cases where it is necessary to have a continuous record during any particular period, say for twenty-four hours, it is necessary to employ the instrument known as the pressure or exhaust "register."

This instrument (fig. 33) consists of a tinned plate tank about 3 feet deep and 16 inches diameter, in the centre of which rises a vertical pipe attached to the gas supply. On the top of the tank there is a receptacle for a vertical revolving cylinder, about 16 inches long and 5 inches diameter, which is enclosed in a glass case.

The cylinder is connected to a clock which is placed on the top of the apparatus, and which causes the cylinder to revolve once in twenty-four hours.

Within the tank a gas-holder of about 14 inches diameter and 16 inches high is placed; this holder has a float throughout its length, which gives it the required degree of buoyancy when immersed, but causes a gradually increasing weight, and consequently pressure, on rising.

The holder is provided with rollers, working in guides, so that it may rise and fall with freedom.

Attached to the top of the holder is a rod, which passes through the centre of the cover of the tank, and on the top of this rod a spring pencil is fixed, for the purpose of recording the variations in pressure.

A sheet of paper is coiled round the vertical cylinder, and this paper is divided off into twenty-four vertical divisions, which indicate the hours of "day" and "night" corresponding with the clock fixed above. On the same paper there are a series of horizontal lines corresponding with the pressure; these horizontal lines are also numbered, commencing from the bottom, which is the zero point, and indicating tenths, continued to forty or fifty tenths. As the holder in rising, by reason of its having less buoyancy, gets gradually heavier, on ascertaining this.

weight at various positions, the scale of pressure can be arranged accordingly by the horizontal lines.

The action of the instrument is as follows:-On the tank being supplied with water to the desired height, if there be no pressure, the holder will of course remain stationary, and the pencil will remain at zero; but supposing we admit gas, say at a pressure equal to 1 inch, the holder will rise, carrying with it the pencil which records the pressure on the horizontal lines of the coil of paper, while at the same time the vertical lines indicate the time at which that particular pressure was given.

The difference between a "pressure " and "exhaust" register is simply one of detail in their construction. The zero line of the " pressure" register is at the bottom of the scale, while in the "exhaust" register it is placed midway on the scale, which necessitates the spindle being made longer, the area of the float being also increased.

A pressure register should be placed in communication with the street mains after the gas has passed through the governor. When employed to register "exhaust it should be connected to the “exhaust" or inlet side of the exhauster.

CHAPTER XIII.

THE GAS-HOLDER AND ITS TANK.

THE scope

of this little work will not permit of any extended account of the construction of gas-holders and tanks, which calls for the exercise of the highest engineering skill; all that will be attempted is to give a brief sketch of the general features of such constructions, referring the student for fuller details to King's "Treatise," Cripps, "The Guide Framing of Gas-holders," Newbigging's "Handbook," and the technical journals.

The tank of a gas-holder is a circular reservoir of water, slightly larger in diameter than the gas-holder itself, and in this reservoir the gas-holder works, rising or falling above or below the water as the gas is stored or taken away for distribution.

Tanks are generally built under the ground level, but in some instances, where it is requisite, owing to special circumstances, to construct them of iron, they are placed entirely above ground. Tanks are sometimes "formed by making a circular cutting in the ground, and erecting therein an iron or brick annular channel to contain the water, the intervening or central space being also covered with water, but only to a few inches depth. These are called annular tanks."

Tanks are constructed of brick, stone, or concrete, according to circumstances, the material employed largely depending upon the materials most cheaply obtainable in

the locality, and the nature of the ground in which it is proposed to construct the tank, which in some instances necessitates the employment of tanks constructed of cast or wrought-iron plates bolted or riveted together, "but such tanks are only employed under very special circumstances, as their cost is much greater than either brick or stone."

Tanks other than those made of iron are generally sunk level with the ground, or to within a few feet of the surface, the portion of the walls which is above ground being strengthened by an earthern backing. They are made water-tight by being surrounded on the outside with clay puddle of from 12 to 24 inches in thickness, or as a substitute for this, the tank is rendered on the inside with cement.

"In a brick and puddle tank the pressure due to the water it contains is transmitted through the porous brickwork to the clay puddle and earth backing, which are slightly elastic. When, however, cement rendering is employed, the tank walls have to sustain the bursting pressure of the water; and their stability largely depends upon the selection of the material to form the earth backing, and the care with which this is filled in, rammed, and watered" (Hunt).

Before adopting any particular site for the construction of a tank, it is necessary to sink a well or shaft in the vicinity, or to make a number of trial borings in close proximity to the site, in order to ascertain the nature of the strata in which the excavation for the proposed tank has to be made. If any considerable quantity of water is found, it will then be necessary to make provision for its continuous removal during the progress of the work in the -following manner.1

1 Newbigging, "Trans. Incorporated Gas Institute, 1892."