about a central nucleus of compact snow. They are called hailstones. Hailstones sometimes are very large, being not infrequently as large as a pigeon's egg, and it is said they sometimes weigh several ounces.

A fall of hail is often preceded by a noise like that of rattling nuts in a bag. This noise is attributed to collisions between the hailstones. A hailstorm is always accompanied by electrical phenomena, and thunder generally precedes or accompanies the fall of hail. From this circumstance it is inferred that hailstorms are in some way due to electrical action. As yet no satisfactory theory has been advanced to account for the formation of hailstones, and especially those enormous ones that are sometimes seen.

VOLTA supposed them to be formed between two clouds oppositely electrified, and that they were alternately repelled from one to the other, like electrical puppets, during which time they were continually increasing in size by congealing the moisture of the clouds upon their surface, till at last they became heavy enough to break through the lower cloud and descend to the earth. This theory is now rejected.

The Tornado.

407. A TORNADO is a violent whirlwind, attended with rain, thunder, and lightning. Tornados often travel considerable distances, overturning buildings and uprooting trees; they are accompanied with a noise like that of heavily-loaded carts driven over a stony road. The flashes of lightning and balls of electrical fire that accompany tornados, indicate their electrical origin.

Two species of tornado are recognized: terrestrial and marine. according as they take place on land or on water. The latter class present remarkable phenomena. The rotary force of the wind raises the water in the form of a cone, whilst a second cone forms in the cloud, having its apex downwards. These cones move to meet each other, forming a column of water reaching from the ocean to the

Describe a hailstone. Explain the rattling sound preceding a hailstorm. What was VOLTA's theory of the formation of hail? (407.) What is a Tornado? Why is it regarded as of electrical origin? How many species of tornados?

cloud. In this form the column of fluid is called a water-spout. When a water-spout strikes a ship, it does immense damage.

The Aurora Borealis.

408. 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. 282.

Since the aurora is always accompanied by a disturbance of the magnetic needle, and is generally arranged in the direction of the dip, and because the chemical action of electricity is accompanied by precisely analogous phenomena, it is inferred that it is due to electrical action. Such is at present the generally received belief.

Why is it regarded as of electrical origin?

409. 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. 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.

In 1790, GALVANI observed that the body of a frog recently killed, when placed near an electrical machine, manifested signs of excitation whenever sparks were drawn from it. The cause of action was, in fact, the return shock, as has been explained; but GALVANI, ignorant of this fact, began to seek for an explanation of the phenomena. One day he saw a dead frog suspended from a copper hook in a window, and noticed a muscular contraction whenever the wind blew the lower extremities against the iron bars of the window. Here was a case of electrical manifestation which was entirely independent of any electrical machine, and it furnished a clew to one of the most important discoveries in 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 back

(409.) What is Galvanism? Why so called? Explain the method of its discovery. How may GALVANI's experiment be repeated?

bone and the nerves which run on each side of it, as shown in

Fig. 283; 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 then touch the legs of the frog with the other end. At every contact the muscles contract, reproducing all the motions of life.

GALVANI attributed the phenomena observed, to the electricity existing in animal tissues, which, passing from the nerves to the muscles, through the metals, produced the muscular contractions.

410. VOLTA repeated the experiment of GALVANI, and after much study, advanced the theory of contact. According to this theory, when two metals or other dissimilar substances are simply brought in contact, there is always a decomposition of the natural electricity of both bodies, the positive fluid going to one and the negative fluid to the other.

In the case of the frog, the electricity was supposed to be developed by the contact of the copper hook and zinc plate, the nerves and muscles serving simply as conductors.

VOLTA called the force which separates the two electricities in cases of contact, the electro-motive force, which he supposed to act

To what did GALVANI attribute the phenomena observed? (410.) What was VOLTA's theory? What is the electro-motive force?