on 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.

378. _Q._--What is the best length of the furnaces of marine boilers?

_A._--It has already been stated that furnace bars should not much exceed
six feet in length, as it is difficult to manage long furnaces; but it is a
frequent practice to make the furnaces long and narrow, the consequence of
which is, that it is impossible to fire them effectually at the after end,
especially upon long voyages and in stormy weather, and air escap