One

considerable time elapsed without any result. very promising cloud passed over his head without producing any effect upon his simple apparatus, when, being about to give up the attempt in despair, he observed, after a sharp shower, some loose threads of the string to repel one another; on this he fastened a key to the string, and was gratified by drawing an electric spark from it. He afterward raised an insulated metallic rod from the end of the house, so arranged as to communicate with two bells by means of a pendulum, which, striking against them as they were alternately attracted and repelled, warned him of the passage of an electric cloud.(51.)

The experiment with the kite has often since been repeated, with the addition of a thin copper wire twisted with the string; but it is not unaccompanied with danger, for experimenters have frequently received violent shocks, and the electricity has been known to discharge itself to the ground in sparks ten feet long and two or three inches in diameter. A fatal catastrophe from incautious experiments upon atmospheric_electricity occurred to Professor Richman, of St. Petersburg, in 1753. He had erected an apparatus in the air, and was examining it with a friend, when a flash of lightning passed from the insulating rod through his body, and instant death was the consequence. His com

(51) The chime of bells here represented is suspended to the electrified body by the metallic hook, a. The two exterior bells, b b, are in metallic communication with it, and the centre one is insulated by a silk thread, but it is in metallic communication with the ground by the chain, c. The metallic clappers, c c, are suspended by silk threads; and, when the exterior bells are charged, they are alternately attracted and repelled, and discharge the elecbtricity by convection to the centre conducting

bell.

panion was at the same time struck senseless to the ground.

240. The snapping noise which the electric spark makes in passing through a portion of the atmosphere, is due to the sudden compression of the air; and there can be no doubt that the awful thunder-clap is produced by the same action. The report is in this extreme instance modified by a variety of circumstances, such as distance, echo, &c.; and the sudden dying away and return of the sound may be accounted for on well-known principles.

We have already stated (§ 12) that sound travels in air with a velocity of only 1130 feet in a second, but light at the rate of 195,000 miles in the same period of time (§ 365). The time in which the flash of lightning reaches us, from the different points of its course, may therefore be taken as instantaneous; but the time which the explosion occupies will be very appreciable, and will vary with the distance of the several parts of the long line which the discharge traverses. By a calculation founded upon the interval between the flash and the sound, and the duration of the thunder-clap, it has been found that a flash of lightning frequently traverses a space of nine or ten miles; and, when we take into account the zigzag path which it ordinarily follows, its alternate approach and recession will account for the phenomenon in question. Such would be the effect produced upon an observer placed at the end of a long file of soldiers, who were to discharge their muskets at the same moment. He would not hear a single report, but a succession of reports, which would produce an irregular rolling sound. (52) § 241. If a house stand in the way of an electric discharge, that is to say, if it form by induction from the charged clouds a part of that line of particles

(52) The usual zigzag path of a flash of lightning is represented on the following page. The sound will reach the ob

which have attained the highest degree of tension, the course which the lightning will take will be determined by the accidental position of different conducting bodies within it, which will change the line

servers, placed at a and a, from the different points, in very unequal times, and will sometimes recede and then approach alter

a

nately, as it reaches them from the farther or nearer portions of the atmosphere in which the concussion is produced.

Sometimes the discharge follows a path approaching to a seg ment of a circle; and then, if the observer be placed in a situation nearly equidistant from every part of the flash, it will affect him with a single crash.

of greatest tension according to their relative situations. Thus it is probable that it may descend a chimney in which the air is rarefied; or it may strike the same object from being the most elevated conducting body in its course. It may then leap to different metallic articles in the chambers, or even to persons whose good conducting properties may present an easy transit in the position in which they may be placed; it will finally pass to the earth, after having thus followed the path which had been previously determined by the arrangement of the particles under the highest state of inductive influence. As long as the discharge is confined to the good conducting substance of metals, lightning produces no injurious effects; but, whenever it passes from these into imperfect conductors, its course is marked by destruction,

§ 242. To Dr. Franklin belongs the honour of having suggested the application of the known principles of electrical science to the defence of buildings against the dangers of a thunder-storm. He proposed the erection of a continuous metallic rod in perfect communication with the earth, by the side of any building intended to be secured against the effects of lightning; and experience has fully demonstrated the utility of this precaution. The great points to be attended to in the construction of such conductors are, that they should be carried above the highest point of the building to be protected; that they should be of sufficient substance not to be melted; and the experience of a century has proved that a copper rod of half an inch diameter has never yet been fused; that there should be good metallic contact in all their parts; and that they should terminate at a sufficient depth in the ground, and be led, if possible, into some piece of moist earth or large body of water. It has been proved that conductors erected with these precautions will protect a circular space of a radius double their height above

the highest point of the building to which they are attached.

§ 243. An objection, founded in ignorance, has sometimes been made to lightning-rods, that they may, by their attractive power, invite a discharge where otherwise it would not have taken place. But when it is considered that one of the conditions of a thunder-storm is an intense electrical induction of a portion of the earth's surface through a dry stratum of air to the surface of a cloud of some thousands of acres in extent, in the manner of a Leyden jar, it will be perceived that any such elevation, as compared with the distance and extent of the charged clouds, is utterly inconsiderable, and perfectly inadequate to influence the charge in the slightest degree. The conductor is perfectly passive, and its efficacy consists in opening an easy path by which the force may be transmitted. Its action is at best but of a negative kind, and it can no more be said to attract the lightning than a water-course can be said to attract the water which necessarily flows through it at the time of heavy rain.

It is true that a pointed conductor will silently draw off a considerable portion of the electric charge from a distant cloud, and this action will sometimes be indicated by a glow, or even brush, upon its extremity; but this would tend to diminish the state of tension of the charged masses; and if inadequate to produce an appreciable diminution of the enormous quantities collected, can have no influence in exalting the tension to the point of disruption.

§ 244. The beautiful meteor called the aurora borealis, there can be little doubt, is another form of electrical discharge taking place in the upper regions of the atmosphere, where the decreased density of the air admits of those brushes and coruscations which can be so well imitated in the receiver of the airpump. Luminous brushes, aigrettes, and glows have not unfrequently been observed also, previous