II. MEASURE

ELASTIC
THE
OF

FORCE OF GASES.

Mariotte's Law.

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 varies 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.

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 com. pressed 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 vertical 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.

124.

The Closed Manometer.

The CLOSED MANOMETER is shown in Fig. 86, and differs from the one just described, in having its vertical tube closed at the top. It is graduated on the principle enunciated in MARIOTTE'S law.

(124.) Describe the Closed Manometer. How is it graduated?

When the pressure in the boiler is one atmosphere, the mercury in the cistern and tube are at the same level, the tension of the steam and the elastic force of the air just balancing each other. When the pressure becomes two, three, four, &c., atmospheres, the air in the closed tube will occupy one half, one third, one fourth, &c., the space it did before, allowance being made for the weight of the mercury which is forced up into the tube. The instrument having been graduated, its use is evident. When it is desired to ascertain the tension of the steam in the boiler, the cock is turned, and the height to which the mercury ascends in the tube, indicates the tension in atmospheres. Any number of subdivisions may be made in either of the two manometers described.

Besides these, there is a metallic manometer, invented by M. BOURDON, and known as BOURDON'S Metallic Manometer.

It is not so reliable as those described.

Fig. S6.

III. APPLICATION то PUMPS AND OTHER MACHINES.

The Air-pump.

125. An AIR-PUMP is a machine for exhausting the air from a closed space. The air-pump was invented by OTTO VON GUÉRICKE, in 1650.

A perspective view of one of the most common forms of the airpump is given in Fig. 87. The details of its construction will be best studied from Figs. 88 and 89; the former represents a longi

Illustrate. How is this manometer used? (125.) What is an Air-pump? When invented, and by whom?