BOILERS EOR MARINE PURPOSES.

63

counting the number of squares in each line of plating in the lengthand the breadth we arrive at the true length and breadth of eachplate without using a scale, the ruled paper being the universalscale for all flat surfaces; all the other parts of the boiler wherepracticable should be set out in like manner, and after they aremarked on the drawing No. 1, No. 2, &c., these are the marksthat must be numbered on each plate as delivered from the rolling-mills. No. 1 plate will contain so many rivets in the length andbreadth, thus the machinemen need never apply a foot-rule, butsimply adjust them on the travelling table of the drilling or punchingmachine, and the machine itself will do the work of drilling orpunching the plates with mathematical precision, the plates beingpreviously planed on the edges to the correct size.

In all Government contracts the top of the boiler must be atleast 1 foot below the water line. To insure ample steam room3 feet 6 inches is allowed from the top of the boiler to the top ofthe flues. For boilers intended to run out about four times thenominal horse-power make an allowance of - 68 to 7 of a squarefoot of fire-grate surface, per nominal horse-power, and from 18'8to 19 square feet of heating surface, taking the whole circumferenceof the small tubes that in multitubular arrangements is available,including the flues, sides, and the tops of the furnaces above thefire-grate, and one half of the back tube-plate may be included inthe total heating surface. Composition tubes are usually adopted,having an external diameter of 2^4 to 3^ inches, and the pitch ofthe tubes to 4^ inches. The length of the tubes may varyfrom 5 feet 6 inches to 7 feet, but should never exceed that length.

Fire Grate and Heating Surface for Indicated Horse-pozver .—Forevery indicated horse-power the engine is intended to work at, makean allowance of about 3 to ^/4 square feet of heating surface, andone-eighth of a square foot, or 18 square inches, of fire-grate forevery horse-power indicated. The consumption of coal per squarefoot of grate is about 20 Ibs. per hour, and the water evaporatedabout 9 lbs. per lb. of coal.

Writing on this subject, Professor Rankine gives as follows:—“Thegreatest available heat, or the rate of expenditure of heat upon thesteam, is to be compared in units of work, by dividing the greatestindicated power required in units of work, per unit of time (say infoot-pounds per hour), by the probable efficiency of the engine; or,otherwise, multiply the pressure equivalent to the rate of expendi-