A.—­Brass faces are put upon valves or cylinders by means of small brass screws tapped into the iron, with conical necks for the retention of the brass:  they are screwed by means of a square head, which, when the screw is in its place, is cut off and filed smooth.  In some cases the face is made of extra thickness, and a rim not so thick runs round it, forming a step or recess for the reception of brass rivets, the heads of which are clear of the face.

712. Q.—­What is the best material for valve faces?

A.—­Much trouble is experienced with every modification of valve face; but cast iron working upon cast iron is, perhaps, the best combination yet introduced.  A usual practice is to pin brass faces on the cylinder, allowing the valve to retain its cast iron face.  Some makers employ brass valves, and others pin brass on the valves, leaving the cylinder with a cast iron face.  If brass valves are used, it is advisable to plane out two grooves across the face, and to fill them up with hard cast iron to prevent rutting.  Speculum metal and steel have been tried for the cylinder faces, but only with moderate success.  In some cases the brass gets into ruts; but the most prevalent affection is a degradation of the iron, owing to the action of the steam, and the face assuming a granular appearance, something like loaf sugar.  This action shows itself only at particular spots, and chiefly about the angles of the port or valve face.  At first the action is slow; but when once the steam has worked a passage for itself, the cutting away becomes very rapid, and, in a short time, it will be impossible to prevent the engine from heating when stopped, owing to the leakage of steam through the valve into the condenser.  Copper steam pipes seem to have some galvanic action on valve faces, and malleable iron pipes have sometimes been substituted; but they are speedily worn out by oxidation, and the scales of rust which are carried on by the steam scratch the valves and cylinders, so that the use of copper pipes is the least evil.

713. Q.—­Will you explain in what manner the joints of an engine are made?

A.—­Rust joints are not now much used in engines of any kind, yet it is necessary that the engineer should be acquainted with the manner of their formation.  One ounce of sal-ammoniac in powder is mingled with 18 ounces or a pound of borings of cast iron, and a sufficiency of water is added to wet the mixture thoroughly, which should be done some hours before it is wanted for use.  Some persons add about half an ounce of flowers of brimstone to the above proportions, and a little sludge from the grindstone trough.  This cement is caulked into the joints with a caulking iron, about three quarters of an inch wide and one quarter of an inch thick, and after the caulking is finished the bolts of the joints may be tried to see if they cannot be further tightened.  The skin of the iron must, in all cases, be broken