to establish on the surface of the work, depends on the use to which the surface is to be applied; but whether it is to be finished with great elaboration, or otherwise, the bearing points should be distributed equally over the surface.  Face plates, or planometers, as they are sometimes termed, are supplied by most of the makers of engineering tools.  Every factory should be abundantly supplied with them, and also with steel straight edges; and there should be a master face plate, and a master straight edge, for the sole purpose of testing, from time to time, the accuracy of those in use.

708. Q.—­Is the operation of surfacing, which you have described, necessary in the case of all slide valves?

A.—­Yes; and in fitting the faces of a D valve, great care must, in addition, be taken that the valve is not made conical; for unless the back be exactly parallel with the face, it will be impossible to keep the packing from being rapidly cut away.  When the valve is laid upon the face plate, the back must be made quite fair along the whole length, by draw filing, according to the indications of a straight edge; and the distance from the face to the extreme height of the back must be made identical at each extremity.

709. Q.—­When you described the operation of boring the cylinder, you stated that the cylinder, when laid upon its side, became oval; will not this change of figure distort the cylinder face?

A.—­It is not only in the boring of the cylinder that it is necessary to be careful that there is no change of figure, for it will be impossible to face the valves truly in the case of large cylinders, unless the cylinder be placed on end, or internal props be introduced to prevent the collapse due to the cylinder’s weight.  It may be added, that the change of figure is not instantaneous, but becomes greater after some continuance of the strain than it was at first, so that in gauging a cylinder to ascertain the difference of diameter when it is placed on its side, it should have lain some days upon its side to ensure the accuracy of the operation.

710. Q.—­How is any flaw in the valve or cylinder face remedied?

A.—­Should a hole occur either in the valve, in the cylinder, or any other part where the surface requires to be smooth, it may be plugged up with a piece of cast iron, as nearly as possible of the same texture.  Bore out the faulty part, and afterward widen the hole with an eccentric drill, so that it will be of the least diameter at the mouth.  The hole may go more than half through the iron:  fit then a plug of cast iron roughly by filing, and hammer it into the hole, whereby the plug will become riveted in it, and its surface may then be filed smooth.  Square pieces may be let in after the same fashion, the hole being made dovetailed, and the pieces thus fitted will never come out.

711. Q.—­When cylinders are faced with brass, how is the face attached to the cylinder?