into vibration by being attached by a silken thread to a light steel spring, which carried at its extremity a blunt metallic point, which was held by the elasticity of the spring against the tin-foil covering the cylinder, but with sufficient lightness to allow it to be thrown into vibration as the indentations on the tin-foil passed beneath it. Mr. Edison has, however, in his more recent instrument, of which Fig. I is an illustration, dispensed with this second membrane, making the one metallic diaphragm do double duty, first by receiving vibrations from the voice and impressing them upon the tin-foil, and afterwards by being thrown into vibration by the passing below its projecting pin of the indentations so produced, and thus giving out a repetition of the original sound. The diagram Fig. 2 will serve to illustrate the principle upon which the tin-foil is impressed by the action of the vibration of the diaphragm, and how the latter is again thrown into precisely similar vibration by the movement of FIG. 2. the embossed foil below it. It represents a magnified section of the tin-foil taken along the line of the indentations, showing the position of the pin as it rides over its surface while the foil is travelling below it from right to left. The indentations, of which three only are shown on the diagram, having been produced on the foil by the sonorous vibrations ot the diaphragm, it follows, from the construction of the instrument, that if the foil be drawn under the pin at precisely the same speed as it was travelling when the impressions were made upon it in the first instance, it will cause the diaphragm to vibrate in an exactly similar manner to that in which it vibrated under the influence of the voice, and, from what was pointed out at the beginning of this article, it would therefore emit a similar sound. This diagram is, of course, greatly exaggerated, and must be taken only as an illustration of a possible explanation of what goes on in the action of the instrument, but which is by no means certain. It presupposes that each indentation is made up of a minute structural surface, the details of which are so small as to be quite indistinguishable under comparatively high powers of the microscope, and yet must be sufficiently pronounced to impart to the diaphragm through its projecting pin those minute variations of vibration by which the proper form is given to the sound-bearing waves of the air, and the exact quality of the sound is conveyed to the ear. It is almost impossible to conceive that microscopical striæ (for they can be nothing more) upon such a substance as tin-foil can impart, by mechanical means, to a diaphragm as rigid as that employed in the phonograph, such niceties of motion; but the phenomena connected with the telephone have shown that metallic diaphragms are capable of imparting to the air, and the human ear is capable of detecting, sonorous vibrations whose amplitude is so minute as to have been altogether unsus FIG. 3. pected before. Some idea of the minuteness of the indentations may be formed from Fig. 3, which is printed from an electrotype cast of a piece of the foil, and is therefore a facsimile on a plane surface of the marks recorded by the instrument. It will readily be understood that, in order to obtain a perfect reproduction of the original sounds, the tin-foil must travel below the diaphragm at precisely the same speed as it was turning when it was receiving the impressions, and therefore in Mr. Edison's second instrument, which we have been describing, the heavy flywheel was added to render the speed of rotation as uniform as possible; but in turning the instrument by hand it is impossible, notwithstanding this addition, to insure the surface speed of the cylinder being always the same. In order to meet this requirement Mr. Edison has since applied clockwork mechanism for driving FIG. 4. the apparatus, with such marked success that twelve clerks were lately able to take down correctly portions of newspaper articles from dictation spoken to them by the instru ment. At a recent exhibition of the phonograph in New York its articulation was distinctly heard and understood at a distance of 425 feet from the instrument. Still more recently Mr. Edison has made records on copper-foil that could be read at a distance of 275 feet in the open air. Fig. 4 is a view of a very beautifully arranged instrument, designed and constructed for his own use and instruction, by Mr. Augustus Stroh. The cylinder is driven round at a surface speed of about I foot in a second, by means of exceedingly simple controlled clockwork mechanism actuated by a descending weight, attached, upon Huyghen's maintaining principle, to an endless chain passing over a pulley fixed upon the principal axis of the instrument, so that it is possible to wind up the weight while the cylinder is rotating without affecting its speed. The controlling fan, or governor, which is beautifully simple and efficient, consists of two circular disks of brass mounted at the upper ends of two light levers, which are geared together at their lower ends, so as to cause them to fly out symmetrically on each side of the axis of rotation of the vertical fly-shaft to which they are pivoted. When the machine is started (by taking off the pressure of a small cork-lined brake-block, shown in small detail sketch, which presses against the cylindrical head of the fly-spindle) the disks fly out under the influence of centrifugal force, and the resistance of the air to the motion of the spindle is increased by the increase in the diameter of their path of rotation if the speed become too great. Should, however, the speed of rotation tend to fall off, a spiral spring, which can be attached to the fan levers at any position in their length, draws them together, and, by reducing the circle of their path, offers to the mechanism a diminished resistance to rotation. At a recent meeting of the Society of Telegraph Engineers both the instruments which we here describe were exhibited in illustration of a very interesting paper, by Mr. W. H. Preece, C.E., upon this last and perhaps greatest marvel of the application of Science which this or any other age has seen. Mr. Edison's first form of phonograph was represented by a very successful instrument made by an amateur, Mr. Pigeon, from descriptions received from America, and in which the two diaphragms, the one of paper and the other of metal, were employed. The second form of apparatus was represented by Mr. Edison's own instrument (Fig. 1), and the more perfect form driven by controlled mechanism was represented by Mr. Stroh's instrument, which has been described, and which is illustrated in Fig. 4. The first words were spoken into the mouthpiece by Mr. Puscus. The mouthpiece was then withdrawn, the cylinder turned back until the pin was at the beginning of the groove, a cone of paper or speaking trumpet was put on in front of the mouthpiece, and the handle once more rotated, when the instrument shouted out, in a perfectly clear voice, "The Phonograph presents its compliments to the audience." This was heard in every portion of the hall of the Institution of Civil Engineers, and brought forth rounds of applause, to which were added roars of laughter, when it again called out in a voice still clearer than before, "How do you do? How do you like the Phonograph ?" and then began to laugh in veritable hearty human laughter, "Ha! ha ha! ha! ha! hurray!" Mr. Pigeon's instrument was next tried, and the sublime words of the national war song "We don't want to fight, but by Jingo if we do," followed by the recital of the equally ennobling creation of the poet "Twinkle, twinkle, little star, were given by the instrument with an emphasis entirely its own, which caused great merriment. A song was next sung into the mouthpiece, and was reproduced amazingly out of tune, in consequence of the impossibility of obtaining a perfectly uniform speed of rotation. When, however, Mr. Stroh's instrument was brought into use, the value to the phonograph of controlling the speed of the cylinder by mechanical means was at once apparent, for not only was the articulation of spoken words more perfect, but songs sung into it by Mr. Spagnoletti, Mr. Edmunds, and Mr. Preece, were reproduced with very respectable correctness; and even the breakdown of one of the singers at a high note, accompanied by a little impatient remark, was faithfully recorded, and given out again with exasperating fidelity. At the Physical Society, on March 2nd, the instrument was again described by Mr. Preece, followed by a similar series of experiments, with the addition of causing the instrument to perform the wonderful feat of reproducing a duet |