Every quarter of a mile of pipe should thus be tried separately. In this manner we become also enabled to detect instantly, whether any collateral branch pipe has been left open by the workmen, a neglect by no means uncommon in this department of the gas light business.
In order to guard against the danger of water entering from the external surface into the pipes, a reservoir should always be placed at the lowest point, where two or more descending mains meet and form an angle, so as to receive the water that may happen to collect at this angular point, an accumulation of which would cut off the communication between the two pipes; this reservoir is usually called a siphon, see [page 221]. It ought to be at least twice the diameter of the bore of the mains, between which it is interposed, and four times that diameter in depth. These reservoirs afford the best indication to show the sound or leaky state of the system of the mains. In all instances where the pipes are perfectly sound, observation has shown, that half a mile of gas mains, three inches in the bore, does not deposit more than a quart of water in a year; on the other hand, if the mains are leaky, the water of the reservoir requires to be pumped out, sometimes as frequently as every fortnight, and during wet weather, much oftener. The loss of gas by such leakage is much greater than is generally imagined. Instances might be mentioned where, in order to keep the common air out of a system of faulty pipes, a constant influx of gas which a pipe two inches in diameter can supply has been found necessary, and this of course is just so much gas lost to the economy of the establishment.
With regard to the diameter of the mains, no general rule can be given. It must vary according to the number of branch pipes and lamps which the main has to supply within a given distance,—the angular direction of the mains,—the pressure of the gas holder, and above all, with the relative altitude of the place where the gas holder is situated, and the place at which the gas is to be supplied, or where the lamps are placed. Indeed this is one of the most important considerations with regard to the economical distribution of gas mains, and by attending to this circumstance, a prodigious saving may be effected.
If the gas flows through a main placed at an altitude of the gas holder, and with a pressure to support a column of water half an inch high, this gas at an altitude of 100 feet, will support a column of water 11⁄10 inch high, and as the velocity of the gas is as the 2√ of the height, or pressure, the quantity of gas which will flow through a given opening at an elevation of 100 feet, will be very nearly in the proportion of two to three. Hence if a gas burner, or gas lamp, produces a flame two inches high, at a place situated on a level with the base of the gas holder, the lamp, if supplied by the same main, but situated 100 feet higher, will burn with a flame three inches high.
This important fact may be rendered obvious in the following simple manner:
Take a tube ten or fifteen feet long, and one inch in diameter, place it horizontally; let one end of the tube be open, and close the other with a plate pierced with a hole, of about 1⁄32 of an inch in diameter, and then fill the tube with gas. If a lighted taper be applied to the hole, when the tube is lying horizontally, the gas will not take fire; but on raising the end of the tube where the small aperture is, the gas will take fire, and the magnitude of the flame will become enlarged in proportion as the tube approaches towards the perpendicular.
Hence the diameter of gas mains must be varied, according to the altitude of the place to be supplied with gas. And it is in consequence of neglecting this principle that we observe so frequently certain parts of large towns scantily supplied with gas, whilst other parts furnished from the same mains, situated considerably above the level of the gas holder, have the gas in the greatest profusion, but at the expense of those places situated at a lower level. And so true is this, that if a main were to descend 100 feet below the base of the gas holder, and if the pressure of the gas in the main was only equal to sustain a column of water half an inch in height, the gas lamps could not be lighted at all, at a point so low, because the pressure of the gas is then in an equilibrium with the pressure of the atmosphere. Hence in lighting a town or district with coal gas, the best situation for the gas apparatus, as far at least as it regards the economy of the mains for distributing the gas, is the lowest part of the town or district. For if the mains are placed at an elevated situation, they require to be proportionally larger, and if situated at a lower place than the level of the gas holder, they must be smaller; but in either case the mains must bear a proper proportion to each other, according to the conditions and circumstances already stated, and it is here, where the skill of the gas light engineer becomes conspicuous, for the saving that may thus be effected in the lighting of a district or town with gas, is very considerable.
The requisite pressure of the gas for different situations with regard to the altitude of the place to be lighted, may be readily known by ascertaining the altitude of the place by means of the mountain barometer. The Englefield mountain barometer is most commodious and suitable for that purpose. This instrument is not liable to be out of order, it may be used by a single observer, and affords an easy method of ascertaining the elevations and depressions of the surfaces of the earth with the greatest facility, and to a degree of precision, that may vie with trigonometrical mensuration. Thus supposing the pressure of the gas at the level of the gas holder to be equal to a column of water half an inch high, by inspecting the height of the barometer, the requisite pressure of the gas at that place may readily be found.
That part of a gas main which does not supply any gas to a branch pipe or lamps, as it proceeds in its course need only be a quarter of the capacity which is necessary at the part where the branch pipe or pipes commence. For no inconvenience can arise from the increased velocity which the gas must assume in proportion to the diminution of the bore of the main, provided that the velocity of the gas is lessened by passing into a main of a greater bore, prior to it being conveyed into the pipe or pipes immediately connected with or supplying the lamps. The enlargement of the pipes should be in the proportion to the diameter of the two pipes, as four to one.