BOILERS FOR MARINE PURPOSES.

67

in. Find the number of square inches in the surface, multiply bythe pressure per square inch, say 20 lbs.; thus, 10 x 11 X 144= 15,840X 20=316,800 lbs., equal pressure on the whole surface, which,divided by 3000, gives the total area of the stays; and again by 56,gives the area in square inches for each stay at the bottom of the

thread: io5’6-F 56= i - 8 square inch area for each stay, say

inch in diameter. For the screwed stays in the furnace side-plates the diameter is generally 13^ inch, with spaces to suit; and thestays which secure the sides and bind the top and bottom together—i.e. the stays passing through the water spaces between the small tubes—are made of flat bar-iron, with screwed ends and nuts inside and out.

Fire-bars. —Wrought-iron fire-bars have a breadth of iY inchat the top and 34 inch at the bottom, and are 3 inches deep.

Weight of Wrought-iron Bars,.

. 3 3 in-

long = 30 lbs.

Weight of Cast-iron Bars,.

„ = 19

Weight of

do. ..

. 2 6

„ = 24

Weight of

do. .

. 3 0

„ =275

Weight of

do. ..

. 3 6

„ = 3 2 '°

Weight of

do. .

. 4 0

,, =36'5

Cast-iron bars have a breadth of full T/% inch at the top and full% inch at the bottom, the depth ranges from 3 to 434 inches, andthe distance between bars from Y to inch.

Tube Area , Furnaces, &c .—The calorimeter of the boiler or sec-tional area of the tubes is a subject no one need trouble himselfabout, as the combined area of the tubes is greatly in excess ofwhat is required, and in which we have no choice, as it depends onthe length of the tubes that can be introduced. The ordinary sizeof tubes for the merchant Service varies from 3 to 4 inches externaldiameter, and from 6 feet to 7 feet in length; while for the Royal Navy the tubes are 234 to 334 inches external diameter, and 5 feet6 inches to 7 feet in length, consequently it will be seen that the longerthe tubes are the greater the heating surface, while the combined areathrough them may be the same. The only thing to be consideredis to arrange them so as to get the greatest effect from the fuel. Forthe Royal Navy composition tubes are usually adopted, as their con-ductive power is greater, and the water spaces are not so liable tochoke up with deposit; but for the mercantile marine iron and Steeltubes are extensively used. The tubes in either case are driven hardinto the holes in the tube-plates, and, after they are all in their places,are widened out with a suitable tool, and the edges neatly laid over.