cannot illustrate the use of this instrument at sea better than to give the following extract from Dr. Arnot, who was himself present at the time. "It was," says he, "in a southern latitude. The sun had just set with a placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch proceeding, when the captain's orders came, to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the oldest sailors had not per ceived even a threatening in the sky, and were surprised at the extent, and hurry of the preparations; but the required measures were not completed, when a more awful hurricane burst upon them, than the most experienced had ever braved. Nothing could withstand it; the sails already furled, and closely bound to the yards, were riven into tatters; even the bare yards and masts were in a great measure disabled; and at one time the whole rigging had nearly fallen by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and the incessant peals of thunder, that no human voice could be heard, and amidst the general consternation, even the trumpet sounded in`vain. On that awful night, but for a little tube of mercury, which had given the warning, neither the strength of the noble ship, nor the skill and energies of her commander, could have saved one man to tell the tale."

Pumps.

There is a philosophical experiment, of which no one in this country is ignorant. If one end of a straw be introduced into a barrel of cider, and the other end sucked with the mouth, the cider will rise up through the straw, and may be swallowed.

The principles which this experiment involve, are exactly the same as those concerned in raising water by the pump. The barrel of cider answers to the well, the straw to the pump log, and the mouth acts as the piston, by which the air is removed.

The efficacy of the common pump, in raising water, depends upon the principle of atmospheric pressure, which has been fully illustrated under the articles air pump and ba

rometer.

What remarkable instance is stated, where a ship seemed to be saved by the use of the barometer? What experiment is stated, as illustrating the principle of the common pump?

These machines are of three kinds, namely, the sucking, or common pump, the lifting pump, and the forcing pump.

Fig. 107.

b

a

Of these, the common or household pump cis the most common, and for ordinary pur-poses, the most convenient. It consists of a long tube, or barrel, called the pump log, which reaches from a few feet above the ground to near the bottom of the well. At a, fig. 107, is a valve, opening upwards, called the pump box. When the pump is not in action, this is always shut. The piston b, has an aperture through it, which is closed by a valve, also opening upwards.

By the pupil who has learned what has been explained under the articles air pump and barometer, the action of this machine will be readily understood.

Suppose the piston b, to be down to a, then on depressing the lever c, a vacuum would be formed between a, and b, did not the water in the well rise, in consequence of the pressure of the atmosphere on that around the pump log in the well, and take the place of the air thus removed. Then on raising the end of the lever, the valve a closes, because the water is forced upon it, in consequence of the descent of the piston, and at the same time the valve in the piston b opens, and the water, which cannot descend, now passes above the valve b. Next, on raising the piston, by again depressing the lever, this portion of water is lifted up to b, or a little above it, while another portion rushes through the valve a, to fill its place. After a few strokes of the lever, the space from the piston b to the spout is filled with the water, where, on continuing to work the lever, it is discharged in a constant stream.

Although, in common language, this is called the suction pump, still it will be observed, that the water is elevated by suction, or in more philosophical terms, by atmospheric pres

On what does the action of the common pump depend? How many kinds of pumps are mentioned? Which kind is the common? Describe the common pump. Explain how the common pump acts. When the lever is depressed, what takes place in the pump barrel? When the lever is elevated, what takes place? How far is the water raised by atmospheric pressure, and how far by lifting?

sure, only above the valve a, after which it is raised by lifting up to the spout. The water, therefore, is pressed into the pump barrel by the atmosphere, and thrown out by lifting.

The lifting pump, properly so called, has the piston in the lower end of the barrel, and raises the water through the whole distance, by forcing it upward without the agency of the atmosphere.

In the suction pump, the pressure of the atmosphere will raise the water 33 or 34 feet, and no more, after which it may be lifted to any height required.

The forcing pump differs from both these in having its piston solid, or without a valve, and also in having a side pipe, through which the water is forced, instead of rising in a perpendicular direction, as in the others.

