the empirical nature of his method will be apparent from an examination of the above items; but in pursuing it he leaves all uncertainty behind and, trusting nothing to theory, he acquires absolute knowledge. whatever may be the mental processes by which he arrives at the starting-point of any specific line of research, the final results almost invariably prove that he does not plunge in at random; indeed, as an old associate remarked: "when edison takes up any proposition in natural science, his perceptions seem to be elementally broad and analytical, that is to say, in addition to the knowledge he has acquired from books and observation, he appears to have an intuitive apprehension of the general order of things, as they might be supposed to exist in natural relation to each other. it has always seemed to me that he goes to the core of things at once."

although nothing less than results from actual experiments are acceptable to him as established facts, this view of edison may also account for his peculiar and somewhat weird ability to "guess" correctly, a faculty which has frequently enabled him to take short cuts to lines of investigation whose outcome has verified in a most remarkable degree statements apparently made offhand and without calculation. mr. upton says: "one of the main impressions left upon me, after knowing mr. edison for many years, is the marvellous accuracy of his guesses. he will see the general nature of a result long before it can be reached by mathematical calculation." this was supplemented by one of his engineering staff, who remarked: "mr. edison can guess better than a good many men can figure, and so far as my experience goes, i have found that he is almost invariably correct. his guess is more than a mere starting-point, and often turns out to be the final solution of a problem. i can only account for it by his remarkable insight and wonderful natural sense of the proportion of things, in addition to which he seems to carry in his head determining factors of all kinds, and has the ability to apply them instantly in considering any mechanical problem."

while this mysterious intuitive power has been of the greatest advantage in connection with the vast number of technical problems that have entered into his life-work, there have been many remarkable instances in which it has seemed little less than prophecy, and it is deemed worth while to digress to the extent of relating two of them. one day in the summer of 1881, when the incandescent lamp-industry was still in swaddling clothes, edison was seated in the room of major eaton, vice-president of the edison electric light company, talking over business matters, when mr. upton came in from the lamp factory at menlo park, and said: "well, mr. edison, we completed a thousand lamps to-day." edison looked up and said "good," then relapsed into a thoughtful mood. in about two minutes he raised his head, and said: "upton, in fifteen years you will be making forty thousand lamps a day." none of those present ventured to make any remark on this assertion, although all felt that it was merely a random guess, based on the sanguine dream of an inventor. the business had not then really made a start, and being entirely new was without precedent upon which to base any such statement, but, as a matter of fact, the records of the lamp factory show that in 1896 its daily output of lamps was actually about forty thousand.

the other instance referred to occurred shortly after the edison machine works was moved up to schenectady, in 1886. one day, when he was at the works, edison sat down and wrote on a sheet of paper fifteen separate predictions of the growth and future of the electrical business. notwithstanding the fact that the industry was then in an immature state, and that the great boom did not set in until a few years afterward, twelve of these predictions have been fully verified by the enormous growth and development in all branches of the art.

what the explanation of this gift, power, or intuition may be, is perhaps better left to the psychologist to speculate upon. if one were to ask edison, he would probably say, "hard work, not too much sleep, and free use of the imagination." whether or not it would be possible for the average mortal to arrive at such perfection of "guessing" by faithfully following this formula, even reinforced by the edison recipe for stimulating a slow imagination with pastry, is open for demonstration.

somewhat allied to this curious faculty is another no less remarkable, and that is, the ability to point out instantly an error in a mass of reported experimental results. while many instances could be definitely named, a typical one, related by mr. j. d. flack, formerly master mechanic at the lamp factory, may be quoted: "during the many years of lamp experimentation, batches of lamps were sent to the photometer department for test, and edison would examine the tabulated test sheets. he ran over every item of the tabulations rapidly, and, apparently without any calculation whatever, would check off errors as fast as he came to them, saying: 'you have made a mistake; try this one over.' in every case the second test proved that he was right. this wonderful aptitude for infallibly locating an error without an instant's hesitation for mental calculation, has always appealed to me very forcibly."

the ability to detect errors quickly in a series of experiments is one of the things that has enabled edison to accomplish such a vast amount of work as the records show. examples of the minuteness of detail into which his researches extend have already been mentioned, and as there are always a number of such investigations in progress at the laboratory, this ability stands edison in good stead, for he is thus enabled to follow, and, if necessary, correct each one step by step. in this he is aided by the great powers of a mind that is able to free itself from absorbed concentration on the details of one problem, and instantly to shift over and become deeply and intelligently concentrated in another and entirely different one. for instance, he may have been busy for hours on chemical experiments, and be called upon suddenly to determine some mechanical questions. the complete and easy transition is the constant wonder of his associates, for there is no confusion of ideas resulting from these quick changes, no hesitation or apparent effort, but a plunge into the midst of the new subject, and an instant acquaintance with all its details, as if he had been studying it for hours.

