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it is the pressure of the atmosphere, on the water around the tube, which forces the fluid up to fill the space, thus left by the air. It is also proved, that the weight, or pressure of the atmosphere, is equal to the weight of a perpendicular column of water, 33 feet high, for it is found (fig. 102), that the pressure of the atmosphere will not raise water more than 33 feet, though a perfect vacuum be formed to any height above this point. Experiments on other fluids, prove that this is the weight of the atmosphere, for if the end of a tube be dipped in any fluid, and the air be removed from the tube, above the fluid, it will rise to a greater or less height, than water, in proportion as its specific gravity is less or greater than that of water.

Mercury, or quicksilver has a specific gravity of about 13 times greater than that of water, and mercury is found to rise about 29 inches in a tube under the same circumstances that water rises 33 feet. Now 33 feet is 396 inches, which, being divided by 29 gives nearly 131, so that mercury being 13 times heavier than water, the water will rise under the same pressure 13 times higher than the mercury.

The barometer is constructed on the principle of atmospheric pressure, which we have thus endeavoured to explain and Fig. 105. illustrate to common comprehension. This term is compounded of two Greek words, baros, weight, and metron, measure, the instrument being designed to measure the weight of the atmosphere.

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Its construction is simple, and easily understood, being merely a tube of glass nearly filled with mercury, with its lower end placed in a dish of the same fluid, and the upper end furnished with a scale, to measure the height of the mercury.

Let a, fig. 105, be such a tube, 34 or 35 inches long, closed at one end and open at the other. To fill the tube, set it upright, and pour the mercury in at the open end, and

What conclusion follows from the experiments above described? How is it proved, that the pressure of the atmosphere is equal to the weight of a column of water, 33 feet high? How do experiments on other fluids show that the pressure of the atmosphere is equal to the weight of a column of water 33 feet high? How high does mercury rise in an exhausted tube? What is the principle on which the baro meter is constructed? What does the barometer measure?

when it is entirely full, place the fore finger forcibly on this end, and in this state plunge the tube and finger under the surface of the mercury before prepared in the cup b. Then withdraw the finger, taking care that in doing this, the end of the tube is not raised above the mercury in the cup. When the finger is removed, the mercury will descend four or five inches, and after several vibrations, up and down, will rest at an elevation of 29 or 30 inches above the surface of that in the cup, as at c. Having fixed a scale to the upper part of the tube, to indicate the rise and fall of the mercury, the barometer would be finished, if intended to remain stationary. It is usual, however, to have the tube inclosed in a mahogany or brass case, to prevent its breaking, and to have the cup closed on the top, and fastened to the tube, so that it can be trans. ported without danger of spilling the mercury.

The cup of the portable barometer also differs from that represented in the figure, for, were the mercury inclosed on all sides, in a cup of wood, or brass, the air would be prevented from acting upon it, and therefore the instrument would be useless. To remedy this defect, and still have the mercury perfectly inclosed, the bottom of the cup is made of leather, which, being elastic, the pressure of the atmosphere acts upon the mercury in the same manner as though it was not inclosed at all. Below the leather bottom there is a round plate of metal an inch in diameter, which is fixed on the top of a screw, so that when the instrument is to be transported, by elevating this piece of metal, the mercury is thrown up to the top of the tube and thus kept from playing backwards and forwards, when the barometer is in motion.

A person not acquainted with the principle of this instrument, on secing the tube turned bottom upwards, will be perplexed to understand why the mercury does not follow the common law of gravity, and descend into the cup; were the tube of glass 33 feet high, and filled with water, the lower end being dipped into a tumbler of the same fluid, the wonder would be still greater. But as philosophical facts, one is no more wonderful than the other, and both are readily explained by the principles already illustrated.

Describe the construction of the barometer, as represented by fig. 105. How is the cup of the portable barometer made, so as to retain the mercury, and still allow the air to press upon it? What is the use of the metallic plate and screw, under the bottom of the cup? Explain the reason why the mercury does not fall out of the barometer tube, when its open end is downwards.

It has already been shown, that it is the pressure of the atmosphere on the fluid around the tube, by which the fluid within it, is forced upward, when the pump is exhausted of its air. The pressure of the air we have also seen, is equal to a column of water 33 feet high, or of a column of mercury 29 inches high. Suppose, then, a tube 33 feet high is filled with water, the air would then be entirely excluded, and were one of its ends closed, and the other end dipped in water, the effect would be the same as though both ends were closed, for the water would not escape, unless the air were permitted to push in and fill up its place. The upper end being closed, the air could gain no access in that direction, and the open end being under water, is equally secure. The quantity of water in which the end of the tube is placed, is not essential, since the pressure of a column of water an inch in diameter, provided it be 33 feet high, is just equal to a column of air of an inch in diameter, of the whole height of the atmosphere. Hence the water on the outside of the tube serves merely to guard against the entrance of the air.

