The sounds caused by a fly walking on a microphone may be made to sound as loud as the tramp of a horse. [Illustration: Fig. 144.—A Very Sensitive Form of Microphone, with which the Footsteps of a Fly can be heard.] The electrical _stethoscopes_ used by physicians to listen to the action of the heart are in principle only a microphone and telephone receiver connected to a battery. The drawing in Figure 144 illustrates a very sensitive microphone that is quite easy to make. With this instrument it is possible to hear the tramping of a fly’s feet or the noise of its wings. The base upon which the apparatus is mounted serves as the sounding-board and is made in the form of a hollow wooden box. It can be made from an ordinary cigar-box by removing the paper and taking the box apart. The piece forming the top of the box must be planed down until it is only three thirty-seconds of an inch thick. The box should measure about five inches square and three-quarters of an inch thick when finished. Do not use any nails or small brads whatsoever in its construction, but fasten it together with glue. If you use any nails you will decrease the sensitiveness of the instrument quite appreciably. The bottom of the box should be left open. The result is a sounding-board of the same principles as that of the banjo head. Small feet, one-quarter of an inch square, are glued to the four under corners so as to raise the bottom clear of the table, or whatever the microphone may be placed upon. The bottom of each one of the small feet is cushioned with a layer of felt so that no jars will be transmitted to the instrument by any object upon which it is resting. The carbon pencil used on this type of instrument is pivoted in the center and rests at one end upon a carbon block. The carbon block is made about one inch long, one-quarter of an inch thick, and one-half of an inch wide. A small hole is drilled near each end to receive a screw which fastens the block to the sounding-board. A fine wire is led from one of these screws to a binding-post mounted at the side of the box. Another wire leads from a second binding-post to a standard which is also fastened to the sounding-board with a small screw. The standard is made from a sheet of thin brass and is bent into the shape shown in the illustration. The pencil is a piece of one-quarter-inch carbon rod, two and three-quarter inches long. A small hole is drilled one and five-eighths of an inch from one end with a sewing-needle, and a piece of fine brass wire, pointed at both ends, pushed in. The wire should be a tight fit in the hole. It should be about one-half of an inch long, and may be made from an ordinary pin.