length fixed at, say, 7 inches, to observe the pressure which is required to give this length of flame, and the corresponding illuminating power.

TENTHS

AN 'RCH

FIG. 31.

In order to rate the instrument, it is connected to the same service pipe from which the gas is supplied to a Bunsen photometer. Ten observations are made with the latter instrument; and, as nearly simultaneously as possible with each of the ten observations, the length of the flame

of the jet, or the pressure required to give a certain length of flame is noted. The mean of the lengths of the jet, or the pressure required to give a certain length are then taken and recorded. The illuminating power as determined by the Bunsen photometer is also recorded. The experiment is then repeated with a second pair of candles if that standard is employed.

Comparative experiments made in the above manner are continued over a sufficiently long period, and with varying qualities of gas, until a comprehensive table is obtained. The most suitable position for fixing the jet photometer is on the outlet of the exhauster, as, when fixed in that position, a continuous record is obtained of the quality of the gas as it comes from the retort-house, hence any falling off in the quality of the gas can be at once detected and remedied; it is necessary to take out the impurities SH, and CO2, before the gas enters the instrument, by passing it through two small purifiers filled with oxide of iron and lime respectively.

CHAPTER XII.

A DESCRIPTION OF THE VARIOUS INSTRUMENTS USED IN GAS-WORKS FOR ASCERTAINING AND RECORDING PRESSURE AND EXHAUST.

IN

or

N the conduct of a gas-works it is very essential to be able to know at any moment the " pressure or the "exhaust," as the case may be, at any particular portion of the gas-making plant, and this information is obtained by means of the instrument known as a "pressure gauge,” which, although of very simple construction, is an exceedingly valuable instrument, as it acts as a sentinel by which we are enabled to tell if there is a clear course for the gas, any obstruction or stoppage being shown by an increase in the pressure.

The pressure gauge in its simplest form consists of a glass tube of about five-eighths of an inch internal diameter, bent in the form of the letter U. The tube is provided with a scale, divided into inches and tenths of an inch, which is placed between the two limbs of the U. The zero point of this scale is in the centre, and the divisions are marked above and below the zero mark. The tube is filled with coloured water to the level of the zero mark.

One of the ends of the tube is provided with a brass cap, pierced with a small hole, so that the pressure of the atmosphere is capable of pressing on the liquid contained in that limb, while the other limb is provided with a suitable attachment for connecting to any apparatus, the pressure

of gas in which we require to know. On allowing the gas to enter the gauge, the pressure of the same will depress the water in the limb to which it is connected, and raise it to the same extent in the opposite limb, the distance between the two levels indicating the amount of pressure. The amount of pressure up to two or three inches is generally spoken of as being so many tenths; when greater than this the term inches is applied. Fig. 32 shows one form of pressure gauge.

Sometimes in place of having one limb open to the atmosphere and the other to the gas supply, one limb is connected to the inlet of an apparatus and the other limb to the outlet of the same apparatus; in this case the difference in level shows the pressure thrown by the apparatus, for should the apparatus not be throwing any pressure, the gauge would not show any difference in level, as the two pressures would balance one another; but as the inlet pressure is always the greater, we always find in practice that there is a difference, and, consequently, we find a difference in the level of the gauge. For instance, if we attached such an instrument to the inlet and outlet of, say, a scrubber which is showing 30 inches pressure on the inlet, and 29 inches on the outlet side, the pressure gauge thus connected would show a pressure of 1 inch which would be the amount of pressure thrown by the vessel. As in many cases we simply wish to know the pressure thrown by a particular vessel, this form of gauge, which is known as a differential pressure gauge," is extremely useful, particularly when fitted with a special arrangement of cocks, by means of which either limb of the gauge can be put into communication with the atmosphere, or gas supply at will; by this arrange

FIG. 32.

66

ment we obtain, in addition, the actual amount of pressure as with an ordinary gauge.

When the pressure gauge is connected to the inlet side of the "exhauster" as, say, in the retort-house, the difference in level between the two limbs of the gauge then indicates "vacuum" or exhaust.

Thomas's pressure gauge consists of two tubes of different diameters, placed concentric to each other, the gas entering the smaller tube. In this form of gauge the height to which the column of liquid is depressed and elevated is not the same in both tubes, but is in inverse ratio to their respective sectional areas.

gas

In cases where we wish to ascertain the amount of pressure very exactly, as for example, when using the jet photometer, or making an ordinary photometrical test, an instrument known as King's gauge, is employed. This consists of a waterchest divided into two chambers, one of which, into which is admitted, being closed to the atmosphere, while the other chamber, which is in communication at the bottom with the closed chamber, is open at the top to the atmosphere. When gas is allowed to enter, the water in the closed chamber is depressed by the pressure of the gas, and elevated in the open chamber. A metal float rests on the surface of the water and is connected to a pulley wheel by a cord provided with a counterbalance weight. On the pulley spindle a pointer is fixed which traverses over a semicircular scale graduated to hundredths of an inch. The action of the instrument is exceedingly simple, as the water in the open chamber rises and falls according to the pressure of gas acting on the water in the closed or gas chamber; the float also rises and falls, and its movements are transmitted to the pointer, which indicates on the scale behind it the pressure exerted by the gas.

Pressure gauges, however, only indicate the pressure to which they are subjected at any particular moment; so in