A.—­Yes, this is very important, else the piston will have to force out the steam from the cylinder, and will be much resisted.  Near the end of the stroke the piston begins to travel slowly, and if the steam be then permitted to escape, very little of the effective stroke is lost, and time is afforded to the steam, before the motion of the piston is again accelerated, to make its escape by the port.  In some locomotives, from inattention to this adjustment, and from a contracted area of tube section, which involved a strong blast, about half the power of the engine has been lost; but in more recent engines, by using enlarged ports and by giving sufficient lead, this loss has been greatly diminished.

322. Q.—­What do you call sufficient lead?

A.—­In fast going engines I would call it sufficient lead, when the eduction port was nearly open at the end of the stroke.

323. Q.—­Can you give any example of the benefit of increasing the lead?

A.—­The early locomotives were made with very little lead, and the proportions were in fact very much the same as those previously existing in land engines.  About 1832, the benefits of lap upon the valve, which had been employed by Boulton and Watt more than twenty years before, were beginning to be pretty generally apprehended; and, in the following year, this expedient of economy was applied to the steamer Manchester, in the Clyde, and to some other vessels, with very marked success.  Shortly after this time, lap began to be applied to the valves of locomotives, and it was found that not only was there a benefit from the operation of expansion, but that there was a still greater benefit from the superior facility of escape given to the steam, inasmuch as the application of lap involved the necessity of turning the eccentric round upon the shaft, which caused the eduction to take place before the end of the stroke.  In 1840, one of the engines of the Liverpool and Manchester Railway was altered so as to have 1 inch lap on the valve, and 1 inch opening on the eduction side at the end of the stroke, the valve having a total travel of 4-1/4 inches.  The consumption of fuel per mile fell from 36.3 lbs. to 28.6 lbs, or about 25 per cent., and a softer blast sufficed.  By using larger exhaust passages, larger tubes, and closer fire bars, the consumption was subsequently brought down to 15 lbs. per mile.

AIR PUMP, CONDENSER, AND HOT AND COLD WATER PUMPS.

324. Q.—­Will you state the proper dimensions of the air pump and condenser in laud and marine engines?

A—­Mr. Watt made the air pump of his engine half the diameter of the cylinder and half the stroke, or one eighth of the capacity, and the condenser was usually made about the same size as the air pump; but as the pressure of the steam has been increased in all modern engines, it is better to make the air pump a little larger than this proportion. 0.6 of the diameter of the cylinder and half the stroke answers very well, and the condenser may be made as large as it can be got with convenience, though the same size as the air pump will suffice.