the electricity of the same name and attracting that of an opposite name. As soon as the tension of the two electricities becomes greater than the resistance of the intervening air, a spark or flash passes, and a thunderbolt is said to full, or the lightning to strike. The flash generally passes from the cloud to the earth, but sometimes the reverse is the case. When the lightning ascends, the clouds are probably negatively and the earth positively electrified, since it has been shown by experiment that at the ordinary pressure positive electricity passes through the air more easily than negative.

The attraction between the two electricities increases as the distance diminishes. Hence it is that clevated objects are most likely to be struck, such as spires, high trees, lofty buildings, and the like. Good conductors, like metals, moist bodies, trees, and the like, are more likely to be struck than bad conductors. Hence the danger of taking refuge under a tree in a thunder-storm.

The effects of the thunderbolt are extremely various and wonderful. It crushes or fractures bad conductors, inflames combustible bodies, melts metals, reverses the poles of magnets, and often kills men and animals. Sometimes it falls slowly in the form of a globe of fire, and then explodes with a noise like a battery of cannon. It is this form of lightning that is most likely to inflame the edifices which it chances to strike.

528. The Return Shock is a violent, and sometimes fatal shock, felt by men and animals even at a great distance from the place where the lightning strikes.

This phenomenon is due to the inductive influence exerted by an electrified cloud upon bodies beneath it, which are all strongly charged with electricity contrary to that of the cloud. Now, if a discharge takes place at any point, the cloud returns to its neutral state, induction ceases instantly, and all of the bodies electrified by induction instantly return to a neutral state. The suddenness of this return is what constitutes the return shock.

The return shock may be illustrated on a small scale by placing a living frog near an electrical machine in motion. Every time that the machine is discharged by placing the finger upon it, the frog experiences a shock, which is nothing else than the return shock above described.

529. Lightning-Rods. - A LIGHTNING-ROD is a rod of metal, placed upon a building or ship to preserve it from the effect of lightning. Galvanized iron or copper is now generally used.

A lightning-rod should fulfil the following conditions:

1. It should be of sufficient size so as not to be melted while carrying the charge off.

A copper rod of half an inch in diameter, or an iron one of three fourths of an inch in diameter, is large enough to protect any building.

2. They should be of one piece throughout.

3. They should terminate in points to give readier egress for the electricity that is set free by induction.

4. The rod should be carried down into the earth till it meets with a good conducting medium, such as a layer of wet or moist earth.

When no such medium can be reached, a pit should be dug, and after the lower end of the rod has been carried to the bottom, it should be nearly filled with some good conductor, as coke. This will also prevent rusting.

A rod is supposed to protect a circular space about it, whose radius is about twice the length of that portion of the rod that extends above the building. The lightning-rod was invented by FRANKLIN, who thought that its protective action consisted in drawing off the electricity from the cloud, and conducting it to the earth.

The real explanation of its utility is just the reverse. The cloud acts by induction upon the earth, repelling the electricity of the same name as that in the cloud, and attracting that of an opposite name, which accumulates upon the bodies under the cloud. Now, by arming a body with metallic points communicating with the earth, we permit a passage of electricity from the earth to the cloud. This not only prevents the accumulation of electricity upon the body, but it tends gradually to neutralize the electricity of the cloud itself, and thus the rod acts in a double way to prevent the body from being struck.

When the electricity set free is more than the conductor can discharge the lightning strikes, but the rod receives the discharge, owing to its higher conducting power, and protects the building.

530. The Aurora Borealis. The AURORA is a luminous phenomenon, which appears most frequently about the poles of the earth, and more particularly about the boreal or northern pole, whence its name.

At the close of twilight a vague and dim light appears in the horizon in the direction of the magnetic meridian. This light gradually assumes the form of an arch of a pale yellowish color, having its concave side turned towards the earth. From this arch streams of

light shoot forth, passing from yellow to pale green, and then to the most brilliant violet purple. These rays or streams of light generally converge to that point of the heavens which is indicated by the dipping needle, and they then appear to form a fragment of an immense cupola, as shown in Fig. 364.

Since the aurora is always accompanied by a disturbance of the magnetic needle, and is generally arranged in the direction of the dip, and acts upon telegraph wires, it is inferred that it is due to electrical action. Such is at present the generally received belief.

Summary.

Identity of Lightning and the Electric Spark.

Discovered by Dr. Franklin.

Method of its Discovery.

Atmospheric Electricity.

Found in the Clouds and in the Atmosphere when free from Clouds.

Method of determining the Electrical Condition of Clouds. Causes of Atmospheric Electricity.

Friction of the Air.

Evaporation and Condensation of Water.

Vegetation.
Combustion.

Lightning.

Definition.

Different Kinds.

Thunder.

Definition.

Method of ascertaining the Distance of Thunder.

Rolling of Thunder explained.

Effects of Lightning.

Why Lightning strikes.

Examples of the Destructive Effects of Lightning.

The Return Shock.

Definition and Cause.

Experiment with Frog.

Lightning-Rods.

Definition.

Conditions of a Good Rod.

Explanation of the Action of a Lightning-Rod.

The Aurora Borealis.

Definition.

Illustrated by Figure.

531. Galvani's Experiment. It has been observed that chemical combinations are sources of electricity. The form of electricity thus developed is different, but its nature is the same as that produced by friction; but inasmuch as its manifestations of power are continuous from the very moment of its production, it is called dynamical electricity, the word dynamical being derived from a Greek word meaning power. The name of GALVANISM has been given to electricity developed by certain chemical combinations, in honor of GALVANI, who first discovered this new way of generating it. It is also called Voltaic electricity, from VOLTA, who added to the discoveries of GALVANI.

GALVANI observed one day that a dead frog, which was suspended from a copper hook in a window, exhibited muscular contractions whenever the wind blew the lower extremities against the iron bars of the window. Here was a case of electrical manifestation which

Fig. 365.

furnished a clew to one of the most important discoveries of modern science.

This discovery led to an experiment which may be repeated as follows: Having killed a frog and cut off the hinder half of the body, we suspend it by a copper hook, c, passed between the backbone and the nerves which run on each side of it, as shown in Fig. 365; then holding a small plate of zinc, z, in the hand, we bring one end of it in contact with the copper stem that holds the hook, and