water, through which the pressure of the steam is transmitted to the mercury.

To graduate the instrument. All communication with the boiler is cut off, by closing the stop-cock E, and communication with the external air is made by opening the stopcock D. The point of the tube AB, to which the mercury rises, is noted, and a distance is laid off, upwards, from this point, equal to what the barometric column wants of 30 inches, and the point I thus determined, is marked 1. This point will be very near the surface of the mercury in the cistern. From the point H, distances of 30, 60, 90, &c., inches are laid off upwards, and the corresponding points numbered 2, 3, 4, &c. These divisions correspond to atmospheres, and may be subdivided into tenths and hundredths.

To use the instrument, the stop-cock D is closed, and a communication made with the boiler, by opening the stopcock E. The height to which the mercury rises in the tube, will indicate the tension of the steam in the boiler, which may be read from the scale in terms of atmospheres and decimals of an atmosphere. If the pressure in pounds is wished, it may at once be found, by multiplying the reading of the instrument by 15.

The principal objection to this kind of manometer, is its want of portability, and the great length of tube required, when high tensions are to be measured.

The closed Manometer.

The general construction of the closed manometer is the same as that of the open manometer, with the exception that the tube AB is closed at the top. The air which is confined in the tube, is then compressed in the same way as in MARIOTTE'S tube.

To graduate this instrument. We determine the division H, as before. The remaining divisions are found by applying MARIOTTE's law.

Denote the distance in inches, from H to the top of the

tube, by 7; the pressure on the mercury, expressed in atmosphere, by n, and the distance in inches, from H to the upper surface of the mercury in the tube, by x.

The tension of the air in the tube will be equal to that on the mercury in the cistern, diminished by the weight of a column of mercury, whose altitude is x. Hence, in atmospheres, it is

х

n

30

The bore of the tube being uniform, the volume occupied by the compressed air will be proportional to its height. When the pressure is 1 atmosphere, the height is 7; when

The upper sign of the radical is not used, as it would give a value for x, greater than 7. Taking the lower sign, and, as a particular case, assuming = 30 in., we have,

x = 15n+ 15

√ — 900(n − 1) + (15n + 15)2.

Making n = 2, 3, 4, &c., in succession, we find for x, the corresponding values, 11.46 in., 17.58 in., 20.92 in., &c. These distances being set off from H, upwards, and marked 2, 3, 4, &c., indicate atmospheres. The intermediate spaces are subdivided by means of the same formula.

The use of this instrument is the same as that of the manometer last described.

In making the graduation, we have supposed the tem perature to remain the same. If, however, it does not remain the same, the reading of the instrument must be corrected by means of a table computed for the purpose.

The instruments already described, can only be used for measuring tensions greater than one atmosphere.

The Siphon Guage.

194. The SIPHON GUAGE is an instrument employed to measure tensions of gases and vapors, when they are less than an atmosphere. It consists of a tube ABC, bent so that its two branches are parallel. The branch BC is closed at the top, and filled with mercury, which is retained by the pressure of the atmosphere, whilst the branch AB is open at the top. If, now, the air be rarified in any manner, or if the mouth

B

Fig. 166.

A of the tube, be exposed to the action of any gas whose tension is sufficiently small, the mercury will no longer be supported in the branch BC, but will fall in that and rise in the other. The distance between the surfaces of the mercury in the two branches, as given by a scale placed between them, will indicate the tension of the gas. If this distance is expressed in inches, the tension can be found, in atmospheres, by dividing by 30, or, in pounds, by dividing by 2.

The Diving-Bell.

B

195. The DIVING-BELL is a bell-shaped vessel, open at the bottom, used for descending below the surface of the water. The bell is placed so that its mouth shall continue horizontal, and is let down by means of a rope AB, and the whole apparatus is sunk by weights properly adjusted. The air contained in the bell before immersion, will be compressed by the weight of the

E

H

Fig. 167.

water, but its increased elasticity will prevent the water from rising to the top of the bell, which is provided with seats for the accommodation of those wishing to descend. The air within is constantly contaminated by breathing, and is continually replaced by fresh air, pumped in through a tube FG. Were there no additional air introduced,, the volume of the compressed air, at any depth, might be computed by MARIOTTE'S law. The unit of the compressing force, in this case, is the weight of a column of water whose cross-section is a square inch, and whose height is the distance from DC, to the surface of the water.

The Barometer.

196. The BAROMETER is an instrument for measuring the pressure of the atmosphere. As already explained, it consists of a glass tube, hermetically sealed at one extremity, which is filled with mercury, and inverted in a basin of that fluid. The pressure of the air is indicated by the height of the column of mercury which it supports.

A great variety of forms of the mercurial barometer have been devised, all involving the same mechanical principle. The two most important of these are the siphon and the cistern barometer.

The Siphon Barometer.

197. The siphon barometer consists essentially of a tube CDE, bent so that its two branches, CD

A

E

and DE, shall be parallel to each other. scale of equal parts is placed between them, and attached to the same frame with the tube. The longer branch CD, is about 32 or 33 inches in length, hermetically sealed at the top, and filled with mercury; the shorter one is open to the action of the air. When the instrument is placed vertically, the mercury sinks in the longer branch and rises in the shorter one. The distance between the surface of the mercury in the two branches, as measured by the scale of equal parts, indicates the pressure of the atmosphere at the particular time and place.

D

Fig. 168.

The Cistern Barometer.

198. The cistern barometer consists of a glass tube, filled and inverted in a cistern of mercury, as already explained. The tube is surrounded by a frame of metal, firmly attached to the cistern. Two opposite longitudinal openings, near the upper part of the frame, permit the upper surface of the mercury to be seen. A slide, moved up and down by means of a rack and pinion, may be brought exactly to the upper level of the mercury. The height of the column is then read from a scale, so adjusted as to have its 0 at the surface of the mercury in the cistern. The scale is graduated to inches and tenths, and the smaller divisions are read by means of a vernier.

The figure shows the arrangement of parts in a complete cistern barometer. KK represents the frame of the barometer; HH that of the cistern, open at the upper part, that the level of the mercury in the cistern may be seen through the glass; L, an attached thermometer, to show the temperature of the mercury in the tube; N, a part of the sliding ring bearing the vernier, and moved up and down by the milled-headed screw M.

The particular arrangement of the cistern is shown on an enlarged scale in Fig. 170. A represents the barometer tube, terminating in a small opening, to prevent too sudden shocks when the instrument is moved from place to place; H represents the frame of the cistern; B, the upper portion of the cistern, made of glass, that the surface of the mercury may be seen; E, a conical piece of ivory, projecting from the upper surface of the cistern: when the surface of the mercury just touches the point of the ivory, it is at the 0 of the scale; CC represents the lower part of the cistern, and is made of leather, or some other

AK

N

M

K

E

H

Fig. 169.

B

D

E

Fig. 170.