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The soft and springy button of lamp-black became no longer necessary, since it was not so much the resistance of the material which varied as the resistance at the contacts of its parts and the platinum electrodes. Two metals, or two pieces of hard carbon, or a piece of metal and a piece of hard carbon, were found to regulate the current in accordance with the vibrations of the voice. Edison therefore discarded the soft and fragile button, replacing it by contacts of hard carbon and metal, in short, by a form of microphone. The carbon, or microphone transmitter, was found superior to the magneto-electric transmitter of Bell; but the latter was preferable as a receiver to the louder but less convenient chemical receiver of Edison, and the most successful telephonic system of the day is a combination of the microphone, or new carbon transmitter, with the Bell receiver.

The 'micro-tasimeter,' a delicate thermoscope, was constructed in 1878, and is the outcome of Edison's experiments with the carbon button.

Knowing the latter to be extremely sensitive to minute changes of pressure, for example, those of sonorous vibrations, he conceived the idea of measuring radiant heat by causing it to elongate a thin bar or strip of metal or vulcanite, bearing at one end on the button. To indicate the effect, he included a galvanometer in the circuit of the battery and the button. The apparatus consisted of a telephone button placed between two discs of platinum and connected in circuit with the battery and a sensitive galvanometer. The strip was supported so that one end bore upon the button with a pressure which could be regulated by an adjustable screw at the other. The strip expanded or contracted when exposed to heat or cold, and thrust itself upon the button more or less, thereby varying the electric current and deflecting the needle of the galvanometer to one side or the other. The instrument was said to indicate a change of temperature equivalent to one-millionth of a degree Fahrenheit. It was tested by Edison on the sun's corona during the eclipse observations of July 29, 1875, at Rawlings, in the territory of Wyoming. The trial was not satisfactory, however, for the apparatus was mounted on a hen-house, which trembled to the gale, and before he could get it properly adjusted the eclipse was over.

It is reported that on another trial the light from the star Arcturus, when focussed on the vulcanite, was capable of deflecting the needle of the galvanometer. When gelatine is substituted for vulcanite, the humidity of the atmosphere can also be measured in the same way.

Edison's crowning discovery at Menlo Park was the celebrated 'phonograph,' or talking machine. It was first announced by one of his assistants in the pages of the SCIENTIFIC AMERICAN for 1878. The startling news created a general feeling of astonishment, mingled with incredulity or faith. People had indeed heard of the talking heads of antiquity, and seen the articulating machines of De Kempelen and Faber, with their artificial vocal organs and complicated levers, manipulated by an operator. But the phonograph was automatic, and returned the words which had been spoken into it by a purely mechanical mimicry. It captured and imprisoned the sounds as the photograph retained the images of light. The colours of Nature were lost in the photograph, but the phonograph was said to preserve the qualities even of the human voice.

Yet this wonderful appliance had neither tongue nor teeth, larynx nor pharynx. It appeared as simple as a coffee-mill. A vibrating diaphragm to collect the sounds, and a stylus to impress them on a sheet of tinfoil, were its essential parts. Looking on the record of the sound, one could see only the scoring of the stylus on the yielding surface of the metal, like the track of an Alpine traveller across the virgin snow.

These puzzling scratches were the foot-prints of the voice.

Speech is the most perfect utterance of man; but its powers are limited both in time and space. The sounds of the voice are fleeting, and do not carry far; hence the invention of letters to record them, and of signals to extend their range. These twin lines of invention, continued through the ages, have in our own day reached their consummation. The smoke of the savage, the semaphore, and the telegraph have ended in the telephone, by which the actual voice can speak to a distance; and now at length the clay tablet of the Assyrian, the wax of the ancient Greek, the papyrus of the Egyptian, and the modern printing-press have culminated in the phonograph, by which the living words can be preserved into the future. In the light of a new discovery, we are apt to wonder why our fathers were so blind as not to see it. When a new invention has been made, we ask ourselves, Why was it not thought of before? The discovery seems obvious, and the invention simple, after we know them.

Now that speech itself can be sent a thousand miles away, or heard a thousand years after, we discern in these achievements two goals toward which we have been making, and at which we should arrive some day. We marvel that we had no prescience of these, and that we did not attain to them sooner. Why has it taken so many generations to reach a foregone conclusion? Alas! they neither knew the conclusion nor the means of attaining to it. Man works from ignorance towards greater knowledge with very limited powers. His little circle of light is surrounded by a wall of darkness, which he strives to penetrate and lighten, now groping blindly on its verge, now advancing his taper light and peering forward;yet unable to go far, and even afraid to venture, in case he should be lost.