cape too fast from the cloud, and never collect in sufficient strength.

336. Thunder-storms are usually attended by a change in the direction of the wind, which accounts for the condensation of atmospheric vapor; indeed, one of the most sublime elements of a storm of this nature, is the conflict and raging of opposing currents.

In the Meteorological Register of Yale College are recorded 116 thunder-storms, which occurred between 1804 and 1823. Of this number, ninety-nine were either preceded or followed by an alteration in the direction of the wind; the change in fifty instances being from a south-westerly breeze to a north-westerly.

Since the air abounds with vapor when its temperature is high, the condensation will be most copious if a loss of heat then suddenly takes place. We therefore easily perceive the reason, why thunder-storms are more frequent in summer than in winter, in low than in high latitudes, and their intensity greatest in the tropic climes.

For the same reason they happen more frequently after mid-day than in the morning.

337. ELECTRICAL STATE OF THUNder-clouds. Since the air surrounding it is a non-conductor, a single thunder-cloud floats in the atmosphere a vast insulated conductor (C. 963); its electricity being spread over the surface of the globules of which it is composed, and there retained by the pressure of the atmosphere.

338. Thunder-clouds may be either positively or negatively electrified; and the observations of Mr. Crosse lead to the conclusion, that at times a cloud of this kind is complex, consisting of a series of concentric bands or zones, alternately positive and negative; the electricity being weakest at the edges of the cloud, and strongest at the centre.

How is this condensation effected?

What fact is stated in proof?

Explain the cause of the differences that exist in the frequency and vio

lence of thunder-storms.

What is the electric state of a single thunder-cloud?

State Mr. Crosse's opinion.

Thus in figure 18., which represents a section of such a cloud obliquely seen, P P' P", &c., are positive zones, N N' N", &c., negative, and the number of dashes show the increase of intensity.

339. ELECTRIC ACTION OF THUNDER-CLOUDS. The earth may be regarded as a reservoir of electricity when, therefore, an electrified cloud floats near its surface, it induces the opposite electricity upon the ground immediately beneath it.

Fig. 18.

b

The cloud may approach so near, that the mutual at traction of the two electricities overcomes the pressure of the atmosphere; a union then occurs, and the lightning, at the same moment, is seen darting between the cloud and the earth, and soon after the rolling of thunder is heard.

340. A similar inductive action arises between the clouds themselves; for, if two clouds differently electrified approach each other, the electricity upon the nearest opposite surfaces augments in intensity, and often increases to such a degree that a discharge takes place, the lightning then flashing from cloud to cloud. It may sometimes happen, that the path of least resistance will not be directly through the air, but from the first cloud. to the earth, and from the earth to the second cloud, and under these circumstances the lightning will take the latter route.

341. RETURN-STROKE. When a highly charged thunder-cloud approaches the earth, it induces, as already stated, the opposite kind of electricity upon the ground below, and repels that of the same kind. Should

Describe the electric action of thunder-clouds.

When does a flash occur?

What is the influence of one cloud upon another?

Why does the lightning in passing from cloud to cloud sometimes take the earth in its course? What is the return-stroke?

the cloud be extended, and come within striking distance, either of the earth or of another cloud, a flash at one extremnity is often followed by a flash at the other. This is called the return-stroke, which sometimes occurs with such violence as to destroy life, even at the distance of several miles from the place of the first discharge. The mode of action may be explained by means of the following figure.

been explained, which happened on the 10th of July, 1785, in the vicinity of Coldstream, in Berwickshire. After a fine morning, clouds were seen in the northwest by the observer Brydone, at about eleven o'clock. Between twelve and one o'clock, the storm being still distant, lightnings were seen darting from cloud to cloud, followed by thunders. Immediately after, Brydone was startled by several loud explosions near his house, like the reports of a gun. At this moment two carts loaded with coals were passing by. The driver and horses of the first were instantly killed, and the coal scattered in all directions, while the driver of the second wagon, which was about twenty yards behind, neither perceived any lightning nor experienced any shock. Upon examination, the hair on the legs and bellies of the horses was found to be singed, and where the wheels rested at the time of the explosions, the tire was melted, and two round holes were discovered in the ground. A quarter of an hour before this event, and at a spot nearly a mile and three-quarters distant, a shepherd of the name of Bell perceived a lamb suddenly fall, while a flame passed before his face. Upon raising the lamb he found it to be dead. A woman, who was cutting grass upon the bank of the Tweed, felt a violent shock upon the soles of her feet, and was thrown to the ground.

During a storm which happened near Manchester, in June, 1835, loud discharges were heard at different points of a road, like the reports of a pistol, and electric flashes distinctly seen; a person is said to have been killed at this time, by an explosion under his right foot.

344. HEIGHT OF THUNDER-STORMS. Though thunder-storms prevail in the lower regions of the atmosphere, they have often been seen at a very great altitude. A storm, observed by Kaemtz, amid the mountains of Switzerland, rose to the height of more than 10,000 feet, and the dwellers in the vale of Chamouni assured him, that storms frequently swept over the

What is said respecting the height of thunder-storms?

summit of Mont Blanc. On the peaks of the Cordilleras, a violent thunder-storm was encountered by La Condamine and Boguer, at an elevation of even 16,000 feet. Vitrified rocks have at times been discovered at lofty heights, and as this change is supposed by some to have been effected by lightning, they have sought to determine the altitude of thunder-storms from facts of this kind. The reasoning, however, is inconclusive, for these vitrifications may be owing to other causes, and were we even to grant that they are produced by lightning, the case is by no means proved; since a flash sometimes passes between the clouds and the earth, when the former are below the point that is struck.

Thus, on the first of May, 1800, a church situated on Mount St. Ursula, a lofty peak in Styria, was struck; and seven persons were killed by a flash of lightning darting upwards from a thunder-storm below.

345. From the observations of Peytier and Hossard among the Pyrenees, it appears, that the upper and lower surfaces of thunder-clouds bear no resemblance to each other, for while the latter are perfectly level, the former are broken and uneven, presenting the appearance of mountains and ridges; whence, during seasons of great heat, lofty peaks and pinnacles of clouds shoot far up into the sky.

LIGHTNING.

346. ORIGIN. When a portion of air is subjected to a very sudden and powerful compression, a spark is elicited (Art. 551): that electricity produces such a compression can be proved by experiment, and to the energetic condensation of the atmosphere before the electric fluid, in its rapid progress from point to point, is attributed the vivid flashes that illumine the stormy sky.

347. KINDS. Lightning has been divided by Arago into three kinds, principally distinguished by their form,

What did Peytier and Hossard observe?

What is the cause of lightning?

Into how many kinds has it been divided by Arago?