in the construction of the link motion, is a problem requiring a good deal of attention and care for its satisfactory solution; and to make sure that this result is attained, the engine must be turned round a sufficient number of times to enable us to ascertain if the valve occupies the desired position, both at the top and bottom centres, whether the engine is going ahead or astern.  This should also be tried with the starting handle in the different notches, or, in other words, with the sliding block in the slot or opening of the link in different positions.

MANAGEMENT OF MARINE BOILERS.

726. Q.—­You have already stated that the formation of salt or scale in marine boilers is to be prevented by blowing out into the sea at frequent intervals a portion of the concentrated water.  Will you now explain how the proper quantity of water to be blown out is determined?

A.—­By means of the salinometer, which is an instrument for determining the density of the water, constructed on the principle of the hydrometer for telling the strength of spirits.  Some of the water is drawn off from the boiler from time to time, and the salinometer is immersed in it after it has been cooled.  By the graduations of the salinometer the saltness of this water is at once discovered; and if the saltness exceeds 8 ounces of salt in the gallon, more water should be blown out of the boiler to be replenished with fresher water from the sea, until the prescribed limit of freshness is attained.  Should the salinometer be accidentally broken, a temporary one may be constructed of a phial weighted with a few grains of shot or other convenient weight.  The weighted phial is first to be floated in fresh water, and its line of floatation marked; then to be floated in salt water, and its line of floatation marked; and another mark of an equal height above the salt water mark will be the blow off point.

727. Q.—­HOW often should boilers be blown off in order to keep them free from incrustation?

A.—­Flue boilers generally require to be blown off about twice every watch, or about twice in the four hours; but tubular boilers may require to be blown off once every twenty minutes, and such an amount of blowing off should in every case be adopted, as will effectually prevent any injurious amount of incrustation.

728. Q.—­In the event of scale accumulating on the flues of a boiler, what is the best way of removing it?

A.—­If the boilers require to be scaled, the best method of performing the operation appears to be the following:—­Lay a train of shavings along the flues, open the safety valve to prevent the existence of any pressure within the boiler, and light the train of shavings, which, by expanding rapidly the metal of the flues, while the scale, from its imperfect conducting power, can only expand slowly, will crack off the scale; by washing