pelled; it also prevents the gas from escaping from the holder.

There are two descriptions of gas-holders, the "single lift" and the "telescopic." The single-lift holder is shown in fig. 34, and simply consists of one single cylindrical vessel floating in a tank, as previously described. The single-lift gas-holder shown in the figure may be taken as a type of this class of construction, and is made up of

the framing and the sheeting. The framing at the top consists of an angle-iron ring, known as the top curb, upon which the roof or crown of the holder is formed. The roof is frequently supported by trussing, the trussing consisting of a series of radiating bars of T or flat bar-iron diverging at equal distances from the centre plate of the holder, which is known as the crown plate, to the top curb. The roof is sustained in the centre by a short column known as the "king post," into the bottom of which are inserted, and

held by it, a series of tension rods which radiate to the top curb; there are also suspension rods, which carry struts for sustaining the centre of the radiating bars. The roof of a holder has a certain amount of rise (about one-twentieth of the diameter) from the top curb to the crown plate. To the top curb a series of vertical stays, usually of T-iron, are fixed, distributed at equal distances round the circumference for the purpose of securing the side sheets. The bottom of the holder is known as the "bottom curb," and is frequently formed of flat bar-iron shaped to the proper curve, and placed between two angle-iron rings, to which it is securely riveted, the vertical stays being also secured to the bottom curb. The whole of the top and side framing is covered with thin sheets of plate iron, the thickness of the plates employed depending upon the size of the holder and other circumstances. It is necessary to note that it is only when the holder is empty or grounded that the roof and sides require any support, the holder when inflated having very little strain upon it, consequently the trussing is in a measure superfluous, as adding unnecessary weight to the holder; for this reason many engineers dispense with the trussing, the holder when empty being supported by a timber framework, shown in fig. 35. In untrussed holders the top curb is considerably strengthened in order to enable it to resist the additional strain to which it is subjected, and with the same object a greater rise is given to

the crown.

Placed at equal distances around the holder are the guide columns, to which the vertical guides are fixed for guiding the holder up and down as it rises and falls. These were formerly made of cast-iron, but at the present day they are almost always made of wrought-iron, in the form of lattice-work standards, as wrought-iron is a much lighter and more reliable material. These standards are braced together at the top and in the centre by wrought

iron girders, which connect one standard with another all round the holder, and fixed to the standards in a truly vertical position are the guides, which are usually of Tiron. The carriages containing the guide rollers are secured to the roof sheets of the holder close to the top curb, so as to admit of the rollers working against the guides. The carriages are constructed so as to allow of their being adjusted when required, and they are fixed in such a manner

that the rollers work quite freely along the T-iron vertical guides. Corresponding carriages are also fixed on the bottom curb which carry the bottom guide rollers; these work in the vertical guides placed in the tank, the latter being usually made of channel iron.

The telescopic gas-holder is shown in fig. 35, and may be said to consist of an ordinary single-lift gas-holder, called the upper lift, surrounded by another similar cylinder of about the same height, but minus the roof. These are both immersed in the same tank.

The bottom of the upper holder or inner lift is so arranged as to form a deep annular channel all round the lift, which is shown in fig. 36. This channel, which is known as the "hydraulic cup," in rising is filled with water. The surrounding cylinder, or lower lift, is about from 12 to 18 inches larger in diameter than the inner lift, and is furnished with a similar annular cup, but inverted, called the "grip," so that when the inner holder rises to the top the inverted cup "grips" on to the water lute of the latter, so that the gas is prevented from escaping. The inner holder in rising carries with it the second lift.

The bottom guide-roller carriages of the inner lift are fixed on the under side of the hydraulic cup, and on the under side of the outer lift suitable guides are fixed for the rollers to travel along, the guides also serving as vertical stays to the outer lift. The top guideroller carriages of the outer lift are fixed on the hydraulic grip. These rollers answer to the rollers of an ordinary single-lift gas-holder. From the above description it will be seen that "telescopic" holders derive their name from the fact that the various lifts slide within one another, something after the style of the tubes of a telescope. Telescopic holders are sometimes made with as many as four lifts. The advantages of the telescopic over the single-lift holder are, that a telescopic holder is capable of storing nearly two or three times (according to the number of lifts) more gas in a given area, so that a considerable economy is effected by reason of the same tank sufficing for a greatly increased holder capacity.

FIG. 36.

It is necessary to give attention to the lutes of telescopic holders in frosty weather, so that any ice which may form is at once broken up, otherwise there is a risk of seriously damaging the holder.

One of the most recent developments of gas-holder construction is that of the abolition of the guide framing as being superfluous; and of the various contrivances which have been designed to act as substitutes for the guide framing, those of Mr. Gadd and Mr. Lloyd Pease are specially worthy of mention as being in practical operation.

In Mr. Gadd's system the ordinary vertical tank guides are placed at an angle of not less than 45° instead of in the ordinary vertical position.

The guides thus placed form a portion of a screw thread, and by the employment of a pair of tangential rollers, one on each side of the guide, fixed to the bottom curb, the holder on rising and falling has a screw-like motion given to it to the extent of about one quarter of a circle. The effect of arranging the guides in such a manner, with a roller on either side, is to cause the holder to lock, so to speak, and thus resist any overturning force that would be likely to be exerted upon it, whilst the gripping or locking action on the angular guides allows the rollers free scope to ascend or descend in the guides.

There are one or two calculations connected with the subject of gas-holders with which the student should make himself familiar. In order to find the cubical capacity of a gas-holder, the rule is, multiply the area of the diameter by the working depth in feet, and in order to obtain the area, we multiply the square of the diameter in feet by the constant 7854; this product multiplied by the depth gives the cubical capacity.

When calculating the capacity of a telescopic gas-holder it is necessary to take out the capacities of the several lifts