Another of the experimental staff says: "Edison is never hindered by theory, but resorts to actual experiment for proof. For instance, when he conceived the idea of pouring a complete concrete house it was universally held that it would be impossible because the pieces of stone in the mixture would not rise to the level of the pouring-point, but would gravitate to a lower plane in the soft cement. This, however, did not hinder him from making a series of experiments which resulted in an invention that proved conclusively the contrary." Having conceived some new idea and read everything obtainable relating to the subject in general, Edison's fertility of resource and originality come into play. He will write in one of the laboratory note-books a memorandum of the experiments to be tried, and, if necessary, will illustrate by sketches. This book is then given to one of the large staff of experimenters. Here strenuousness and a prompt carrying on of the work are required. The results of each experiment must be recorded in the notebook, and daily or more frequent reports are expected. Edison does not forget what is going on, but in his daily tours through the laboratory keeps in touch with the work of all the experimenters. His memory is so keen and retentive that he is as fully aware of the progress and details of each of the numerous experiments constantly going on as if he had made them all himself. The use of laboratory note-books was begun early in the Menlo Park days and has continued ever since. They are plain blank-books, each about eight and a half by six inches, containing about two hundred pages. At the present time there are more than one thousand of these books in the series. On their pages are noted Edison's ideas, sketches, and memoranda, together with records of countless thousands of experiments made by him or under his direction during more than thirty years. These two hundred thousand or more pages cover investigations into every department of science, showing the operations of a master mind seeking to surprise Nature into a betrayal of her secrets by asking her the same question in a hundred different ways. The breadth of thought, thoroughness of method, infinite detail, and minuteness of investigation proceeding from the workings of one mind would surpass belief were they not shown by this wonderful collection of note-books. A remark made by one of the staff, who has been experimenting at the laboratory for over twenty years, is suggestive. He said: "Edison can think of more ways of doing a thing than any man I ever saw or heard of. He tries everything and never lets up, even though failure is apparently staring him in the face. He only stops when he simply can't go any farther on that particular line. When he decides on any mode of procedure he gives his notes to the experimenter and lets him alone, only stopping in from time to time to look at the operations and receive reports of progress."