lightning-a fate which Professor Richman of St. Petersburg experienced, while performing incautiously the sublime experiment of Franklin, measuring the strength of the electricity brought down by a metallic rod in a thunder-storm, being instantly killed. Pliny likewise mentions the laurel as the only earthly production which lightning does not strike; hence, as a protection, these trees were planted around the temple of Apollo. Columella, however, mentions white vines surrounding the house of Tarchon, the Etruscan, for the same purpose. These expedients may provoke a smile without deserving one; for there can be no doubt that trees sufficiently high around a temple, or succulent plants covering a dwelling, will exercise to some extent a protective power, and act as a regular system of conductors. Salverte mentions several medals which appear to have reference to this subject, particularly one which represents the temple of Juno, the goddess of the air, the roof of which is armed with pointed rods. He quotes also Michaelis, upon the temple of Jerusalem, to show that the Jews were not unacquainted with the art of protecting their public buildings-a position grounded upon the following facts:"1. That there is nothing to indicate that the lightning ever struck the temple of Jerusalem during the lapse of a thousand years." This, of course, does not make the fact certain; but when, as M. Arago justly remarks, we consider how carefully the ancient authors recorded the cases in which their public buildings were injured by lightning, we may accept the silence observed respecting the temple of Jerusalem, as proof that it was never struck. For three centuries the cathedral of Geneva, the most elevated in the city, has enjoyed a similar immunity, although inferior buildings have been repeatedly damaged. Saussure discovered the reason of this, in the tower being entirely covered with tinned iron plates, connected with different masses of metal on the roof, and again communicating with the ground by means of metallic pipes. "2. That according to the account of Josephus, a forest of spikes with golden or gilt points, and very sharp, covered the roof of this temple; a remarkable feature of resemblance with the temple of Juno represented on the Roman medals. 3. That this roof communicated with the caverns in the hill of the temple, by means of metallic tubes, placed in connection with the thick gilding that covered the whole exterior of the building; the points of the spikes there necessarily producing the effect of lightning rods. How are we to suppose that it was only by chance they discharged so important a function; that the advantage received from it had not been calculated; that the spikes were erected in such great numbers only to prevent the birds from lodging upon and defiling the roof of the temple? Yet this is the sole utility which the historian Josephus attributes to them." Upon a sober review of these facts, it is difficult to resist the conclusion that the ancient world had some proficiency in the art of guiding the electric fluid from the bosom of the clouds, conducting it in a prescribed course, and thus disarming it of its terrors.

The subject of electrical agency is intimately connected with that of magnetism, to which this is the fittest place to glance, one of the most recondite points of physical science. The relation between the two is evident, from the notorious fact that lightning often renders steel magnetic, and disturbs the magnetism of the magnetised needle, so that in thunder-storms the compass needles of a ship have frequently been seriously injured. The magnetic agency, like electricity, has a general distribution over the earth, but the phenomena differ in different parts of the world, and are subject to periodical differences in the same place, the cause of which is very little understood. Every one is acquainted with the polarity of a freely suspended magnetic needle, or its tendency to lie parallel with the earth's axis, pointing nearly north and south in every region of the globe. What is called the dip or inclination of the needle is its divergence from a perfectly horizontal position. Thus the north pole of the needle inclines downwards in the latitude of London at an angle of 70°, but conveyed towards the equator, the dip diminishes, till no inclina

tion at all appears. Transported farther towards the south, the dip again discovers itself, but in an opposite direction, the south pole of the needle inclining downwards. "To understand the reason of this dip of the magnetic needle, and of its general direction, we have only to consider that the earth itself operates as a great magnet, the poles of which are situated beneath its surface. The directive property of the needle is owing to these poles; and when the needle is on the north side of the equator, the north pole of the earth having the greatest effect, the needle is attracted downwards towards the north pole; hence exactly over the magnetic pole the needle would be vertical. Similar phenomena occur in the southern hemisphere; but here the south pole predominates, and of course depresses the corresponding pole of the needle; while at the magnetic equator, from the equal action of both poles, the needle will assume an exactly horizontal position."

