Chapter 5

There is not, I suppose, in the mind of any intelligent man at the present day a doubt as to the electrical origin of a lightning flash. The questions to be considered are rather whence comes the electricity, and in what way is the thunderstorm brought about. In attempting to answer these questions, sight must not be lost of the fact that the very nature of electricity is in itself almost sufficient to baffle any effort put forth to ascertain from lightning, as such, its whence and its whither.

It is possible, however, with the aid of our knowledge of static electricity, to arrive at hypotheses of a more than chimerical nature. In the first place, that our sphere is a more or less electrified body is generally admitted. More than this, it is demonstrated that the different parts of the earth's surface and its enveloping atmosphere are variously charged. As a consequence of these varying charges, there is a constant series of currents flowing through the various parts of the earth, which show themselves in such telegraph wires as may lie in the direction followed by the currents. Such currents are known as earth currents, and present phenomena of a highly interesting nature. But, apart from these electrical manifestations, there is generally a difference of electrical condition between the various parts of the earth's surface and those portions of the atmosphere adjacent to or above them. Inasmuch as air is one of the very best insulators, this difference of condition (or potential) in any particular region is in most cases incapable of being neutralized or equilibrated by an electric flow. Consequently the air remains more or less continually charged. With these points admitted as facts, the question arises, Whence this electricity? There have been very many and various opinions expressed as to the cause of terrestrial electricity, but far the greater portion of such theories lack fundamental probability, and indicate causes which cannot be regarded as sufficiently extensive or operative to produce such tremendous effects as are occasionally witnessed. I take it that we may safely regard the evolution of electricity as one of the ways in which force exhibits itself, that, in other words, when work is performed electricity may result. When two bodies are rubbed together, electricity is produced, so also is it when two connected metals are immersed in water and one of them is dissolved, or when one of the junctions of two metals is raised to a higher temperature than the other junction. I will go further than this, so far, in fact, as to maintain that there is a reasonable ground for supposing that every movement, whether it be of the mass or among the constituent particles, is attended by a change of electrical distribution; and if this is true, it may easily be conceived that inasmuch as motion is the rule of the universe, there must be a constant series of electrical changes. Now, these changes do not all operate in one direction, nor are they all of similar character, whence it is that not only are there earth currents of feeble electro-motive force, but that this E.M.F. is constantly varying, and that, furthermore, electricity of high E.M.F. is to be met with in various parts of the atmosphere.

With earth currents we have here very little to do. The rotation of the earth is in itself sufficient to generate small currents, and the fact that they vary in strength at regular periods of the day and of the year enforces the suggestion that the sun exerts considerable electrical influence on the earth. Letting it be granted, however, that the earth is variously charged, how comes it that the air is also charged, and with electricity of greater tension than that of the earth itself? It was pointed out by Sir W. Grove that if the extremities of a piece of platinum wire be placed in a candle flame, one at the bottom and the other near the top, an electric current will flow through the wire, indicating the presence of electricity. If an electrified body be heated, the electricity escapes more rapidly as the temperature rises. If a vessel of water be electrified, and the water then converted into steam, the electric charge will be rapidly dissipated. If a vessel containing water be electrified, and the water allowed to escape drop by drop, electricity will escape with each drop, and the vessel will soon be discharged.

We regard it as an established fact that the earth has always a greater or less charge; whence it is safe to assume that in the process of evaporation which is going on all over the surface of the globe, more particularly in equatorial regions, every particle of water, as it rises into the air, carries with it its portion, however minute that portion may be, of the earth's electric charge. This small charge distributes itself over the surface of the aqueous particle, and the vapor rises higher and higher until it reaches that point above which the air is too rare to support it. It then flows away laterally, and as it approaches colder regions gets denser, sinking lower and nearer to the earth's surface. The aqueous particles becoming reduced in size, the extent of their surfaces is proportionately reduced. It follows that as the particles and their surfaces are reduced, the charge is confined to a smaller surface, and attains, therefore, a greater "surface density," or in simpler language, a greater amount of electricity per unit of surface.

Electricity, as above set forth, is in what is known as the "static" condition (to distinguish it from electricity which is being transferred in the form of a current), when it has the property of "repelling itself" to the utmost limits of any conductor upon which it may be confined. This will account for the charge finding its way to the surface of the water particles, and will furthermore account for the greater density of the charge as the particle gets smaller and has the extent of its surface rapidly diminished. It may be mentioned that the surface of a sphere varies as the cube of its radius.

