To mark the centenary of practical marine screw propulsion, a special exhibit was opened at the Science Museum, South Kensington, on February 11. In accordance with the usual practice of the Museum, this shows both the history of the pioneer experimental work, as well as the subsequent developments and current practice in marine screw propellers. In February 1837, Francis Pettit Smith successfully tried his first screw-propelled steam launch (a model of which is shown) on the Paddington Canal. The screw had two complete turns; on one of the trips it struck an obstacle, and about one-half of it was broken off. To the inventor's astonishment, this accident materially increased the speed of the boat. The S.S. Archimedes (1838) was fitted with a double-threaded screw of half a turn, in accordance with Smith's amended patent. Other historical exhibits include models of Ericsson's experimental screw of 1837, and those of the S.S. James Lowe (1838), the S.S. Novelty (1839-40), and the first French screw steamer Napoleon (1843). Screws were at first used as an auxiliary means of propulsion, and in order that they should not impede the vessel when under sail only, lifting screws (an example by Maudslay, 1846, is shown) were introduced, which could be disconnected from the shaft and raised clear of the water. Other developments represented are the ‘common screw’, which was the most successful form of propeller up to about 1860, Hirsch's propeller (1860-66), and the adjustable pitch propeller patented by Griffiths in 1868. Various examples of modern practice are also included, notably a model of the four-bladed bronze propellers of the Cunard White Star liner R.M.S. Queen Mary, which are 20 ft. in diameter, each weighing 35 tons. These are the largest screw propellers which have ever been cast in one piece.