velocity with which this weight is being moved, or the distance through which it travels in one minute.

The former is found by multiplying the area of the piston by the average pressure of the steam upon each square inch; this average having been either found by the Indicator, or assumed from a knowledge of the pressure of the steam as generated in the boiler and used in the engine.

The distance through which the piston travels in a minute is found by multiplying the length of stroke which it makes in going from one end of the cylinder to the other, and back again, by the number of such strokes made in a minute. The total pressure on the piston of any engine, and the velocity or total distance travelled by the piston in one minute being obtained or known, the power of that engine is then calculated as a common rule of three sum.

:

Thus As 33,000 lbs. multiplied by 1 foot high in 1 minute (which represents one horse power) is to the total pressure in lbs. on the piston, multiplied by the velocity of the piston in feet in 1 minute, so is 1 horse power to the horse power of the engine as sought.

In speaking however of an engine generally, the length of stroke is the distance travelled by the piston, from the top to the bottom of the cylinder, or vice versâ, and not a whole stroke, or the distance from a starting point back again to that starting point, which is equal to a revolution of the crank.

The duty of an engine is the amount of work done in reference to the quantity of fuel consumed. The Modulus or useful effect of a machine is the fraction which expresses the amount of work compared with the power applied, which is expressed by unity. Thus if the work performed be only, the other third would have been lost in friction, and the actual product would be the modulus.

The consumption of fuel is computed at 8 lbs. per horse power per ton. The capacity of coal boxes at 48 cubic feet to

the ton.

The cost of engines varies from 20l. to 607. per horse power, according to the intrinsic value of the materials used.

An engine of 450 horse power, weighs on an average, altogether, about 300 tons. Thus, for example :

The Marine Propeller is a screw placed on an axis parallel to the keel, having its thread more or less inclined to the perpendicular. Turned in one direction it bears against the water and the vessel; the water offering the greatest resistance, the vessel recedes and is moved ahead. Turned in the opposite direction its tendency is to draw the water towards the ship; but again the water asserts its superiority, the ship follows the screw and goes astern. When the screw is tried in the basin, the ship's bows bear against, and are secured to, the wall. The ship then of course becomes the fixture and the water the body that moves. The enormous power of the screw is never seen to greater advantage than during this experiment. The scene presented by the agitation of the water in this struggle for supremacy reminds a spectator of the eddies and convulsions of a maelstrom, and he is inclined at some moments to fancy that either the vessel will break up under the pressure, or the strong opposition of stone blocks themselves be forced to give way. The propelling action of the screw, unable to project the ship, flings the water astern with tremendous force, and the opposite action, after straining the bow fastenings to the uttermost, expends its force on the water, which it draws towards the stern in rapid and tumultuous currents.

The annexed figure will probably convey a better idea of the propeller than can be gathered from a mere description. In this piece of a screw there are two threads, and it is evident there might be more or less. But as the propellers used in the navy are generally two threaded, that is the class represented here.

*These contained 220 tons of coal.

Body Post.-After side.

Fig. 233. represents the propeller in its frame, on its bearings, and connected with the shaft.

A propeller cut out of a screw of this kind, would appear end on and sideways, as in the sections.

It must be observed that a propeller is cast in the form of a section, and not cut out of a great length of screw. The length is merely drawn for the sake of illustration, because a section alone is not very readily comprehended.

The mode in which the propeller is suspended in its frame, by which it is raised, lowered, and supported in its place by bearings, and by which it is connected with, and disengaged from the driving shaft are shown in the drawings.

"The pitch of a screw is the distance in a straight line, from one point of the outer line (periphery) to the point that completes one spiral turn around the axis or shaft, or it is the distance that a screw advances in a solid by one turn.

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During its revolution the propeller does not advance the whole distance of its pitch as is the case of a screw entering a solid substance; because the water yields to some extent. This deficiency of progress is called the slip."

HANDLING THE PROPELLER.

When the Propeller is heavy in proportion to the yards, place the main yard, lash the bunt particularly well, put on the lee rolling tackle, pull up the lifts, trusses, &c. Have two top tackles and their pendants as in the first way described of getting in guns; use the main jeers rove in the upper and spare jeer blocks for an up and down tackle; also, have two preventer tackles to separate strops on the yards for lifts. They must be from the cap, but the ends of the pendants should, after going over the cap, come on deck and be secured there. It is not impossible to wring a mast head. If you have to carry the screw aft, land it on an 8-inch gun carriage.

In dealing with the mizen mast, as in getting the screw on the poop from the quarter deck, or lifting it out of the chamber, the greater rake of the mast and shortness of howsing demand considerable support from forward, and it is a matter of opinion whether the runners and tackles should do so from the mainmast head or the deck.

To lift the screw into the banjo and place it on the racks in the chamber, make a derrick of the spanker boom, stepping the