375. Q.—­Is it not important to have the holes in the plates opposite to one another?

A.—­The whole of the plates of a boiler should have the holes for the rivets punched, and the edges cut straight, by means of self-acting machinery, in which a travelling table carries forward the plate with an equal progression every stroke of the punch or shears; and machinery of this kind is now extensively employed.  The practice of forcing the parts of boilers together with violence, by means of screw-jacks, and drifts through the holes, should not be permitted; as a great strain may thus be thrown upon the rivets, even when there is no steam in the boiler.  All rivets should be of the best Lowmoor iron.  The work should be caulked both within and without wherever it is accessible, but in the more confined situations within the flues the caulking will in many cases have to be done with the hand or chipping hammer, instead of the heavy hammer previously prescribed.

376. Q.—­How is the setting of marine boilers with internal furnaces effected?

A.—­In the setting of marine boilers care must be taken that no copper bolts or nails project above the wooden platform upon which they rest, and also that no projecting copper bolts in the sides of the ship touch the boiler, as the galvanic action in such a case would probably soon wear the points of contact into holes.  The platform may consist of three inch planking laid across the keelsons nailed with iron, nails, the heads of which are well punched down, and caulked and puttied like a deck.  The surface may then be painted over with thin putty, and fore and aft boards of half the thickness may then be laid down and nailed securely with iron nails, having the heads well punched down.  This platform must then be covered thinly and evenly with mastic cement and the boiler be set down upon it, and the cement must be caulked beneath the boiler by means of wooden caulking tools, so as completely to fill every vacuity.  Coomings of wood sloped on the top must next be set round the boiler, and the space between the coomings and the boiler must be caulked full of cement, and be smoothed off on the top to the slope of the coomings, so as to throw off any water that might be disposed to enter between the coomings and the boiler.

377. Q.—­How is the cement used for setting marine boilers compounded?

A.—­Mastic cement proper for the setting of boilers is sold in many places ready made.  Hamelin’s mastic is compounded as follows:—­to any given weight of sand or pulverized earthenware add two thirds such given weight of powdered Bath, Portland, or other similar stone, and to every 560 lbs. weight of the mixture add 40 lbs. weight of litharge, 2 lbs. of powdered glass or flint, 1 lb. of minium, and 2 lbs. of gray oxide of lead; pass the mixture through a sieve, and keep it in a powder for use.  When wanted for use, a sufficient quantity of the powder is mixed with some vegetable oil upon a board or in a trough in the manner of mortar, in the proportion of 605 lbs. of the powder to 5 gallons of linseed, walnut, or pink oil, and the mixture is stirred and trodden upon until it assumes the appearance of moistened sand, when it is ready for use.  The cement should be used on the same day as the oil is added, else it will be set into a solid mass.