-g

Fig. 108.

upper

The forcing pump is represented by fig. 108, where a is a solid piston, working air tight in its barrel. The tube c, leads from the barrel to the air vessel d. Through the pipe p the water is thrown into the open air. g is a gauge, by which the pressure of the water in the air vessel is ascertained. Through the pipe i, the water ascends into the barrel, the end being furnished Z with a valve opening upwards. To explain the action of this pump, suppose the piston to be down to the bottom of the barrel, and then to be raised upward by the lever l; the tendency to form a vacuum in the barrel will bring the water up through the pipe i, by the pressure of the atmosphere. Then on depressing the piston, the valve at the bottom of the barrel will be closed, and the water, not finding admittance through the pipe whence it came, will be forced through the pipe c, and opening the valve at its upper end, will enter

How does the lifting pump differ from the common pump? How does the forcing pump differ from the common pump? Explain fig. 108, and show in what manner the water is brought up through the pipe i and afterwards thrown out at the pipe p.

into the air vessel d, and be discharged through the pipe p, into the open air.

The water is therefore elevated to the piston barrel by the pressure of the atmosphere, and afterwards thrown out by the piston. It is obvious that by this arrangement, the height to which the fluid may be thrown, will depend on the power applied to the lever, and the strength with which the pump is made.

The air vessel d, contains air in its upper part only, the lower part, as we have already seen, being filled with water. The pipe p, called the discharging pipe, passes down into the water so that the air cannot escape. The air is therefore compressed, as, the water is forced into the lower part of the vessel, and reacting upon the fluid by its elasticity, throws it out of the pipe in a continued stream. The constant stream which is emitted from the direction pipe of the fire engine is entirely owing to the compression, and elasticity of the air in its air vessel. In pumps without such a vessel, as the water is forced upwards, only while the piston is acting upon it, there must be an interruption of the stream while the piston is ascending, as in the common pump. The air vessel is a remedy for this defect, and is found also to render the labour of pumping more easy, because the force with which the air in the vessel acts on the water, is always in addition to that given by the force of the piston.

b

Fig. 109.

The fire engine is a modification of the forcing pump. It consists of two such pumps, the pistons of which are moved by a lever with equal arms, the common fulcrum being at c, fig. 109. While the piston a is descending, the other piston b, is ascenda ing. The water is forced by

pressure of the atmosphere, through the common pipe p, and then dividing, ascends into the working barrels of each piston, where the valves, on both sides, prevent its return. By the alternate depression of the pistons, it is then forced into the air box

Why does not the air escape from the air vessel in this pump?

d, and then by the direction pipe e, is thrown where it is want. ed. This machine acts precisely like the forcing pump, only that its power is doubled by having two pistons instead of one. Fig. 110.

a

There is a beautiful fountain, called the fountain of Hiero, which acts by the elasticity of the air, and on the same principle as that already describ. ed. Its construction will be understood by fig. 110, but its form may be varied according to the dictates of fancy or taste. The boxes a and b, together with the two tubes, are made air tight, and strong, in proportion to the height it is desired the fountain should play.

To prepare the fountain for action, fill the box a, through the spouting tube nearly full of water. The tube c, passing nearly to the top of the box, will prevent the water from passing downwards, while the spouting pipe will prevent the air from escaping up. wards, after the vessel is about half filled with the water. Next shut the stop cock, of the spouting pipe, and pour water into the open vessel d. This will descend into the vessel b, through the tube, which nearly reaches its bottom, so that after a few inches of water are poured in, no air can escape except by the tube c, up into the vessel a. The air will then

be compressed by the weight of the column of water in the tube e, and therefore the force of the water from the jet pipe will be in proportion to the height of this tube. On turning the stop cock, a jet of water will spout from the pipe that will amuse and astonish those who have never before seen such an experiment. [For other properties of air, see Chemistry.]

ACOUSTICS.

Acoustics is that branch of natural philosophy which treats of the origin, propagation, and effects of sound.

What effect does the air vessel have on the stream discharged? Why does the air vessel render the labor of raising the water more easy? Explain fig. 109, and describe the action of the fire engine. What cau ses the continued stream from the direction pipe of this engine? How is the fountain of Hiero constructed?