The reversibility of the dynamo was enunciated by Werner Siemens in 1867; but it was not experimentally demonstrated on any practical scale until 1870, when M. Hippolite Fontaine succeeded in pumping water at the Vienna international exhibition by the aid of two dynamos connected in circuit; one, the generator, deriving motion from a hydraulic engine, and in turn setting in motion the receiving dynamo which worked the pump. Professor Clerk Maxwell thought this discovery the greatest of the century; and the remark has been repeated more than once. But it is a remark which derives its chief importance from the man who made it, and its credentials from the paradoxical surprise it causes. The discovery in question is certainly fraught with very great consequences to the mechanical world; but in itself it is no discovery of importance, and naturally follows from Faraday's far greater and more original discovery of magneto-electric generation. In 1874, Dr. Siemens published a treatise on the laying and testing of submarine cables. In 1875, 1876 and 1877, he investigated the action of light on crystalline selenium, and in 1878 he studied the action of the telephone. The recent work of Dr. Siemens has been to improve the pneumatic railway, railway signalling, electric lamps, dynamos, electro-plating and electric railways. The electric railway at Berlin in 1880, and Paris in 1881, was the beginning of electric locomotion, a subject of great importance and destined in all probability, to very wide extension in the immediate future. Dr. Siemens has received many honours from learned societies at home and abroad; and a title equivalent to knighthood from the German Government. VI. LATIMER CLARK. MR. Clark was born at Great Marlow in 1822, and probably acquired his scientific bent while engaged at a manufacturing chemist's business in Dublin. On the outbreak of the railway mania in 1845 he took to surveying, and through his brother, Mr. Edwin Clark, became assistant engineer to the late Robert Stephenson on the Britannia Bridge. While thus employed, he made the acquaintance of Mr. Ricardo, founder of the Electric Telegraph Company, and joined that Company as an engineer in 1850. He rose to be chief engineer in 1854, and held the post till 1861, when he entered into a partnership with Mr. Charles T. Bright. Prior to this, he had made several original researches; in 1853, he found that the retardation of current on insulated wires was independent of the strength of current, and his experiments formed the subject of a Friday evening lecture by Faraday at the Royal Institution--a sufficient mark of their importance.