IT is an interesting phenomenon that many inventions have been made two or more times by different inventors, each working without knowledge of the other's research. There are a number of cases of such duplicate inventions or discoveries that are of common knowledge. It is well known, for instance, that both Newton and Leibnitz invented calculus. The theory of natural selection was developed practically identically by Wallace and by Darwin. It is claimed that both Langley and Wright invented the airplane. And we all know that the telephone was invented by Gray and by Bell. A good many such cases of duplication in discovery are part of the stock of knowledge of the general reader. There are, however, a large number of very important instances that are not so well known. For example, the invention of decimal fractions is credited to Rudolph, Stevinus and Biurgi. Oxygen was discovered by Scheele and by Priestley in I774. The molecular theory is due to Avagadro in I8II and to Ampere in i8I4. Both Cros and du Hauron invented color photography in I869. The trolley car resulted from the work of Van Doeple and also Sprague, and the essential elements were devised independently by Siemens and Daft. We think of Napier and Briggs as the inventors of logarithms, but it is not generally known that Biirgi also invented them three years previously. We associate the origin of photography with Daguerre but it was also independently invented by Talbot. Boyle's Law is known in French textbooks as Marriotte's Law. The existence of Neptune was discovered independently by Adams and Leverrier, before the planet was actually observed, the work of these two mathematical astronomers leading to its observation by others. Gauss is frequently recognized as re-sponsible for the principle of least squares. Legendre pub-lished his account of the principle three years before Gauss did, although Gauss had used the principle still earlier. 83