down the flues with a hose, the scale will be carried to the bottom of the boiler, or issue, with the water, from the mud-hole doors.  This method of scaling must be practised only by the engineer himself, and must not be intrusted to the firemen who, in their ignorance, might damage the boiler by overheating the plates.  It is only where the incrustation upon the flues is considerable that this method of removing it need be practised; in partial cases the scale may be chipped off by a hatched faced hammer, and the flues may then be washed down with the hose in the manner before described.

729. Q.—­Should the steam be let out of the boiler, after it has blown out the water, when the engine is stopped?

A.—­No; it is better to retain the steam in the boiler, as the heat and moisture it occasions soften any scale adhering to the boiler, and cause it to peel off.  Care must, however, be taken not to form a vacuum in the boiler; and the gauge cocks, if opened, will prevent this.

730. Q.—­Are tubular boilers liable to the formation of scale in certain places, though generally free from it?

A.—­In tubular boilers a good deal of care is required to prevent the ends of the tubes next the furnace from becoming coated with scale.  Even when the boiler is tolerably clean in other places the scale will collect here; and in many cases where the amount of blowing off previously found to suffice for flue boilers has been adopted, an incrustation five eighths of an inch in thickness has formed in twelve months round the furnace ends of the tubes, and the stony husks enveloping them have actually grown together in some parts so as totally to exclude the water.

731. Q.—­When a tubular boiler gets incrusted in the manner you have described, what is the best course to be adopted for the removal of the scale?

A.—­When a boiler gets into this state the whole of the tubes must be pulled out, which may be done by a Spanish windlass combined with a pair of blocks; and three men, when thus provided, will be able to draw out from 50 to 70 tubes per day,—­those tubes with the thickest and firmest incrustations being, of course, the most difficult to remove.  The act of drawing out the tubes removes the incrustation; but the tubes should afterward be scraped by drawing them backward and forward between the old files, fixed in a vice, in the form of the letter V. The ends of the tube should then be heated and dressed with the hammer, and plunged while at a blood heat into a bed of sawdust to make them cool soft, so that they may be riveted again with facility.  A few of the tubes will be so far damaged at the ends by the act of drawing them out, as to be too short for reinsertion:  this result might be to a considerable extent obviated by setting the tube plates at different angles, so that the several horizontal rows of tubes would not be originally of the same length, and the damaged tubes of the long rows would serve to replace the short ones; but the practice would be attended with other inconveniences.