was pulled up by double 20-inch blocks, double stropt, the eye of the shroud being taken through the strop and toggled. The lower blocks strops went over the end of the out-riggers, the falls were 7-inch. The fore-mast was secured just as the main, excepting that it had but one fish, and that was on the after side. The runners were carried forward on each mast to support the stays. The mizen mast was additionally supported by the long tackles.

The wedges being removed, the masts were steadied over against the port partners, after which the starboard rigging was tautened.

The purchase lashings were set up by the runner pendant from one mast-head to the other. The tripping cables were brought from the opposite side of the harbour, under the bottom, and secured through the starboard upper deck ports to the port side, one being forward and the other aft. The Fore Chains were broken downwards (although previously shored) by the pressure of the foremost of these cables when heaving down.

The heaving down occupied twenty minutes, and the ship was eased up once to set up rigging; on which occasion, it was found that the long shore of the main-mast had butted against and injured the tressle-tree.

When first down, the fulness of the buttocks caused the stern to rise so high as to be inconvenient for the workmen, therefore eighty butts were secured under and below the fore chains before heaving down the second time. This had the desired effect of keeping the keel parallel, and enabling the repairs to be carried on at the gripe and stern at the same time.*

A very instructive lesson on this subject will be found in an account of the heaving down of the "Melville," published by Captain Harris. That officer remarks that the ship leaked when down as much as 268 tons in ninety-six minutes, although nine hand-pumps and five engines were at work all that time, a quantity which it took 210 minutes to pump out when righted; but that she leaked less when the main deck ports, which we observe were open in the Melville and Formidable in the first instance, were closed. He also calls attention to the importance of preserving the original distance between the fore

It is well to nail battens fore and aft on the deck before heaving down, otherwise it is most difficult to pass along.

and main-mast heads, when setting up the rigging afresh; otherwise in heaving down, the masts spread apart, and the purchase falls do not look straight into their sheaves. In the case of the Melville's pumps, the lower ones discharged into tubs on the lower deck, into which the lower ends of the upper pumps were placed.

In cases where the vessel has been dismasted, or where it would be impossible to procure sufficient length of purchase falls, &c., the bottom is turned out of the water by means of Spur derricks. H.M. ship "Success," for instance, was thus repaired. The upper ends of the derricks were cleated on the ship's side, the lower, to which the purchase blocks were lashed,

Fig. 229.

were secured from rising by turns of the chain cable, that were passed under the bottom from the opposite side, being steadied by guys led from forward and aft. (Fig. 229.)

The after bearings of the "Croesus," a screw ship of 2500 tons, were, in the absence of a dock, recently repaired by means of a caisson, which, when placed, enclosed the heel of the ship from the foremost stern post aft.

It was formed sloping at the fore part from the base to the top, and sufficiently open at that part to admit the heel, the dimensions being 22 feet at base, 15 feet at top, 20 feet in depth, and 9 feet in breadth. Displacement about 100 tons. It was sunk by loading it with chain cable, which was removed when the caisson was drawn forward into position by guys, and was kept free by constantly working two seven-inch pumps;

FUEL.

Coal is usually supplied in bags containing about 224 lbs., and therefore running ten to the ton. An average ton of coal requires 48 cubic feet of stowage room; but as coal of equal evaporative powers varies in degrees of compactness so much that 100 tons of one kind will stow in a space that 80 of another would fill, bulk must not be taken as a sure mode of ascertaining the number of days' fuel that may be on board the ship.

"Patent fuel is a mixture of coal and other substances. Its object being to combine great evaporating power, with freedom from sulphurous or other noxious qualities, with durability in form and nature, compactness for stowage, non-liability to spontaneous combustion, or generation of gases, and economy; a combination of qualities possessed by no one kind of coal."

When coal is closely confined, the bunkers become charged with dangerous inflammable gases; and when exposed to moisture, heat is generated, and what is called spontaneous combustion produced. Ventilation and dryness are therefore to be studied.

A quarter of a pound of good coal will produce 25 cubic feet of steam, which is equal to one horse power.

The evaporative power of coal is dependent on the quantity of carbon it contains; Welsh is superior to and less bulky than Newcastle, and all coal is superior to wood as one to three, and to peat as one to six.

At whatever pressure steam may be generated, it requires the same quantity of fuel to evaporate a given volume of water. Hence it is economical to use high pressure steam.

Oxygen, as we have already shown, is the principle of burning, and one pound of coals requires as much as 240 cubic feet of air for its combustion. Hence it is, that if there be delay in clearing the stoke hole of ashes, or neglect of ventilation by windsails, removing hatchway gratings, &c., the fires get low, and then the steam.

The same consequences follow when the fires are only replenished at great intervals, and then so profusely as to retard combustion.

By good stoking is meant little at a time and frequent; and

as this cannot be done without great toil, the ship will often move slowly, when a few spare hands lent from the watch would make more difference than shaking two reefs out with all hands.

The relative consumption of fuel is independent of the power of the engine; and is modified by the construction of the furnace and handling of the fires. In marine engines it has been found to vary from seven to twelve pounds per horse power per hour.

"The draft of a chimney arises from the difference between the weights of the column of rarified air within, and of the column of air without; the greater the perpendicular height of the chimney and the hotter the air within it, the greater this difference, and the greater therefore the draft. Horizontal or oblique passages diminish the draft by cooling the air, before it arrives at the effective portion of the flue. Sometimes as in locomotives, the waste steam escapes through the chimney, being thrown up into it by the blast-pipe." A pipe led from the steam chest to the uptake in a marine engine is a partial adoption of this mode. Should the fires burn more fiercely than necessary, the draft is diminished by regulating a damper placed across the inside of the funnel.

STEAM.

The two leading properties of Steam are its expansive force and its facility of condensation, both which are shown by the use of a glass tube as in the figure.

Let a represent a glass tube with a bulb at its lower end. It is held in a brass ring to which a wooden handle is attached, and contains a piston c, which (as well as its rod) is perforated, and may be opened or closed by the screw at top, d; it is kept central by passing through a slice of cork

at e.

When used, a little water is poured into the bulb and carefully heated over a spirit lamp; the aperture in the piston rod being

d

Fig. 230.

open, the air is thus expelled, and when steam freely follows it, the screw may be closed, when, on applying cold to the bulb (as