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After the laborer has completed the section assigned to him, he is moved forward to the front of the gang. This method of working always insures that each laborer will do his share of the work, and all of them will work nearly as well as the best man you have.

II.

Depth of Main. The trench should be dug to approximately the depth at which the main is to be laid, and no fixed rule can be prescribed for the depth at which mains should be laid. In a general way it is governed by these conditions: The question of getting below the frost line, the matter of avoiding obstructions and the question of securing sufficient pitch for draining. In addition to these, the mains should also be laid deep enough to avoid any trouble caused by the jar of traffic on the street. (Frank Hellen, Shelton, Simpson, '04, Q. B., No. 105.)

The first three conditions are purely local, but the frost line has been given as 5 feet deep for 30 degrees below zero. (Ohio Q. B., 1906.)

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'In order to be below the jar of traffic, the pipe should be laid to cover at least three feet deep in large cities where there

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is heavy traffic, and thirty inches in the smaller towns. (Simpson and Walton Forstall.-O. Q. B.-'04-No. 105.) For difference in opinion and some estimates, see original

reference.

12.

Grade of Mains. In the laying of street mains it is of the utmost importance to see that all pipes are on a slight incline or gradient, so as to drain all condensation to a given point which is situated at the lowest part of the main, where all the condensation is collected by the means of drip wells. If the pipes were not laid on a perfect gradient there would be a collection of water in the various parts of the pipes where sags or traps occurred, which would hinder or stop the flow of gas, according to the depth of the trap and the amount of water therein.

The concensus of opinion appears to be that 2 inches in each 100 feet is the minimum permissible grade for main lines, and that services should have at least 6 inches in 100 feet. In case the minimum grade is used, great care should be taken in laying the pipe. It is also observed that a lesser grade can be used when the flow of gas and condensation are in the same direction than would otherwise be advisable. Large mains can also be given a less grade than smaller ones. For variations and a number of individual opinions, see original references. ('03-Q. B., No. 110 and '04-Q. B., No. 120.)

In streets where the ground has sufficient fall it is comparatively easy to lay gas mains to grade. The only point to be looked out for being to see that the grade of the main is continuous, and that there are no sags or hollow places, except where it is intended to place a drip to catch the condensation. On this work an ordinary level may be used; sometimes it is sufficient to put the level on each length of the pipe, look at the level and see that the pipe has a fall in the right direction. It is sometimes advisable to have a straight edge 12 feet long, or one that will reach from one bell to another. At one end, on the under side of this straight edge, a block is fastened of the thickness that it is expected the main will have fall for each length. The straight edge is placed upon the pipe with the end to which the block is attached in the direction in which the main falls; the spirit level is then placed on top of the

straight edge, and when it shows level the pipe will have the amount of fall called for by the thickness of the block.

In laying street mains or services, especially where obstruction exists on the proposed line, the use of three T-pieces, of which a description follows, has been found of great convenience in readily obtaining the line of uniform fall. The tees are of wood, the cross pieces being made all the same size, and set square with the uprights. Two of the latter are just equal length, but the third one is longer by an equal distance to the thickness of the cross-piece.

From four to five feet is a handy length for the uprights. In using them, the two equal tees are set vertically, one at each end of the line, their feet resting in the ditch at the depth desired for a bottom. To find the line for the bottom at any point between these, hold the other tee vertically in the ditch at the place in question, and deepen the trench, following it with the tee until, sighting across the top of that at one end, the middle one just cuts off the light between its under side and the top of the one at the opposite end. Trenching work may thus be carried on past mortar and brick piles, intersecting streets, railway crossings, rock bottoms, or any temporary obstruction, with confidence in the certainty of being able to join the trenches later, and bave all continuous. Whether working with or without instruments, the T-piece will be found valuable.-Wrinkle Dept., Western Gas Association-1893.

Where the ground is very level or it is very hard to determine grade, an engineer's or a surveyor's level should be used. A preliminary survey of the street should be made to determine in which way the main is to fall and where the drips could be placed to the best advantage. The City Engineer should also be consulted. After this is done, and the grade to which the pipe should be laid is determined, 2 x 4 stakes, cut of the required length, can be placed on each side of the trench. These stakes may be driven, if deemed advisable, after the trench has been lined up and before it has been dug. On these stakes is marked a grade which is a given height. above the top of the pipe. If the pipe should be covered 3 ft.,

advisable, for instance, to mark the grade on the stakes,

4 ft. 6 inches above the top of the pipe. This will allow the cross bars to be about 18 inches above the surface of the ground. Cross bars made of 1 x 4 material are then nailed acoss the trench from one stake to another. The top of the cross bar being on the grade lines marked on the stake, each pair of stakes being 50 feet apart along the trench. A line is then stretched from one cross bar to the other.

The ditch may now be bottomed up and the pipe laid at a uniform grade by measuring down from this line to either the bottom of the ditch or the top of the pipe, as the case might require. It will help the work somewhat if the line is not stretched until

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the ditch is dug approximately the required depth. Some authorities advocate the digging of the ditch a trifle deeper, and then the pipe should be graded by being placed on a bed block. In this case it is considered preferable to use wooden blocks instead of either brick or stone, as the wood would be slightly elastic and would be apt to give a little under any stress that would be likely to come on the pipe. (O. Q. B.No. 513 and No. 492-'06.)

After the ditch has been properly bottomed, a hollow space should be dug in the bottom of the ditch for the reception of the bell of each length of pipe. In soft ground the preliminary bell holes may be made comparatively small, and when the

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