Returning to the discussion of the state of affairs existing when the particles have reached their highest position in the atmosphere, we may imagine that they set themselves off on journeys toward either the north or the south pole. As they pass from the hotter to the colder regions, a number of particles coalesce; these again combine with others on the road until the vapor becomes visible as cloud. The increased density implies increased weight, and the cloud particles, as they sail poleward, descend toward the surface of the earth. Assuming that a spherical form is maintained throughout, the condensation of a number of particles implies a considerable reduction of surface. Thus, the contents of two spheres vary as the cubes of their radii, or eight (the cube of 2) drops on combining will form a drop twice the radius of one of the original drops. We may safely conceive hundreds and thousands of such combinations to take place until a cloud mass is formed, in which the constituent parts are more or less in contact, and, therefore, behave electrically as a single conductor of irregular surface, upon which is accumulated all the electricity that was previously distributed over the surfaces of the millions of particles that now compose it.

The tendency of an electric charge upon the surface of a conductor is to take upon itself a position in which it may approach nearest to an equal and opposite charge; or, if possible, to attain neutrality. If, then, a cloud has a charge, and there is no other cloud above or near it, the charge _induces_ on the adjacent earth surface electricity of the opposite kind. Thus, assuming the cloud to be charged with positive electricity, the subjacent earth will be in the negative state. The two electricities[3] exert a strong tendency to combine or to produce neutrality, whence there is a species of stress applied to the intervening air. Possibly the cloud will be drawn bodily toward the earth more or less rapidly, according as the charge is great or small. Or, on the other hand, the cloud may roll on for leagues, carrying its influence with it, so that the various portions of the earth underneath become successively charged and discharged as the cloud progresses on its journey.

[Footnote 3: We may speak of two electricities or two electric states without necessarily implying adherence either to the single or the double "fluid" theory. Whether electricity be of two kinds or no, the fact remains that there are two conditions, and all the features of this paper may be explained with equal facility by the supporters of either hypothesis.]

Should the cloud be near the earth, or should it be very highly charged, the tension of the two electricities may be so great as to overcome the resistance of the intervening air; and if this resistance should prove too weak, what happens? How does the discharge show itself? It takes place in the form of a lightning flash, and passing from the one surface to the other--or, maybe, simultaneously from both--produces neutrality more or less complete.

There has recently been a little discussion in these pages on the subject of lightning, some having stated that they discerned the discharge to take place upward--that is, from the earth toward the cloud. I will not venture so far as to say whether or not the direction of the discharge is discernible; possibly the flash may sometimes be long enough to enable one to tell; but I have never so seen it, and have always looked upon the eye as a deceitful member--very. "The lightning flash itself never lasts more than 1/100000 of a second." It is, however, just as likely that a discharge may travel upward as downward. What controls the discharge? Does the quality of the charge?--that is to say, is the positive or the negative more prone to break disruptively through the insulating medium? Investigations with Geissler's and other tubes containing highly rarefied gases have made it tolerably clear that there is a greater "tearing away" influence at the negative than at the positive pole, and if two equal balls, containing one a positive and the other a negative charge, be equally heated, the negative is more readily dissipated than the positive. But, so far as we at present know, this question enters into the discussion scarcely, if at all. Our knowledge seems rather to point to the substances upon which the charges are collected. The self-repellent nature of electricity compels it to manifest itself at the more prominent parts of the surface, the level being forsaken for the point. The tension of the charge, or its tendency to fly off, is proportionately increased. And if at a given moment the tension attains a certain intensity, the discharge follows, emanating from the surface which offers the greatest facilities for escape. The earth is generally flatter than the cloud, whence, in all probability, the discharge more frequently originates with the cloud.

Should a lightning flash strike the earth and produce direct neutrality, it is possible that no damage will result, although this again is not always certain, because when the cloud charge acts inductively on the earth it produces the opposite (say negative) charge on the nearer parts, the similar (or positive) state is also produced at some place more or less distant. Sometimes this "freed" positive (which, by the way, accumulates gradually and physiologically imperceptibly) is collected at some portion of the earth's surface. When the negative is neutralized by the discharge, the freed positive is no longer confined to a particular region, but tends to dissipate itself, and a shock may be felt more or less severely by any person within the region. Or, again, a similar shock may be experienced by a person standing within the negative zone on the neutralization of the charge.

I may take the opportunity here to mention a highly interesting and instructive incident observed on local telegraph circuits during a thunderstorm. The storm may be taking place at some distance from the point of observation. The electrified cloud induces the opposite charge beneath it, the similar charge being repelled. It is noticeable that the needle of a galvanometer, starting from the middle position, goes gradually over to one side, eventually indicating a considerable deflection. Suddenly, owing apparently to a lightning discharge some distance away, the force which caused the deflection is withdrawn, and the needle rebounds with great violence to the opposite side. In a short time, the cloud becoming again charged on its under surface, and recommencing its inductive effect upon the adjacent earth, the needle starts again, and goes through the same series of movements, a violent counterthrow following every flash of lightning.

