The piston rod should be lubricated at the gland, by means of a cup, or swab, to prevent the piston rod from becoming heated from the piston rod packing. Several defects may be mentioned that will retard the drifting. But the main cause is due to the use of valve cylinder cocks. When an engine is drifting, a vacuum is formed by the piston head, and atmospheric pressure fills the cylinder with air at every revolution. There would be very little, if any, air drawn into the cylinder, if plug cocks were used, because the propelling power would overcome the vacuum, and allow engines to drift as they should.
Relief valves and valve cylinder cocks are impediments to a locomotive, for another reason.
While locomotives are drifting, cold air rushes in at these openings and chills the cylinder and steam chest away below the temperature of steam, thereby causing condensation in cylinder and steam chest, until heated to the same temperature as steam. If any one doubts this statement in regard to valve cocks and relief valves, just let him get the superintendent of motive power to fit up a locomotive with plug cylinder cocks, and it will verify my words. I was talking with a superintendent of motive power a year ago about the different builds of locomotives and their deficiencies. We finally took up the Mallet Compound Locomotive, which, he said, scarcely drifted at all. I asked him if he knew the reason for this.