the case of thunder is due to the fact that the points along the line of the flash are at unequal distances from the observer, a separate sound being produced at each point. The sound from the nearest point first reaches the ear, and then the sounds from points more and more remote in order. If the relation of the line of flash to the observer be such that two or more points are equally distant, the sounds arising from these points blending together at the ear, will give a result proportionally louder, and thus in nearly every peal of thunder maximum and minimum sounds may be observed. Generally after a low rumbling sound for five or ten seconds, a loud crashing sound occurs which continues from five to twenty seconds, when it is succeeded by the rumbling noise again which gradually dies away. Several maxima and minima of sound may occur during the same thunder peal. The rolling of thunder is also undoubtedly due in a considerable degree to the effect of echoes. A cannon fired under a clear sky gives a short, sharp cracking sound, but when fired beneath clouds. from which reflection can take place, the sound becomes prolonged and rolling.

To consider in detail the questions: Does lightning ever occur without thunder and with a clear sky? Is thunder ever heard in perfectly clear weather? Does lightning take place without thunder, with a cloudy sky? Does thunder ever occur without lightning? would be foreign to our present purpose. It may be remarked however, that seemingly well authenticated examples are on record which would give an affirmative answer to each of the above inquiries. Let us now turn our attention from these general points in regard to lightning and thunder to those in which we have a more specific interest.

Other circumstances being equal lightning strikes the most elevated points. Particular instances may occur which seem to be in opposition to this rule-instances in which the causes remain concealed in the masonry of buildings or beneath the surface of the earth. Yet, no one who has compared in a given locality the strokes of church spires and tall trees with those of lower surrounding objects will question the truth of the above statement.

Lightning seeks by preference metallic bodies, whether external or concealed, and it is in making its way to them, or at the moment of quitting them that it does its principal damage. Of all the properties of lightning these are the most important to us

as regards results. From numerous illustrations I select the following instructive example furnished by the stroke of an immense timber spire or tower in Newbury, (Vt.): "Lightning fell upon the upper part of the tower; it shattered and threw to a distance a timber pyramid seventy feet high. After it had overthrown this heavy mass it found in its path a metallic wire which connected the clapper of the bell with the wheelwork of the clock, twenty feet lower down, and threw itself entirely or almost entirely upon this wire, which it melted in some parts. For this length of twenty feet the surrounding timber of the tower suffered absolutely no injury, although the lightning was far from having exhausted itself upon the upper pyramid. As soon as it had reached the lower end of the wire, it threw itself afresh on the timber of the tower and injured it considerably. On reaching the ground, its force was still such, that it tore up several stones from the foundations of the building, and projected them to some distance." So long as the wire was followed, no injury resulted; when that was abandoned, destruction ensued.

Still another example, not less pertinent, is adduced. On the 15th of March, 1773, lightning fell at Naples on the house of Lord Tylney. His apartments at the time contained not less then five hundred persons attending a grand reception. Yet not one of them sustained actual injury. Saussure the next day examined the rooms and found that almost all the gilt parts had been affected. "The gilt mouldings and cornices of the ceilings, metallic rods placed so as to protect the tapestries from the contact of furniture, the gilt portions of sofas and arm-chairs in contact with those rods, the gildings of the door-posts, and lastly the bell wires had all suffered more or less by fusion, discoloration or scaling off of the surface. As usual, the maximum of effect had taken place where the lightning in its course had met with interruptions in metallic continuity. A stroke of lightning capable of melting a bell wire is strong enough to kill a man; yet here, as has been said, no one was hurt. We have thus a sufficient proof that the fulminating matter or lightning, in passing through the nine rooms which formed the suite of apartments, directed itself by preference, or almost in totality to the metallic substances found in the different rooms."

From references already made a well known fact appears, viz. : that lightning often fuses pieces of metal which are struck by it. The important fact, however, for our present purpose, is to de

termine the largest diameter or thickness of metal, that has ever been melted by it. The loose expression of ancient writers, that "the sword is liquified in the scabbard," is not sufficiently definite for modern science. What are the actual dimensions of metals actually fused by lightning? From nearly a score of examples, I find that a copper rod two-tenths of an inch in diameter has been thus melted; that Franklin in 1787, found that a stroke of lightning had melted, at his own house in Philadelphia, a conical copper rod 9 inches long, and rather more then threetenths of an inch thick at its base; that "a thunderstroke may fuse completely and throughout its extent an iron chain of 130 feet in length, the diameter of the iron forming the links being .23 of an inch, and a conical iron rod .43 of an inch thick at its basethat an iron rod .47 of an inch in diameter was broken by a heavy stroke of lightning, but showed no trace whatever of fusion. From such data a tolerably correct idea can be formed of the necessary magnitude of metallic rods to convey away without harm any probable charge of the electric fluid. A stroke not sufficiently powerful to fuse small rods or wires may have the effect of shortening them. A metallic wire 16 feet in length has been thus contracted between two and three inches. Wires stretched between fixed points are thus often broken by lightning strokes. (Is it not possible that the contraction results from a lateral escape of the electric fluid, giving rise to tension in that direction, just as a rope is shortened when from any cause its diameter is increased?)