The same happens to the barometer tube, when filled with mercury. The mercury, in the first place, fills the tube perfectly, and therefore entirely excludes the air, so that when it is inverted in the cup, all the space above 29 inches is left a vacuum. The same effect precisely would be produced, were the tube exhausted of its air, and the open end placed in the cup; the mercury would run up the tube 29 inches, and then stop, all above that point being left a vacuum.

The mercury, therefore, is prevented from falling out of the tube, by the pressure of the atmosphere on that which remains in the cup; for if this be removed, the air will enter, while the mercury will instantly begin to descend.

In the barometer described, the rise and fall of the mercury is indicated by a scale of inches and tenths of inches, fixed behind the tube; but it has been found, that very slight vari. ations in the density of the atmosphere, are not readily per ceived by this method. It being, however, desirable that these minute changes should be rendered more obvious, a contrivance for increasing the scale, called the wheel barometer was invented.

What fills the space above 29 inches, in the barometer tube? In the common barometer how is the rise and fall of the mercury indicated? Why was the wheel barometer invented?

Fig. 106.

The whole length of the tube of the wheel barometer, fig. 106, from c to a, is 34 or 35 inches, and is filled with mercury, as usual. The mercury rises in the short leg to the point o, where there is a small piece of glass floating on its surface, to which there is attached a silk string, passing over the pulley p. To the axis of the

pulley is fixed an index, or hand, and behind this is a graduated circle, as seen in the figure. It is obvious, that a very slight variation in the height of the mercury at o, will be indicated by a considerable motion of the index, and thus changes in the weight of the atmosphere hardly perceptible by the common barometer, will be. come quite apparent by this.

The mercury in the barometer tube being sus. tained by the pressure of the atmosphere, and its medium altitude at the surface of the earth being about 29 inches, it might be expected that if the instrument was carried to a height from the earth's surface, the mercury would suffer a proportionate fall, because the pressure must be less, at a distance from the earth than at its surface, and experiment proves this to be the case. When, therefore, this instrument is elevated to any considerable height, the descent of the mercury becomes perceptible. Even when it is carried to the top of a hill, or high tower, there is a sensible depression of the mercury, so that the barometer is employed to measure the heights of mountains, and the elevation to which balloons ascend from the surface of the earth. On the top of Mont Blanc, which is about 16000 feet above the level of the sea, the medium elevation of the mercury in the tube is only 14 inches, while on the surface of the earth as above stated, it is 29 inches.

The medium range of the barometer in several countries, has generally been stated to be about 29 inches. It appears, however, from observations made at Cambridge, in Massachu setts, for the term of 22 years, that its range there was nearly 30 inches.

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Explain fig. 106, and describe the construction of the wheel baromeWhat is stated to be the medium range of the barometer at the surface of the earth? Suppose the instrument is elevated from the earth, what is the effect on the mercury? How does the barometer indicate the heights of mountains? What is the medium range of the mercury on Mont Blanc? What is stated to be the medium range of the barometer at Cambridge?

While the barometer stands in the same place, near the level of the sea, the mercury seldom falls below 28 inches, or rises above 31 inches, its whole range, while stationary, being only about 3 inches.

These changes in the weight of the atmosphere, indicate corresponding changes in the weather, for it is found, by watching these variations in the height of the mercury, that when it falls, cloudy or falling weather ensues, and that when it rises, fine clear weather may be expected. During the time when the weather is damp and lowering, and the smoke of chimnies descends towards the ground, the mercury remains depressed, indicating that the weight of the atmosphere during such weather is less than it is when the sky is clear. This contradicts the common opinion, that the air is the heaviest when it contains the greatest quantity of fog and smoke, and that it is the uncommon weight of the atmosphere which presses these vapors towards the ground. A little consideration will show, that in this case the popular belief is erroneous, for not only the barometer, but all the experiments we have detailed, on the subject of specific gravity, tend to show that the lighter any fluid is, the deeper any substance of a given weight will sink in it. Common observation, ought, therefore, to cor. rect the error, for every body knows that a heavy body will sink in water while a light one will swim, and by the same kind of reasoning ought to consider, that the particles of vapor would descend through a light atmosphere, while they would be pressed up into the higher regions, by a heavier air. The principal use of the barometar is on board of ships, where it is employed to indicate the approach of storms, and thus to give an opportunity of preparing accordingly; and it is found that the mercury suffers a most remarkable depression before the approach of violent winds, or hurricanes. The watchful captain, particularly in southern latitudes, is always attentive to this monitor, and when he observes the mercury sink suddenly, takes his measures without delay to meet the tempest. During a violent storm, we have seen the wheel barometer sink a hundred degrees in a few hours. But we

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How many inches does a fixed barometer vary in height? When the mercury falls, what kind of weather is indicated? When the mercury rises, what kind of weather may be expected? When fog and smoke descend towards the ground, is it a sign of a light or heavy atmosphere? By what analogy is it shown that the air is lightest when filled with vapor? Of what use is the barometer, on board of ships? When does the mercury suffer the most remarkable depression.?

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