The commonest and at the same time one of the most sensitive forms of the instrument is called the 'pencil microphone,' from the pencil or crayon of carbon which forms the principal part of it. This pencil may be of mercurialised charcoal, but the ordinary gas-carbon, which incrusts the interior of the retorts in gas-works, is usually employed. The crayon is supported in an upright position by two little brackets of carbon, hollowed out so as to receive the pointed ends in shallow cups. The weight of the crayon suffices to give the required pressure on the contacts, both upper and lower, for the upper end of the Pencil should lean against the inner wall of the cup in the upper bracket. The brackets are fixed to an upright board of light, dry, resonant pine-wood, let into a solid base of the same timber. The baseboard is with advantage borne by four rounded india-rubber feet, which insulate it from the table on which it may be placed. To connect the microphone up for use, a small voltaic battery, say three cells (though a single cell will give surprising results), and a Bell speaking telephone are necessary. A wire is led from one of the carbon brackets to one pole of the battery, and another wire is led from the other bracket to one terminal screw of the telephone, and the circuit is completed by a wire from the other terminal of the telephone to the other pole of the battery. If now the slightest mechanical jar be given to the wooden frame of the microphone, to the table, or even to the walls of the room in which the experiment takes place, a corresponding noise will be heard in the microphone. By this delicate arrangement we can play the eavesdropper on those insensible vibrations in the midst of which we exist. If a feather or a camel-hair pencil be stroked along the base-board, we hear a harsh grating sound; if a pin be laid upon it, we hear a blow like a blacksmith's hammer; and, more astonishing than all, if a fly walk across it we hear it tramping like a charger, and even its peculiar cry, which has been likened, with some allowance for imagination, to the snorting of an elephant. Moreover it should not be forgotten that the wires connecting up the telephone may be lengthened to any desired extent, so that, in the words of Professor Hughes, 'the beating of a pulse, the tick of a watch, the tramp of a fly can then be heard at least a hundred miles from the source of sound.' If we whisper or speak distinctly in a monotone to the pencil, our words will be heard in the telephone; but with this defect, that the TIMBRE or quality is, in this particular form of the instrument, apt to be lost, making it difficult to recognise the speaker's voice. But although a single pencil microphone will under favourable circumstances transmit these varied sounds, the best effect for each kind of sound is obtained by one specially adjusted. There is one pressure best adapted for minute sounds, another for speech, and a third for louder sounds. A simple