Lightning frequently fuses and instantly vitrifies certain earthy substances. Lightning tubes or fulgurites (as they are termed) are produced when lightning descends into sandy soils, the path of the lightning being marked by a tube of vitrified sand. These tubes are sometimes three inches in external diameter, and have been known to exceed thirty-three feet in length. They generally descend vertically into the sand, but are often found inclined to the horizon at an angle of 40 degrees. They contract in descending and often terminate in a point; sometimes, however, the principal tube divides into two or three branches, each with smaller lateral branches from an inch to a foot in length. These branches are conical and all terminate in points. The sides of the tubes vary from two-hundredths of an inch to an inch in thickness. The inside part of lightning tubes is smooth and bright. It scratches glass and strikes fire as a flint.

A singular freak of lightning is that of piercing bodies with several holes in opposite directions. Of several recorded instances the following strikingly presents this remarkable characteristic In 1777, lightning struck a church in Cremona, breaking the iron cross on its summit, and throwing to a distance the weathercock which had been placed immediately below the cross. This weathercock was made of tinned copper and covered with a coat of oil paint, and "was pierced by eighteen holes ;" the edges of nine of these holes stood out prominently on one of the faces of the weathercock, and the edges of the other nine holes were equally prominent on the other side. There were no indications which led the inhabitants of Cremona to suppose that the weathercock had received several strokes of lightning. It were remarkable indeed that the strokes should be in pairs; nine on each side, and in essentially parallel lines, as the nearly identical inclination of the projecting edges would seem to require. Is there not more reason for the belief that the eighteen holes pierced in the Cremona weathercock were the result of a single stroke? Other instances of bodies pierced by lightning in a similar manner, largely confirm this view. As matter of fact, in the case of the zigzag path of lightning, it is not always easy to determine whether the stroke be downwards or upwards. There are physicists who regard the very general appearance of downward strokes of lightning an occular illusion. With a movement so rapid, and with the notion or expectation of a downward stroke by which the mind is pre-occupied, it certainly would not be strange if the eye, at times, were deceived. The clouds above and the earth or the objects beneath, at the moment before an electrical discharge, are in opposite electrical conditions, and when the tension of the electric fluids (to use the ordinary mode of expression,) becomes too great for the resistance presented by the atmosphere, they rush together producing the spark or flash— and thus the electrical equilibrum is restored. An object may, therefore, receive the stroke from the earthward side, (of which numerous instances are on record,) or even from both sides simultaneously.

The power of lightning to shatter into pieces the object struck, and to project or transport heavy bodies, is matter of frequent observation. These results can hardly be supposed to follow from the mechanical shock produced by lightning, and hence the hypothesis that some other force is brought into action. This

secondary agent in rending bodies asunder and projecting the pieces to a distance is thought to be the elastic force of steam. A temperature of 400° Fahrenheit converts water into steam with a tension of 17 atmosphere; a temperature of 500° gives rise to steam with a tension of 45 atmosphere.

We know that lightning may fuse small metallic wires, or at least render them incandescent; we know that the heat developed by a stroke of lightning may be competent to fire buildings. Suppose a block of free stone containing moisture in its fissures or between its particles is struck by lightning, steam is at once developed of sufficient tension to shatter the stone into pieces, and project the fragments in all directions. The action of steam is clearly shown in the peculiar and minute division of wood by the passage of lightning. It is split in the direction of its length into " thin laths or still more slender fibres." In the vapor of water suddenly generated at a high temperature we have a force competent to displace the foundations of buildings, to raise and transport heavy masses, and give rise to the other manifestations of power which so frequently accompany a stroke by lightning. The direction of the electric discharge, (whether upwards or downwards or at an oblique angle,) cannot, as matter of course, be determined in those cases in which steam has been the immediate agency in producing mechanical effects.

The magnetic action of lightning cannot be safely disregarded. It always affects the needle of a compass in passing near it, sometimes wholly destroys its magnetism, and sometimes reverses its magnetic poles. The manner in which this discovery was made is always read with interest.

"About the year 1675, two English vessels were sailing in company from London to Barbadoes. Not far from the Bermudas a thunderstroke shattered the mast and rent the sails of one of the ships, while the other sustained no damage. The captain of the latter seeing that his consort had altered her course, as if making for England, asked the cause of this sudden change of purpose, and found much to his astonishment, that her captain and crew believed themselves to be still following the same course as before. An attentive examination of the compasses of the vessel which had been struck by lightning, showed that the characteristic mark on the compass cards, which before the stroke pointed, as is usual, towards the north, now pointed on the contrary, to the south, showing that the poles had been completely reversed by