The screw propeller was invented in 1836, but for a long time it was thought to be inferior to the paddle as a means of propulsion, and there was some difficulty in applying the power to the screw shaft. The side lever in various forms was tried, but proved a failure. The "Great Britain," 3,270 tons, launched in 1843, had engines which worked upward on to a crank shaft, and the power was brought down by endless chains to the screw shaft. This did not prove satisfactory. Then we had oscillating engines working a large geared wheel fitted with wooden teeth to increase the revolutions of the propeller. Then came the direct-acting engines with inverted cylinders, which for years were almost the universal type of engine, and were a very efficient form of low pressure engine. The compound engine revolutionised the steamship trade, ensuring such economy of fuel as to permit of long voyages being successfully undertaken. The compound engine developed into the triple expansion engine; the object being to get the last ounce of power out of the steam by first using it in a high pressure cylinder at 180 lbs., then passing it into a larger cylinder, using it expansively, and finally passing it into a still larger cylinder at about 8 lbs. pressure, and again allowing it to expand. The triple expansion engine came into general use in 1886. The turbine, invented by Sir Charles Parsons in 1897, has effected a revolution in the engines of large size. The principle is simply to allow steam at a high pressure to impinge upon blades fitted to a rotor which it revolves on the principle of the syren. The steam is afterwards used expansively in a second rotor working directly upon the screw shaft. The advantage of a turbine engine is its simplicity--few working parts and a saving in weight and space; its disadvantage is that a separate turbine has to be employed to obtain sternway. Recently, geared turbines have been introduced which are lighter, slightly more economical in fuel, and are sweeter running machines. It is noteworthy that whereas gears were necessary in olden times with engines working at a low pressure to speed up the propeller shaft, with turbines gears are used to reduce the revolutions. [Illustration: SS. "Scotia," 1860]