ON NAVAL ARCHITECTURE. 291 



Three ships, the "Arcthusa," " Octavia," and "Constance," were 

 fitted respectively by Messrs. Pcnn, Messrs. Maudslay, and Messrs. 

 Elder, with engines of large cylinder capacity to admit of great 

 expansion, with surface-condensers and super-heaters to the boilers. 



Those of the "Arcthusa" were djuble-trunk, with two cylinders; those 

 of the " Octavia " were three cylinder engines ; and those of the " Con- 

 stance," illustrated by the drawings referred to, were compound engines, 

 with six cylinders ; the two former were worked with steaiv. of twenty- 

 five pounds pressure per square inch, and the latter \vi:h steam of 

 thirty-two pounds pressure. All these engines gave good results as to 

 economy of fuel, but those of the " Constance," were the best, giving 

 one indicated horse-power wjth two and a-half pounds of fuel. 



But the engines of the " Constance " were excessively complicated 

 and heavy. They weighed, including water in bjilers and fittings, 

 about five and a-half hundred weight per maximum indicated horse- 

 power, whereas ordinary engines varied between three and a-half and 

 four and three-quarter hundred weight. 



For the next ten years engines with low pressure steam, surface- 

 condensers, and large cylinder capacity were employed almost exclu- 

 sively in the ships of the Royal Navy. A few compound engines, with 

 steam of thirty pounds pressure, were used in this period with good results 

 as to economy, but they gave trouble in some of the working parts. 



Compound engines, with high pressure steam (fifty-five pounds), were 

 first used in the Royal Navy in 1867, on Messrs. Maudslay's plan, in 

 the " Sirius." These have been very successful. In the Royal Navy the 

 compound engine is now generally adopted. They are rather heavier 

 than the engines which immediately preceded them, but they are about 

 twenty-five per cent, more economical in fuel, and taking a. total 

 weight of machinery and fuel together, there is from fifteen to twenty 

 per cent, gain in the distance run with a given weignt. We are now 

 introducing wrought iron largely in the framing in the place of cast iron r 

 and hollow propeller shafts made of Whitworth steel. By these means 

 the weight is being reduced, and it is to be hoped that a still further 

 reduction may yet be made by the use of high class materials in the 

 engines, and steel in the boilers. That there is room for improvement 

 is indicated by the marvellous results obtained by Mr.'Thorneycroft, 

 ol Chiswick, and others, by means of high piston speed, forced com 