If we can so far control our imagination, we may conceive the earth to be one large insulated conductor, susceptible to every influence around it. If then the earth, as a mass of matter, behaves as above indicated, there is no plausible reason for declining to regard any other large conducting mass in a similar light, and as a body capable of being subjected more or less completely to the various impulses affecting the earth. In other words, a large mass of conducting material, partially or perfectly insulated, is, during a thunderstorm, in considerable danger. With this portion of the subject I shall, however, deal more fully when discussing the merits of lightning protectors.

Lightning discharges do not take place between cloud and earth only, but also, and perhaps more frequently, between too oppositely charged clouds. We then get atmospheric lightning, the flash often extending for miles. This form of lightning is harmless, and in all probability what we see is only a reflection of the discharge. The oft-told tale of the lightning flying in at the window, across the room, and out of the door, or up the chimney, is all moonshine, and before dealing with lightning protectors I intend to expose some of the fallacies concerning lightning. Were the discharge to pass through a house, it would infallibly leave more decided traces and do more damage than simply scaring a superstitious old lady now and again. Many people are often and unnecessarily frightened during a thunderstorm, but it may be safely predicted that a person under a roof is infinitely safer than one who is standing alone on level ground, and making himself a prominence inviting a discharge. Rain almost invariably accompanies the discharge, and the roof and sides of the house being wet, they form a more or less perfect channel of escape should a flash strike the building.--_Knowledge._

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RESEARCHES ON MAGNETISM.

By M. DUTER.

If we place a thin plate of steel in a uniform magnetic field, so that the lines of force of the field may be normal to the surface of the plate, we have a very flat magnet, the two faces of which are the two polar surfaces. The magnetic distribution thus obtained seems to disappear when the plate is no longer in the field. The following experiments show that this disappearance is not complete. I made use of plates of tempered steel of 1 millimeter in thickness, and varying in diameter from 0.040 to 0.005 meter. With these plates I formed cylindrical batteries. In some of these batteries the plates are directly in contact, and in others they were separated by leaves of pasteboard, the thickness of which varied from that of the thinnest paper to 0.001 meter. The batteries were placed in the central portion of a very powerful magnetic field, and after they have been taken out they formed perfectly regular permanent magnets. The supporting power of these magnets was the greater the nearer its constituent plates were to each other. In a battery of 100 plates, touching each other directly, and strongly pressed into a brass cylinder, the portative force at each extremity rose to 30 grammes. This first result having been obtained, I dismounted the batteries, plate by plate, taking care to mark the upper and under side of each. I found then that each plate retained only an excessively slight magnetism. Yet each of them still constituted a flat magnet, of which the two faces are the polar surfaces; for on rebuilding the battery it gave again a perfectly regular magnet, though weaker than it was at first. The separation of the magnet into its constituent plates, and its reconstruction, maybe repeated indefinitely.--_Comptes Rendus._

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Dr. T. Tommasi (_Cosmos les Mondes_) notes that the thermic constant of thallium is exactly the mean of the thermic constants of potassium and lead, the two metals which it most resembles in its chemical character.

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IMPROVED GAS LIGHT BUOY.