a good stiff difficulty—one which may, perhaps, appear to be an unsurmountable obstacle—only serves to make edison cheerful, and brings out variations of his methods in experimenting. such an occurrence will start him thinking, which soon gives rise to a line of suggestions for approaching the trouble from various sides; or he will sit down and write out a series of eliminations, additions, or changes to be worked out and reported upon, with such variations as may suggest themselves during their progress. it is at such times as these that his unfailing patience and tremendous resourcefulness are in evidence. ideas and expedients are poured forth in a torrent, and although some of them have temporarily appeared to the staff to be ridiculous or irrelevant, they have frequently turned out to be the ones leading to a correct solution of the trouble.

edison's inexhaustible resourcefulness and fertility of ideas have contributed largely to his great success, and have ever been a cause of amazement to those around him. frequently, when it would seem to others that the extreme end of an apparently blind alley had been reached, and that it was impossible to proceed further, he has shown that there were several ways out of it. examples without number could be quoted, but one must suffice by way of illustration. during the progress of the ore-milling work at edison, it became desirable to carry on a certain operation by some special machinery. he requested the proper person on his engineering staff to think this matter up and submit a few sketches of what he would propose to do. he brought three drawings to edison, who examined them and said none of them would answer. the engineer remarked that it was too bad, for there was no other way to do it. mr. edison turned to him quickly, and said: "do you mean to say that these drawings represent the only way to do this work?" to which he received the reply: "i certainly do." edison said nothing. this happened on a saturday. he followed his usual custom of spending sunday at home in orange. when he returned to the works on monday morning, he took with him sketches he had made, showing forty-eight other ways of accomplishing the desired operation, and laid them on the engineer's desk without a word. subsequently one of these ideas, with modifications suggested by some of the others, was put into successful practice.

difficulties seem to have a peculiar charm for edison, whether they relate to large or small things; and although the larger matters have contributed most to the history of the arts, the same carefulness of thought has often been the means of leading to improvements of permanent advantage even in minor details. for instance, in the very earliest days of electric lighting, the safe insulation of two bare wires fastened together was a serious problem that was solved by him. an iron pot over a fire, some insulating material melted therein, and narrow strips of linen drawn through it by means of a wooden clamp, furnished a readily applied and adhesive insulation, which was just as perfect for the purpose as the regular and now well-known insulating tape, of which it was the forerunner.

dubious results are not tolerated for a moment in edison's experimental work. rather than pass upon an uncertainty, the experiment will be dissected and checked minutely in order to obtain absolute knowledge, pro and con. this searching method is followed not only in chemical or other investigations, into which complexities might naturally enter, but also in more mechanical questions, where simplicity of construction might naturally seem to preclude possibilities of uncertainty. for instance, at the time when he was making strenuous endeavors to obtain copper wire of high conductivity, strict laboratory tests were made of samples sent by manufacturers. one of these samples tested out poorer than a previous lot furnished from the same factory. a report of this to edison brought the following note: "perhaps the —— wire had a bad spot in it. please cut it up into lengths and test each one and send results to me immediately." possibly the electrical fraternity does not realize that this earnest work of edison, twenty-eight years ago, resulted in the establishment of the high quality of copper wire that has been the recognized standard since that time. says edison on this point: "i furnished the expert and apparatus to the ansonia brass and copper company in 1883, and he is there yet. it was this expert and this company who pioneered high-conductivity copper for the electrical trade."

nor is it generally appreciated in the industry that the adoption of what is now regarded as a most obvious proposition—the high-economy incandescent lamp—was the result of that characteristic foresight which there has been occasion to mention frequently in the course of this narrative, together with the courage and "horse-sense" which have always been displayed by the inventor in his persistent pushing out with far-reaching ideas, in the face of pessimistic opinions. as is well known, the lamps of the first ten or twelve years of incandescent lighting were of low economy, but had long life. edison's study of the subject had led him to the conviction that the greatest growth of the electric-lighting industry would be favored by a lamp taking less current, but having shorter, though commercially economical life; and after gradually making improvements along this line he developed, finally, a type of high-economy lamp which would introduce a most radical change in existing conditions, and lead ultimately to highly advantageous results. his start on this lamp, and an expressed desire to have it manufactured for regular use, filled even some of his business associates with dismay, for they could see nothing but disaster ahead in forcing such a lamp on the market. his persistence and profound conviction of the ultimate results were so strong and his arguments so sound, however, that the campaign was entered upon. although it took two or three years to convince the public of the correctness of his views, the idea gradually took strong root, and has now become an integral principle of the business.

in this connection it may be noted that with remarkable prescience edison saw the coming of the modern lamps of to-day, which, by reason of their small consumption of energy to produce a given candle-power, have dismayed central-station managers. a few years ago a consumption of 3.1 watts per candle-power might safely be assumed as an excellent average, and many stations fixed their rates and business on such a basis. the results on income when the consumption, as in the new metallic-filament lamps, drops to 1.25 watts per candle can readily be imagined. edison has insisted that central stations are selling light and not current; and he points to the predicament now confronting them as truth of his assertion that when selling light they share in all the benefits of improvement, but that when they sell current the consumer gets all those benefits without division. the dilemma is encountered by central stations in a bewildered way, as a novel and unexpected experience; but edison foresaw the situation and warned against it long ago. it is one of the greatest gifts of statesmanship to see new social problems years before they arise and solve them in advance. it is one of the greatest attributes of invention to foresee and meet its own problems in exactly the same way.