An Æolian Harp.—Make a shallow box of thin dry pine. The top piece should be free from knots and three-sixteenths of an inch thick. This is the sounding board. The sides and bottom of the box may be of wood one quarter of an inch in thickness. The harp should be two inches shorter than the width of the window in which you are going to place it. The width of the box itself may be ten inches, its depth two and a half inches. The ends should be of hard wood, for they have to bear the strain of the strings. In one end put studs or rings or eyes to which are fastened the wires or catgut strings. At the other end should be a corresponding row of violin pegs if you use catgut, or iron piano pins if you use wire. If you do use wire it should be of steel. In the diagram you will see the two bridges of hard wood glued diagonally across each end for the strings to rest upon. If steel wire is employed a piece of wire should run along the top of each bridge to prevent the other wires from cutting into the wood. Four holes, each an inch in diameter, in the sounding board improves the harp. The tuning may be harmonics, thirds, fifths, and octaves. Raise the sash of the window, and place the harp so that the wind blows across the strings.
CHAPTER XXII.
SCIENCE FOR THE PLAY-HOUR
A Home-Made Electrical Machine.—To make a really first-class machine of the modern type would require a good deal of mechanical skill, even supposing my readers to be the happy possessors of the necessary tools and materials; but the older type of machine—though of course not so powerful—will probably do quite well enough for most of their purposes.
I will, therefore, describe one of the simplest forms of these machines, such as any one, with a little care and patience, can make for himself.
The first thing to do is to get a general idea of what you are going to construct, which may be had from the illustration, and from the actual machines you may sometimes see in a shop window or in a scientific collection, like the Science Departments of the South Kensington Museum. It is the making of the cylinder machine we are going to work out, and, therefore, to begin with, the glass cylinder must be procured. This can be had from a dealer in chemical apparatus and costs only a few pence for the smaller size—about 3 inches by 6 inches. At the same time purchase a round glass rod, 3/8 inch diameter by 5 inches long; a sheet or two of tinfoil, and sixpennyworth of amalgam. From a carpenter or timber-merchant you will require a base-board for the machine, say 13 inches by 8 inches, by 1 inch thick, and of heavy wood; also two uprights, which are to stand on the base-board to support the cylinder. These may be 6 inches tall, by 2 inches by ¾ inch.
Having now the principal parts of the frame, the work of fitting together can be begun by making a circular hole (centre about 1¼ inches from the end) in one wooden upright, to take easily one of the projecting glass pieces, or pivots, at the ends of the cylinder—probably ¾ inch diameter will do. This hole may be made with a brace and suitable bit, or failing that, with a round chisel—taking care not to split the wood. In one end of the other upright cut a slot of same width as the hole, the bottoms of both being on the same level. Then rest the two glass pivots in the hole and slot, holding the uprights vertically on the base-board, when the cylinder should be quite horizontal. If it is not so, deepen the slot, or shorten either upright, as required. Drill a hole through the two sides of the slot at the top, and insert a round nail to keep the pivot from having too much play.
It will next be necessary to secure these supports to the board, which may be done by driving stout screws from below, together with the aid of some strong glue. If you have the skill it will be better to sink the supports ½ inch into the surface.
The position should be such, that the cylinder is not quite over the middle of the board. (See illustrations.) Next remove the cylinder by a little side working, and screw a piece of wood, 1½ inches by ¼ inch by about 7 inches, to the supports and base. This is to act as a brace to the supports, and also for holding tightening screws for the rubber.