“The sheet of tin-foil or other plastic material receiving the impressions of sound, will be stereotyped or electrotyped so as to be multiplied and made durable; or the cylinder will be made of a material plastic when used, and hardening afterward. Thin sheets of _papier maché_, or of various substances which soften by heat, would be of this character. Having provided thus for the durability of the Phonograph plate, it will be very easy to make it separable from the cylinder producing it, and attachable to a corresponding cylinder anywhere and at any time. There will doubtless be a standard of diameter and pitch of screw for Phonograph cylinders. Friends at a distance will then send to each other Phonograph letters, which will talk at any time in the friend’s voice when put upon the instrument.” (_Scribner._) The MICROPHONE (an outcome of the Telephone) was discovered by Professor Hughes, of London. It is an instrument which in its main features consists of a carbon “pencil,” so suspended that one end rests upon a carbon “die.” The instrument being connected with a Telephone by the circuit wires, will reproduce faint sounds very distinctly. Once a Microphone was put into a preacher’s pulpit, and joined to a private telegraph wire which led to a gentleman’s house. The owner was thus enabled to hear the sermon. So long as it is thus connected every minute sound, even a fly’s footstep, will be faithfully reproduced. CHAPTER XVII. THE TUNING-FORK—THE SYREN—SOUND FIGURES—SINGING FLAMES. We cannot close the subject of Sound without some mention of the Musical Pitch, and various instruments and experiments which have from time to time been made to discover the pitch, sound, and vibrations, and even to _see_ Sound. To understand the vibrations or “pitch” of a musical note we may study the illustration, which shows us a tuning-fork in vibration. You will perceive that each prong of the tuning-fork beats the air in an opposite direction at the same time, say from _a_ to _b_ (fig. 196). The prong strikes the air, and the wave thus created strikes again outward, and the condensation thus created travels along the back beat, rarefying the air, and both these, the rarefaction and the condensation, move with the same rapidity one behind the other. [Illustration: Fig. 196.] [Illustration: Fig. 197.] The tuning-fork of course vibrates a very great many times in a second, every vibration generating a wave. “Pitch,” in a general sense, is the number of vibrations per second which constitute a note. For instance, the note A, the standard pitch consists of four hundred and thirty-five complete vibrations per second. Concert pitch is slightly higher, for there are a few more vibrations in the second. The lowest sound pitch is forty vibrations, the highest forty thousand. “Pitch” may be determined by an instrument termed the “Syren,” or by a tooth-wheeled apparatus.