If a stone be thrown into a pool of still water, the motion of the stone causes a disturbance on the surface of the water. Circular waves radiate from the point at which the water was struck, diminishing in height until no longer visible. The movement of these waves is slow; the eye can easily follow them and count the number of waves per minute. Other waves in a more elastic medium than water are found to be much more rapid in movement. The striking of a bell causes it to vibrate, which vibration imparts wave motion to the surrounding air. Our ears are so constructed that this wave motion, if the rate be not less than 16 nor more than 44,000 per second, is transmitted through the tympanum and nerves of the ear, and we become sensible of it as Sound. Certain bodies are responsive to a particular rate of vibration. If a violin be played close to a wine-glass in exactly the same tone as the vibration rate of the wine-glass, the wave motion from the violin will set up a vibration in the glass, sometimes so violent as to cause the glass to break in pieces. Many interesting instances of this harmony of vibrating rate are recorded in the various textbooks on Physics.

Sound waves, while much more rapid than the water waves, are still comparatively slow when we consider the rapid vibrating motion of heat waves. The rapidity of these waves is beyond the ability of the mind to comprehend except by comparison. That degree of heat termed " bright red " requires the atoms of the body giving out this heat to vibrate at the rate of 400 billion times per second. It has been discovered that, under certain conditions, electrical waves radiate through space and have the power to influence suitable objects prepared for that purpose. The particular form of electrical wave under consideration is that known as Hertzian waves, so termed from the comprehensive discoveries of Dr. Heinrich Hertz, of Carlsruhe and Bonn. By means of a series of masterly experiments based upon certain phenomena previously discovered by other scientists, Dr. Hertz, between the years 1886 and 1891, added greatly to the knowledge of these electric waves and their effects on adjacent bodies, enabling them to be put to practical use in wireless telegraphy.

These Hertz waves do not have the extremely rapid vibratory rate of heat waves, though., as compared with sound waves, they are still very rapid , their vibrations being, as near as has yet been discovered, approximately 230 millions per second. These waves are set up by any sudden electric discharge, such as a lightning flash, or in a less degree by a spark from a sparking or induction coil or Leyden jar. They are made evident to our senses by suitable apparatus that, being adjusted to the same rate of vibration, receives the wave impulses and acts in unison with them. We may soon be able to learn of the approach of electric storms by means of instruments that will receive the electrical waves set up by the distant lightning flash.

The apparatus for demonstrating electric-wave action is simple and may easily be constructed at small'cost. Procure two sheets of heavy zinc 16" square, and mount them in a light wooden frame.

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Small picture-frame moulding makes a neat-looking frame. At the center of one edge of each plate (z) solder an L-shaped strip of zinc, the projecting piece being about 1/2"long, and having a 1/8" hole through it. To one end of two pieces of brass wire 4" long and 1/8" in diameter, fit brass balls (c) 1" in diameter. The other ends of the wire are then put through the holes in the zinc angle-piece, and when the plates are placed in line, the two balls will face each other. The plates should also be fitted with ebonite or glass feet, raising them 2 1/2" or 3" from the level. At the outside of one plate and in the lower outside corner of the other, bore small holes, and connect, by soldering, two pieces of insulated copper wire, size 16 or 18, which are to connect with the Leyden jar. This Oscillator, as Dr. Hertz named it, if placed on a stand with the plates in line and the balls from 1" to 1" apart, according to conditions, will, when connected to the outer and inner coatings of the charged Leyden jar (l), set up powerful electrical or Hertz waves in the surrounding medium at the instant the discharge takes place between the balls of the " oscillator " plates.

These waves are taken up and made evident by a simple form of receiver known as Hertz's Resonator. This consists of 1/4" brass rod 5 feet long bent into the shape of a nearly complete circle 18" in diameter. The unconnected ends are fitted with two 1" brass balls; the distance between them is adjusted by bending the rod. Wings of thin sheet copper 6" wide and 10" long are fastened to each side of the rod by twisting around the rod extension strips that were left* on the wings when they were cut out. In place of the brass balls the ends of the rod may be turned into two small circles, and soldered to make a perfect joint. The brass balls are the best, and should be polished with emery-cloth before trying experiments. The circular brass rod (d) is held suspended by two round pieces of wood 8" long and 1" thick, the lower ends of which rest in holes bored in the base (b). Two round-headed brass screws on each upright hold the brass rod in place, one screw on each side of the rod. It will add materially to the success of the experiment if one wing is connected by a piece of covered copper wire to a "ground." The nearest gas or water pipe will answer. The base is a heavy block of wood with wooden uprights, upon which to fasten the circular rod.

