_In erecting the Smith Point lighthouse, after the cylinder was set up, it had to be forced down fifteen and a half feet into the sand. The lives of the men who did this, working in the caisson at the bottom of the sea, were absolutely in the hands of the men who managed the engine and the air-compressor at the surface; and twice these latter were entirely deluged by the sea, but still maintained steam and kept everything running as if no sea was playing over them._ Peter Cooper Hewitt 292 _With his interrupter._ Watching a Test of the Hewitt Converter 299 _Lord Kelvin in the centre._ The Hewitt Mercury Vapour Light 305 _The circular piece just above the switch button is one form of "boosting coil" which operates for a fraction of a second when the current is first turned on. The tube shown here is about an inch in diameter and several feet long. Various shapes may be used. Unless broken, the tubes never need renewal._ Testing a Hewitt Converter 311 _The row of incandescent lights is used, together with a voltmeter and ammeter, to measure strength of current, resistance, and loss in converting._ BOYS' SECOND BOOK OF INVENTIONS CHAPTER I THE MIRACLE OF RADIUM _Story of the Marvels and Dangers of the New Element Discovered by Professor and Madame Curie_ No substance ever discovered better deserves the term "Miracle of Science," given it by a famous English experimenter, than radium. Here is a little pinch of white powder that looks much like common table salt. It is one of many similar pinches sealed in little glass tubes and owned by Professor Curie, of Paris. If you should find one of these little tubes in the street you would think it hardly worth carrying away, and yet many a one of them could not be bought for a small fortune. For all the radium in the world to-day could be heaped on a single table-spoon; a pound of it would be worth nearly a million dollars, or more than three thousand times its weight in pure gold. Professor and Madame Curie, who discovered radium, now possess the largest amount of any one, but there are small quantities in the hands of English and German scientists, and perhaps a dozen specimens in America, one owned by the American Museum of Natural History and several by Mr. W. J. Hammer, of New York, who was the first American to experiment with the rare and precious substance. [Illustration: M. Curie Explaining the Wonders of Radium at the Sorbonne.]