wine through the bung-hole, and when it has become filled to the level of the liquor in the cask, the thumb is placed over the upper end, and the instrument is withdrawn. A portion of the wine is

held in the tube, being retained by the atmospheric pressure, and if the tube be placed over a tumbler, and the thumb be raised, the wine will flow out. This is the principle of the dropping tube, employed by druggists and others.

Pressure on the Human Body.

119. The pressure on each square inch of the body is 15 lbs.; hence, on the whole body the pressure is enormous. If we take the surface of the human body equal to 2000 square inches, which is not far from the average in the case of an adult, the pressure amounts to 30,000 pounds, or

15 tons.

If it be asked why the body is not crushed by this enor mous pressure, the answer is, because it is uniformly distributed over the whole surface, and is resisted by the elastic force of air, and other gases, distributed through the tissues of the body.

The following experiment shows that the tissues of the human

Describe it and its use. What is the dropping tube? (119.) What is the amount of atmospheric pressure on the human body? How is this pressure resisted?

body contain air and gases, whose elasticity resists the atmospheric pressure. Let the hand be pressed closely upon the mouth of a glass cylinder, whose interior communicates with the air-pump, as shown in Fig. 82. No inconvenience will be felt. But if the air be exhausted from the cyl

inder, the flesh of the hand will be forced into the cylinder by the pressure from without, which is no longer resisted by the pressure of the air. The hand swells, and the blood tends to flow out through the pores.

The question may be asked, why, when the hand is placed upon a body, it is not retained there by the pressure of the atmosphere. The answer is, there is a thin layer of air between the hand and the body, which exactly counterbalances the effect of the external pressure. Were the air perfectly excluded from between the hand and the body, there would be a strong tendency to adher

ence between them.

How shown that the tissues of the body contain gases? Explain experiment. Principle of cupping.

120. When a given mass of any gas or vapor is compressed, so as to occupy a smaller space, its elastic force is increased; on the contrary, when the volume is increased, its elastic force is diminished.

The law of increase and diminution of elastic force was first made known by MARIOTTE; hence it was called by his MARIOTTE'S law may be enunciated as follows:

name.

The elastic force of any given amount of gas, whose temperature remains the same, varies inversely as its volume. As a consequence of this law it follows that,

If the temperature remains constant, the elastic force raries as the density.

Mariotte's Tube.

121. MARIOTTE's law may be verified by means of an apparatus, shown in Figs. 83 and 84, called Mariotte's Tube. This tube is of glass, bent into the shape of a letter J. The short branch is closed, and the long one open at the top. The tube is attached to a wooden frame, provided with suitable scales for measuring the heights of mercury and air in the two branches.

The instrument having been placed vertical, a sufficient quantity of mercury is poured into the long branch to cut off communication between the two branches, as shown in Fig. 83. The level of the mercury in the two branches is the same, and this level is at the 0 point of the two scales. The air in the short branch is of the same density, and has · the same tension as that of the external atmosphere.

(120.) What is MARIOTTE'S Law? Consequence? (121.) Describe Mariotte's Tube.

ELASTIC FORCE OF GASES.

125

If an additional quantity of mercury be poured into the longer branch of the tube, it will press upon the air in the shorter branch, and compress it. If the difference of level

in the two branches be made equal to the height of the barometrical column, as shown in Fig. 84 (where the difference is 76 centimetres, or 29.92 inches), the air will be compressed into BC, one half of its original bulk.

How used to verify the law?

In the figure, the air in BC is subjected to the pressure of two atmospheres, one from the actual atmosphere, transmitted through the mercury, and an equal pressure from the weight of the mercury, AC, which is equal to that of an atmosphere.

If the difference of height, AC, be made equal to two, three, four, &c., times that of the barometric column, the air in BC will be reduced to one third, one fourth, one fifth, &c., of its original bulk.

Manometers.

122. A MANOMETER is an apparatus for measuring the elastic force of a gas or vapor.

There are two principal kinds of manometers, the open and the closed manometer.

The Open Manometer.

123. Fig. 85 represents an OPEN MANOMETER, such as is often used for measuring the pressure of steam in a boiler.

It consists of a narrow tube of glass fixed against a verti cal wall, and communicating with a cistern of mercury, C. A pipe leads from the boiler to the cistern, C, and by means of a stop-cock, steam may be admitted to the cistern, or cut off at pleasure.

When the tension of the steam in the boiler is just equal to that of the atmosphere, the mercury stands at the same level in the tube and cistern. When the tension of the steam becomes equal to twice that of the atmosphere, the mercury is forced from the cistern into the pipe, where it rises till the difference of level is 30 inches. This is marked 2 on the tube, and when the mercury is at this division, the tension of the steam is two atmospheres. The divisions 3, 4, 5, &c., are placed at distances of 30 inches, and when the mercury stands at any one of them, the manometer indicates a tension of the corresponding number of atmospheres.

(122.) What is a Manometer? How many kinds are employed? What are they? (123.) Describe the Open Manometer. Explain its action.