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The Leyden jar may be made from an ordinary quart glass milk or preserve jar, provided it is made of the right kind of glass : that is, a good insulator. To test this point, carefully clean and dry the bottle. When quite dry (it must also be cold), rub it briskly on the outside with a warm silk handkerchief. Reject any jar that does not quickly become charged so as to give a distinct spark. A sound wooden bung is then fitted to the mouth. The bung should be a new one, entirely free from acid or grease. A hole is bored through the center to admit a piece of brass rod about 1/8" in diameter and one-third longer than the bottle. The rod should fit very tight, and after putting it through the bung the top of the latter is given a liberal coating of red sealing-wax. The outside or top end of the rod is fitted with a brass ball 1" in diameter, and to the inside or lower end is soldered a piece of brass chain 3" long to aid in making a good contact between the inside coating of the jar and the rod. In place of the brass ball the rod may be turned to form a circle, but the joint must be carefully soldered and filed perfectly smooth.

The jar is then given the coating of tin foil, the inside being done first. The tin foil used should be heavy enough to withstand the work without tearing. A piece is first cut into a circle a trifle smaller than the outside diameter of the jar. Carefully cover one side with hot glue, and place it upon a dauber. This is made by wrapping a tuft of cotton wool.to one end of a small stick and covering with cloth. Holding the jar with the mouth down in one hand, press the tin foil, by means of the dauber, firmly up against the bottom of the jar; then turn the jar upright and finish pressing the tin foil smoothly into place. The side coating should cover three-quarters of the distance from the bottom to the bung and lap over the bottom layer slightly. Owing to the difficulty of handling one large piece, it may be cut into halves. One side of the tin foil is covered with glue, placed lengthwise on the dauber and holding the jar horizontally, inserted in the jar. A quick turn of the dauber will allow the foil to drop lightly against the inside of the jar, when it may be set in place and smoothed firmly against the glass. The other half is then placed in a similar manner.

The outside is then coated in the same way and to the same height as the inside. It will not be necessary to divide the outside coating. The coatings should get thoroughly dry before using the jar. The proper apparatus for charging the jar is a Wimshurst machine ; a description of the construction of one will appear in a future issue. In the absence of such a machine the jar may be charged from a rapidly moving leather belt, such as can be found in almost any factory. Hold the jar with one hand around the outside coating and the top of the brass rod about 1" away from the belt. A few minutes in this position will allow the jar to be fully charged. Do not try to discharge the jar by making a circuit with the hand, as a strongly charged jar will give a shock that would be far from comfortable.

The discharge of the Leyden jar is made by bringing the ends of one of the connecting wires to the outside coating and the end of the other wire to the terminal knob of the inside coating. A sharp discharge will then take place between the balls (c and c'), provided all the parts are in proper condition. Some adjusting may be necessary to determine this. The waves set up by these discharges, on impinging on the wings (w) of the Resonator (this being placed eight or ten feet from the oscillator), set up sympathetic surgings in the ring (d) and these overflow at the spark gap between the two balls (e). The walls of a room offer no obstruction to the passage of these waves, but another current of electricity in the path to be traversed will interfere with it. Much interesting instruction may be derived from experiments with this apparatus.

The great objection to electric motors - that they will not run far enough without recharging - is said to be overcome. It is recorded that recently in England a circuit of 94 miles was run without recharging. It was done with a battery of 42 four-plate cells, with a capacity of 180 ampere-hours. The carriage was a four-wheeled dogcart, with two motors of two and one-half horse-power each. In going down grade the motors were reversed, thus making dynamos for charging the accumulators. In this way the current was not only saved, but a new current actually generated, rendering the battery stronger at the bottom of the grade than it was at the top.