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means of screws, the joint being well secured by cement, &c.

But the small wrought-iron pipes, commonly called service-pipes, constitute the greater number of those which are usually attached to the principal mains; and these are generally fixed by boring a hole in the main, in which is inserted a screw, which corresponds with another screw formed on one end of the pipe and connects them together. These pipes are usually well coated with tar, to prevent their being injured by the action of air or water, and each end has an appropriate but corresponding screw, that adapts them for being easily connected. These pipes are commonly placed in an inclined position toward the main, in the ratio of about one inch for every ten feet, that any moisture may drain into the mains; and, before they are put together, the screws are usually dipped in a mixture composed of white-lead and linseed oil, for the purpose of rendering the joints. air-tight. Every length of pipe is proved by a powerful process, to ascertain that it is completely sound, and this is the case whether it be made of iron, copper, or tin.

To regulate the current of the gas, by allowing it to flow, or entirely to interrupt it, a great variety of valves has been invented, but the principles of their construction are simple. Some of

them are sliding valves, others are hydraulic ;* their forms vary at different works, and their dimensions are in proportion to the size of the mains to which they are to be adapted. One of them is usually fixed at the point where the principal pipe of the gasometer communicates with the large mains.

Hydraulic valves were generally employed formerly, and their use is still continued for some purposes. They are usually constructed of a large air-tight iron box, having in its centre an upright iron spindle, which is made in the form of a screw and works in a screw-socket, for the purpose of moving it readily upwards and downwards. The pipe through which the gas passes from the gasometer into the box, is fixed near to the top of its side; but the pipe that conveys the gas to the mains enters at the bottom, and rises several inches perpendicularly, where it is surrounded nearly to its surface, either by tar or water which the lower part of the box contains. To the lower end of the spindle is firmly connected an inverted iron cup, that has a larger diameter than the main pipe, and therefore adapted to pass

* ́All the valves which were employed in the early period of Gas-lighting were hydraulic valves, but some of those now used are improved in their construction. The slide valves are chiefly used in the machinery at the gas works.

over it. When, by means of the screw, this cup is lowered in the tar or water, it covers the end of the pipe, and prevents the gas from entering the mains; on the contrary, when it is raised above the surface of the tar or water, it permits the gas to have a free course. In some of these vessels the pipe passes through the top, and therefore, in that case, the cup contains the tar which, being drawn upwards, stops the communication; but it will be evident that the principle is the same in both.

Another kind of valve consists of a combination of different pieces of wood, in the form of wedges, which are connected together and faced with cork or leather. They are adjusted to a square iron box, and the openings for the gas to pass in and out are in its sides. Through its centre, at the top, a perpendicular screw passes, to which a bent spring is attached; and this spring, having its ends connected with the pieces of wood, lengthens as the screw raises it, and presses them closely against the sides of the vessel, so as effectually to stop the communication; when lowered, the passages of course are opened, and the gas flows freely.

But the sliding valves being less complicated in their construction, more simple in their form, and also occupying but a small space, are now most commonly preferred for some of those pur

poses in which hydraulic valves were formerly employed. They are placed in an upright position; the external part consists of a strong square iron frame, having two flat iron plates to form the sides, and these have proper apertures, adapted to the size of the pipes between which they are to be fixed. The inner surfaces are perfectly smooth, and the distance from each other is such as to allow another plate to slide between them. The middle plate, which forms the valve, is made either of brass or iron, and both its surfaces are ground so perfectly even as to fill up the space between the other two, and render the whole completely air-tight. The valve is moved up and down by means of a screw which is attached to it, and works in a socket formed in the upper part of the frame, but some of the larger are raised or lowered by the aid of a rack and pinion, in a similar manner to that usually adopted for canal locks. Some of these valves are nicely constructed, and have a scale attached to them, which enables the superintendent to calculate the quantity of gas that may flow, accordingly as the valve may be elevated or lowered, to cover a greater or less proportion of the area of the end of the pipe.

OF THE EXPLOSIVE PROPERTIES OF COAL GAS, AND THE MEANS OF PREVENTING ACCIDENTS FROM EXPLOSIONS.

The advantages which society has derived from the introduction of the art of lighting with gas, are so important, that it must be desirable to remove whatever objections or prejudices may still operate against the adoption of this convenient and cheap mode of illumination. Among these, perhaps the most formidable, and that which has created most alarm, is that which arises from the explosive nature of the gas when mixed with atmospheric air. But the apprehensions which some persons have entertained with respect to the danger have been indulged to a most unreasonable extent; for, that it is by no means so great as they have imagined or represented, may be fairly inferred from the very few accidents which have heretofore occurred, notwithstanding the wide extension of the practice, and the number of the works established for its purposes.

Extensively as this plan of lighting is at present adopted, only a few years have elapsed since its first introduction to public use; and however much may have been accomplished by the efforts of ingenious and enterprising men, in the improve

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