream is round and well defined.

Friction between solids and fluids.

The rapidity with which water flows through pipes of the same diameter, is found to depend much on the nature of their internal surfaces. Thus, a lead pipe with a smooth aperture, under the same circumstances, will convey much more water than one of wood, where the surface is rough, or beset with points. In pipes, even where the surface is as smooth as

What two forces act upon the fluid as it is discharged, and how do these forces produce a curved line? Does the velocity with which a fluid is discharged, depend entirely on the pressure? What circumstance, besides pressure, facilitates the discharge of water from an orifice? In a tube discharging water with the greatest velocity, what is the proportion between its diameter and its length? What is the proportion between the quantity of fluid discharged through an orifice of tin, and through a short pipe? Suppose a lead and a glass tube, of the same diameter, which will deliver the greatest quantity of liquid in the same time?