The accompanying engravings represent a light buoy made by the Pintsch's Patent Lighting Company for the river Humber. The chief dimensions of the buoy are given in the engraving, which also shows that the gas holder is placed within the boat in such a way as to be protected from blows likely to cause any leakage. The buoy has a special form to meet its requirements as a lightship, and the conditions of its employment is the fast tidal current of the river. It was designed by Mr. C. Berthon, of Westminster, and is intended to carry a six months' supply of gas, the burner, regulator, and lamp being on the well known Pintsch system. The hull is formed of 3/8 inch plate, 24 feet 3 inches total length, and 9 feet beam at the line of flotation. The laps of the plates are 4 inches wide, and riveted with 3/4 inch rivets, spaced 2-1/4 inch apart center to center. The keel and stem are both in one piece, as shown, and to this the garboard strake is to be fastened. The bilge pieces are riveted on to the bilge, and made of 9 inches by 4-1/2 inches by 9/16 inch T-iron. A wooden fender, 4 inches by 4 inches wood, is fitted on both sides of hull, running from stern to stern, by 3 inches by 3-1/2 inches by 7/16 inch L-iron top and bottom with the sheer as shown. The hull from water line falls in as shown, so as to describe at midships an arc of 4 feet 6 inches, and a circular deck of 1/8 inch plate is riveted on the hull. There are two man-holes, each 16 inches diameter in the clear, placed in end plates of the circular deck as shown, and provided with covers 3/8 inch thick, secured by twenty screws 3/4 inch diameter. The edge of each manhole is stiffened by a welded iron ring. The surface of the mooring link that comes in contact with the shackle and mooring chain is steeled. The gas holder rests upon a plate bent up on each side, and riveted to the keelson, and is prevented from rolling by four gusset plates, with two short pieces of angle iron riveted thereto at the ends and coming in contact with the holder, and at the ends by angular plates, and angle iron riveted on each side and riveted to the keelson. The superstructure consists of four legs of angle iron 2-1/2 inches by 2-1/2 inches by 5/16 inch, the upper ends of the legs being attached to a square flanged plate for supporting the lighting apparatus. Four wooden battens of pitch pine, 4 inches by 1-1/2 inches, and bolted on to each cant of the angle iron superstructure, with 7/8 inch galvanized iron bolts and nuts.

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PROJECT FOR A ROADSTEAD AT HAVRE.

The present port of Havre is absolutely insufficient to answer the ever increasing requirements of commerce. Its entrance, which is too narrow and not deep enough, does not permit steamers to go in, come out, and perform their evolutions with the rapidity required by our epoch. So they are gradually abandoning our port, and going to load and unload at Anvers and elsewhere. A large number of wise heads, who are anxious about the future of this port and our national interests, have devoted themselves to finding a means of enlarging it, not by dredging _new_ basins, which would prove ruinous to the budget and useless in twenty years, but by installing a true roadstead at the entrance to the present basins.

Upon the maps of the hydrographic service may be seen, under the name of the Little Roadstead, a vast extent of sea nearly two kilometers wide by three to four in length, bounded upon one side by the heights of Heve and St. Adresse, and upon the other by the rocky line of Eclat and of the heights of the roadstead (Fig. 1). This Little Roadstead, so called, in order to become a genuine one, would have to be protected against the great waves of the open sea. To thus protect it, to close it as quickly and as cheaply as possible--that is the problem.

In 1838, Charles de Massas presented a project (the first in order of date), which consisted in constructing upon the Eclat reef a semi-lunate dike, and a breakwater at Cape Heve. Moreover, upon the emergent parts of the Eclat reef and heights of the roadstead he proposed to erect two forts.

The defense of the port of Havre is a very important question, and one that appears to be completely abandoned. Since Engineer Degaulle in 1808 advised the erection of a fort upon the Eclat, and requests have periodically been made and projects drawn. The requests are forgotten, but the drawings are in the Ministers' portfolios, and if France should to-morrow have a war with a maritime power our great northern port might be destroyed and burned by the smallest squadron.

Some years after Massas' project, two officers, Deloffre and Bleve, and an engineer named Renaud, received a commission to search for a means of closing a portion of Seine Bay. These gentlemen advised the erection of two dikes, one on the Eclat shoal in the very axis of this reef, and the other at Heve. Between these two masonry dikes was to be placed a floating breakwater. This project, which was submitted to Admiral de Hell in 1845, had a favorable reception, and the Admiral especially applauded the trial of breakwaters, "which were much talked of in England, although the effects that they might produce were not well known." Deloffre, Bleve, and Renauds' project comprised two forts--one to the north and the other to the south of the roadstead. For a long time nothing more was said about it, and it is only during recent years, when the peril has become imminent for Havre (threatened as it is of being abandoned even by the French transatlantics), that the question has again became the order of the day.

Mr. Bert, a merchant, would protect the Little Roadstead by means of two jetties, 1,000 and 1,600 meters in length, built, one of them upon the Eclat and the other upon the eminences of the roadstead. These would be constructed by forming a foundation of loose rocks, and using earth and brick above the level of the water. Mr. Vial has likewise proposed a rockwork of 2,000 meters in length, to form a dike 10 meters in height and width, whose platform would be on a level with the highest tides.

Next comes the more recent project of Mr. Coulon. Seeing that it is the deposits of the ocean and not those of the Seine that accumulate upon the estuary, Mr. Coulon advises the construction of a dike about 2,000 meters in length, starting from the Havre jetty, and ending at the southwest extremity of the shoals at the roadstead heights, and a second one returning toward the northwest, of from 500 to 1,000 meters. A third and very long one of not less than 8 kilometers would be built from Honfleur to the Ratier shoals.