MECHANICS AND USEFUL ARTS. 23 



seven feet long and weighing thirty-five tons, was made at the LaecfieH 

 forge. This portion of the shaft, the heaviest piece of wrought iron in the 

 ship, was manufactured this enormous length in order that the junction of 

 it with the remaining portions should not interfere with the floor of the after- 

 cabins. The other lengths of the propeller-shaft, consisting of different 

 pieces, each twenty-five feet long and sixteen tons weight, were made in 

 London for Messrs. James Watt & Co., the builders of the screw-cn-rines. 



The Screw-Engines. --The, screw-engines, designed and built by^ Messrs. 

 Watt & Co., are horizontal direct-acting engines of 



Nominal horse power, JQQQ 



Number of cylinders, 4 



Diameter of each cylinder, Si in 



Length of stroke, 4f( .' 



Number of revolutions per minute, ... 50 



They are the largest ever made for marine purposes ; and, as is the case 

 with the paddle cylinders, each of the four is in itself a complete and sepa- 

 rate engine, capable of working quite independently of the other three 

 The combined screw-engines work up to an indicatoV-power of four thou- 

 sand five hundred horses of thirty-three thousand pounds, when Avorkin"- at 

 forty-five strokes a minute, with steam in the boiler at fifteen pounds Ind 

 the expansion-valve cutting off at one-third of the stroke. They are how- 

 ever, made to work smoothly, either at forty strokes per minute, with 'steam 

 at twenty-five pounds, without expansion, or at fifty-five strokes a minute 

 with the expansion cutting off at one-fourth of the stroke. Under the<e 

 circumstances, they will be working at the tremendous power of six thou- 

 sand five hundred horses. 



Screiv-Engine Boilers. - - The boilers for these engines are similar to those 

 for the paddle-engines, but a trifle larger and heavier. They are ten in 

 number, and the whole are so arranged that all or any of them can be used 

 with either set of engines. The weight of the screw-engines and boilers is 

 one thousand five hundred tons. To communicate between the different 

 stoke-holes and engine rooms, there are two perfectly water-tight tubes six 

 feet high and four feet wide, running through the ship, the opening ' into 

 which can be closed by water-tight doors. Through one of them the va- 

 rious steam-pipes go, and the other is used as a passage for the engineers 

 and stokers. There are ten donkey-engines to pump water into the boilers 

 and two auxiliary high-pressure engines of seventy horse power working 

 with forty pounds; but these, as with the other auxiliary engines are made 

 > work at sixty pounds. Both these, besides having to do all kinds of odd 

 out the ship, such as work the capstans, attend to the drainage and 

 water supply of the ship, etc., are connected with the screw-shaft abaft the 

 ordinary disconnecting apparatus, so as to enable them to drive the screw ir 

 sary, when disconnected from its main engine. It will thus be seen 

 that the paddle and screw engines, when working together at their highest 

 wer, will exert an effective force of not less than eleven thousand five 

 hundred horse power. 



The Coal Bunkers. - The coal bunkers are on either side, above and be- 



i the boilers, and are capable of containing about twelve thousand tons 



The distance to Port Philip, Australia, is nearly twelve thousand 



miles, which, at the rate of eighteen miles per hour, would take thirty days 



