joints, so as to bring their buttons as near as may be desired to any body that is placed upon the stand, M.

To melt a wire by electricity, we attach it to the two inner buttons at i, then connect one of the wires, A, for example, with the exterior coating of the battery, and complete the circuit by connecting B with the button of one of the jars of the battery.

This is

effected in the manner shown in the figure, the connecting chain being managed by means of a hook with a glass handle. At the instant of contact, the wire, if fine enough, is melted into globules. and even volatilized, that is, reduced to vapor, which disappears in

the air.

When the wire is a little larger, it simply becomes red hot and gives forth a brilliant light; if still larger, it becomes heated without being luminous. Fine and short wires may be melted under

water in the same manner as in air, but the experiment is more difficult to make.

Mechanical Effects of Electricity.

396. The MECHANICAL EFFECTS OF ELECTRICITY are manifested when large charges of electricity are passed through imperfect conductors. They consist of violent expansions, with tearing, fracturing, and the like.

These effects are generally exhibited by placing the body upon the plate, M, of the universal discharger (Fig. 278), and then passing a

Fig. 279.

powerful charge from a battery through it. In this way a small block of wood may be torn to splinters in an instant.

Fig. 279 represents an apparatus by means of which a hole may be torn in a card by using a single Leyden jar. A carù is placed at the top of a glass cylinder, beneath which is a wire projecting from

(396.) What are some of the mechanical effects of electricity? How exhibited! Explain the method of perforating a card by electricity.

a metallic plate. The plate connects by a chain with the exterior coating of the jar. Above the card is a second wire which is insulated in the manner shown in the figure. When the circuit is completed, by touching the upper wire with the button of the jar, a shock follows, and the card is found to have been pierced as if run through by a needle or pin.

To pierce a plate of glass requires a large battery. The battery belonging to Harlem Museum (Art. 394), is capable of piercing a book of four hundred pages.

A partial account of the chemical effects of electricity has been given in speaking of the electrical cannon. More on this subject will be given when we come to treat of the effects of the Voltaic pile.

Identity of Lightning and the Electrical Spark.

397. FRANKLIN published a memoir in 1749, showing the complete parallelism between the electricity of the clouds and that of the electrical machine. In that memoir he suggested that the electricity of the clouds might be attracted to the earth by means of points, and recommended that the experiment should be made.

In accordance with that suggestion, the experiment was first made by DALIBARD, in May, 1752. He erected in his garden a rod of iron about forty feet high, having its upper extremity terminating in a point. After the passage of a thunder cloud, the rod was found to be electrified, and for the space of fifteen minutes sparks were drawn from it, which were used in charging several Leyden jars.

About a month later, FRANKLIN, without any knowledge of the discovery of DALIBARD, succeeded in attracting electricity from a cloud to the earth. He raised a silken kite, just before a coming thunder storm. The string of

Of piercing a plate of glass. (397.) Who first showed the identity of lightning and electricity? Explain DALIBARD's experiments. Explain FRANKLIN's experiments.

the kite was of hemp; attached to the lower end of it was a small key, and fastened to the key was a silken cord, by which the kite might be insulated. It was only after the string became damp from the falling rain that the key showed signs of being electrified. He was at last rewarded by obtaining an electric spark. So great was his joy that he could not refrain from bursting into tears.

The complete identity between lightning and the electric spark was thus established, and all, even DALIBARD himself, unite in attributing to FRANKLIN the honor of the discovery.

Atmospheric Electricity.

398. The existence of atmospheric electricity is not confined to clouds alone, for it often exists in the atmosphere when no trace of a cloud is visible. In this case the electricity is positive. It is most abundant in open spaces and at considerable elevations. In houses, in the streets, under trees, and in sheltered localities, no trace of free electricity is discoverable. During storms the electricity of the air is sometimes positive and sometimes negative. All clouds are supposed to be electrified, some positively and some negatively.

The electrical condition of clouds may be determined by metallic rods, by kites, or by small balloons held by a string in the hand.

The electrical state of the atmosphere may be determined in a great variety of ways. M. BECQUEREL employed for this purpose the gold-leaf electrometer shown in Fig. 259. Instead of the button he used a stem of metal, attaching to its upper end a fine and flexible wire. To the second extremity of the wire he fastened an arrow,

(398.) What is the nature of the electricity of the air? Where is it most abundant? What is the state of the atmosphere during storms? How is the electrical condition of the clouds determined? How is the electrical state of the atmosphere determined?

which, being shot from a bow, ascended into the atmosphere, drawing the wire with it. When the arrow was shot directly upwards, the divergence of the gold leaves indicated the existence of free electricity, and the nature of this electricity was tested as already explained.

Lightning and Thunder.

399. LIGHTNING is nothing else than an elongated electrical spark, which passes between two differently electrified clouds when brought near each other. Sometimes a discharge takes place between a cloud and the earth; this is called a thunderbolt.

A flash of lightning is often of great length, and as it takes place along the line of least resistance, it generally follows a zig-zag path, as is often the case with the spark from a Leyden jar. When a flash of lightning is seen in the lower regions of the atmosphere, it has a brilliant white color; but in the higher regions, where the air is rarefied, it assumes a violet hue, similar to that of the electric egg (Art. 382).

THUNDER is the sound which follows a flash of lightning. It is due to vibrations caused by the passage of the spark through the air.

Thunder is not heard till an appreciable time after the flash is perceived. This arises from the fact that light travels with immense velocity, reaching the eye instantaneously, whilst sound travels more slowly, and reaches the ear only after a sensible interval of time. The distance of a clap of thunder may be ascertained by counting the number of seconds between the flash and the report, and allowing five seconds to a mile.

The intensity of the sound diminishes as the distance becomes greater near by, it is sharp and rattling, like boards falling one

(399.) What is Lightning? What is a thunderbolt? Why is the flash often zig. zag in its shape? What is the color of the flash? What is Thunder? Why is the thunder only heard after an appreciable time? How may the distance of the flash be determined? What effect has distance on the sound of thunder?