The last invention was the most important of the four, the application of the screw propeller to navigation made by John Ericsson. The author is aware of the fact that this invention was claimed by others, and is claimed for others now. The weight of testimony, however seems to be on the side of Ericsson; and as has been pointed out before, the question of the identity of the inventor is not important to our discussion. The first ocean steamship to be propelled by a screw was the _Stockton_, which was built in England under Ericsson and fitted with his screw. The first war-ship to be fitted with a screw was the U. S. S. _Princeton_ in 1841. Its screw was designed by Ericsson. In 1837 Crawford invented a process for "galvanizing" iron; for electro-plating it with a non-oxidizable metal. The value of this invention in preserving iron wire and iron articles in general needs not to be pointed out; it was a contribution to the permanency of the Machine. In the same year, Cooke and Wheatstone in England invented their famous "Needle Telegraph," in which a magnetic needle was made to deflect quickly to the right or left when one of two keys was pressed by an operator and letters thereby signaled. This invention was a valuable contribution; but it was eventually superseded by Morse's telegraph, after that system had established itself in the United States and on the Continent. In 1839 Babbitt invented his celebrated Babbitt metal, which has been successfully used ever since in the bearings of engines and in moving machinery generally, for reducing friction; and in the same year Goodyear made an invention even more important, the art of hardening, or "vulcanizing," rubber by means of sulphur. This invention was a great boon to mankind, but not to Goodyear; for the jackals who lie in wait for great inventions eager to wrest unearned profit for themselves from the men who have truly earned it, made Goodyear's life miserable for many years. Before he died, however, his wrongs were righted at least in part. In the same year Jacobi, in Germany, propelled a boat by electricity using an electric motor of his own invention. But the great contributions made in 1839 were to the art of what we now call photography. About 1834 Talbot had succeeded in taking pictures in a camera by the agency of light on paper washed with nitrate of silver and also in fixing them. Later, he was able to obtain many copies, or "proofs," from one picture or negative. It seems that he did not publicly announce his invention till 1839. To it was given the name "calotype." In May of that year Mr. Mungo Ponton announced that he had been able to copy pictures of engravings and of dried plants on paper that he had soaked in bichromate of potash. A number of other investigators forthwith announced similar feats, using various chemical solutions.