All About Engines

278 All About Engines fore alternately admitted to and released from the cylinder. In the double-acting oscillating engine the cylinder is pivoted at its centre, and there are ports leading to each end. The block, to which the cylinder is pivoted, has one steam and two exhaust ports, and steam is admitted and released by the swinging of the cylinder about the pivots as the crank turns. The advantage of oscillating engines lies in the fact that no connecting rod is used, and that space is, therefore, saved. Even to-day they are to be seen on many paddle steamers plying on lakes and rivers, but for efficiency they cannot be compared with modern engines. The cramped space on board ship led to inclined engines, built on sloping bedplates, being used for paddle steamers, and on screw steamers engines with a return connecting rod were employed. The vertical form, with the cylinders over the shaft, was slow to be adopted, because the A-shaped standards had to be very heavy castings in order to withstand the vibration. But with improved materials and workshop processes this form gradually replaced every other, because it took up less space and the parts were more readily accessible. Ever since they were introduced by John Elder in 1850, compound engines have always been em- ployed on ships. The shipowner wants to carry as little coal as possible in order to provide the largest space for profitable cargo, so he demands engines which will give the greatest amount of power for the