The form of the microphone has been varied in a thousand ways, to suit the purposes to which it was to be applied. In the ‘English Mechanic and World of Science,’ June 28, 1878, we see the drawings of several arrangements, one of which is specially adapted for hearing the steps of a fly. It is a box, with a sheet of straw paper stretched on its upper part; two carbons, separated by a morsel of wood, and connected with the two circuit wires, are fastened to it, and a carbon pencil, placed crosswise between the two, is kept in this position by a groove made in the latter. A very weak battery will be enough to set the instrument at work, and when the fly walks over the sheet of paper it produces vibrations strong enough to react energetically on an ordinary telephone. The instrument must be covered with a glass globe. When a watch is placed on the membrane, with its handle applied to the morsel of wood which divides the two carbons, the noise of its ticking may be heard through a whole room. Two carbon cubes placed side by side, and only divided by a playing-card, may also be used instead of the arrangement of carbons described above. A semicircular cavity, made in the upper part of the two carbons, in which are placed some little carbon balls, smaller than a pea and larger than a mustard seed, will make it possible to obtain multiple contacts which are very mobile and peculiarly fit for telephonic transmissions. This arrangement has been made by Mr. T. Cuttriss. Several other arrangements of microphones have been devised by different makers and inventors, such as those of Messrs. Varey, Trouvé, Vereker, de Combettes, Loiseau, Lippens, de Courtois, Pollard, Voisin, Dumont, Jackson, Paterson, Taylor, &c., and they are more or less satisfactory. The instruments of MM. Varey, Trouvé, Lippens, and de Courtois are the most interesting, and we will describe them. M. Varey’s microphone consists of a sounding box of deal, mounted in a vertical position on a stand, and two microphones are arranged on either side of it, with vertical carbons united for tension. A small Gaiffe cell of chloride of silver, without liquid, is applied to the standard of the instrument, and is enough to make it work perfectly. This system is extremely sensitive. M. Trouvé’s microphones, represented in figs. 46, 47, 48, are extremely simple, so that he is able to sell them at a very moderate price. They generally consist of a small vertical cylindrical box, as we see in the figure, with disks of carbon at its two ends, which are united by a carbon rod, or by a metallic tube tipped with carbon. This rod or tube turns freely in two cavities made in the carbons, and the box, while acting as a sounding box, becomes at the same time a prison for the insects whose movements and noises are the objects of study. [Illustration: FIG. 46.]