On Plate XVI. we illustrate the general arrangement of the machinery in the _Hecate_ and _Hecla_. There were four boilers of the rectangular type, each with two wet-bottomed furnaces at one end and large return flues at the other end. The uptakes passed up inside the boilers through the steam space, uniting in one funnel. Smith's screw-propeller was tried experimentally in 1837, and Ericsson's about the same time. The comparative trials of the _Archimedes_ fitted with Smith's screw against existing paddle-steamers did much to prove the efficiency of the new system.[57] The screw-ship excelled the performance of paddle-steamers on the service, and the screw-propeller was adopted by the Admiralty in 1845; twin-screws followed twenty-five years later. [Illustration: Plate XVI. MACHINERY OF H.M.SS. "HECLA" AND "HECATE," 1839.] The _Greenock_, built in 1848, was the first war vessel by the Scotts fitted with the screw-propeller. We have already referred to her construction in iron, and to her launch. She had a displacement of 1835 tons, and her engines were of 719 indicated horse-power. The speed realised on the trial was 9.6 knots. The _Greenock's_ machinery, which is illustrated on the next page, is specially interesting, as it represents one of the earliest attempts to drive the screw-propeller by gearing. Two horizontal cylinders were fitted, each 71 in. in diameter, with a stroke of piston of 4 ft. The gearing consisted of four sets of massive spur-wheels and pinions, in the ratio of 2.35 to 1, so that 42 revolutions per minute of the engines give 98.7 revolutions to the propeller-shaft. The propeller was 14 ft. in diameter, and was so fitted that it could be detached and raised to the deck. There were four rectangular brass-tube boilers, each with four wet-bottomed furnaces, and all the internal uptakes united in one funnel, which was telescopic, so that when it was lowered and the propeller raised out of the water, the vessel had the appearance, as well as the facility, of a sailing frigate. As will be seen from the drawings, both the engines and boilers were arranged very low in the hull, to be safe from the enemy's fire. The engine and boiler compartment occupied 72 ft. of the length of the ship--about one-third of the total length--and the seating for the machinery was specially constructed, with a very close pitch of frames which were only 1 ft. apart. For comparison with the drawings of the machinery in the _Greenock_, we give on page 49 a similar drawing of the machinery of the _Canopus_, of 12,956 tons displacement, seven times that of the _Greenock_. To double the speed, the power of machinery had to be multiplied twenty times, and yet the space occupied is only about trebled. [Illustration: MACHINERY OF H.M.S. "GREENOCK," 1848.] [Illustration: MACHINERY OF H.M.S. "CANOPUS," 1900.]