But neither the magnetic equator nor the magnetic poles coincide precisely with the geographical equator and poles, and this difference constitutes what is termed the variation of the needle. From calculation, the north magnetic pole had been fixed in latitude 70°, and longitude 98° 30′ west, a spot which Commander Ross approached within the distance of ten miles, in the year 1830, but was unable to verify the site, for want of the requisite instruments. Upon going through a long series of calculations afterwards himself, he concluded the above position to have been erroneously assigned, and that the real point lay in latitude 70° 5′ 17′′ north, and longitude 96° 46′ 45′′ west, a spot on the western coast of Boothia, which he prepared to reach. On the 1st of June 1831, at eight o'clock in the morning, he arrived at the site to which his calculations pointed, and found the same day the amount of the dip to be 89° 59′, only one minute less than 90°, the vertical position, which would have precisely indicated the polar station; and the horizontol needles, suspended in the most delicate manner possible, did not betray the slightest movement. The spot was an unattractive level site along the coast, rising into ridges from fifty to sixty feet high, about a mile inland. The wish expressed by the discoverer was natural, that a place so important had possessed more of mark or note, but Nature had erected no monument to denote the spot which she had chosen as the centre of one of her "great and dark powers." A cairn of some magnitude was constructed by the adventurers, upon which the British flag was planted, and underneath, a canister was buried, containing a record of the interesting enterprise.

The magnetic needle has frequently exhibited violent disturbance when the Aurora Borealis has appeared. This has led to the surmise that these brilliant lights are connected with the electric and magnetic properties of the earth, though in a manner which we cannot explain. It has been remarked that during the appearance of the aurora the electric fluid may often be readily collected from the air. If a current of electricity also be passed through an exhausted receiver, a very correct imitation of the auroral light will be produced, displaying the same variety of colour and intensity, and the same undulating motions. It is highly probable, therefore, that the beautiful and fantastic meteoric display is connected with electricity; but great obscurity rests upon this department of meteorology.

Of all optical phenomena, the Aurora Borealis, or the northern day-break, is one of the most striking, especially in the regions where its full glory is revealed. The site of the appearance, in the north part of the heavens, and its close resemblance to the aspect of the sky before sunrise, have originated the name. The "Derwentwater Lights" was long the appellation common in the north of England, owing to their display on the night after the execution of the unfortunate earl of that name. The scene in the illustration is a picture of the auroral light, as observed from the neighbourhood of Loch Leven— a scene in itself admirably calculated to exhibit the phenomenon; and to convey any

The appearances exhibited by the aurora are so various as to render it impossible to comprehend every particular in a description that must be necessarily brief and general. A cloud, or haze, is commonly seen in the northern region of the heavens, but often bearing towards the east or west, assuming the form of an arc, seldom attaining a greater altitude than 40°, but varying in extent from 5° to 100°. The upper edge of the cloud is luminous, sometimes brilliant, and irregular. The lower part is frequently dark and thick, with the clear sky appearing between it and the horizon. Streams of light shoot up in columnar forms from the upper part of the cloud, now extending but a few degrees, then as far as the zenith, and even beyond it. Instances occur in which the whole hemisphere is covered with these coruscations; but the brilliancy is the greatest, and the light the strongest, in the north, near the main body of the meteor. The streamers have in general a tremulous motion, and when close together present the appearance of waves, or sheets of light, following each other in rapid succession. But no rule obtains with reference to these streaks, which have acquired the name of "the merry dancers," from their volatility, becoming more quick in their motions in stormy weather, as if sympathising with the wildness of the blast. Such is the extraordinary aspect they present, that it is not surprising the rude Indians should gaze upon them as the spirits of their fathers roaming through the land of souls. They are variously white, pale red, or of a deep blood colour, and sometimes the appearance of the whole rainbow as to hue is presented. When several streamers emerging from different points unite at the zenith, a small and dense meteor is formed, which seems to burn with greater violence than the separate parts, and glows with a green, blue, or purple light. The display is over sometimes in a few minutes, or continues for hours, or through the whole night, and appears for several nights in succession. Captain Beechey remarked a sudden illumination to occur at one extremity of the auroral arch, the light passing along the belt with a tremulous hesitating movement towards the opposite end, exhibiting the colours of the rainbow; and as an illustration of this appearance, he refers to that presented by the rays of some molluscous animals in motion. Captain Parry notices the same effect as a common one with the aurora, and compares it, as far as its motion is concerned, to a person holding a long riband by one end, and giving it an undulatory movement through its whole length, though its general position remains the same. Captain Sabine likewise speaks of the arch being bent into convolutions, resembling those of a snake in motion. Both Parry, Franklin, and Beechey agree in the observation that no streamers were ever noticed shooting downwards from the arch.

The preceding statement refers to aurora in high northern latitudes, where the full magnificence of the phenomenon is displayed. It forms a fine compensation for the long and dreary night to which these regions are subject, the gay and varying aspect of the heavens contrasting refreshingly with the repelling and monotonous appearance of the earth. We have already stated that the direction in which the aurora generally makes its first appearance, or the quarter in which the arch formed by this meteor is usually seen, is to the northward. But this does not hold good of very high latitudes, for by the expeditions which have wintered in the ice, it was almost always seen to the southward; while by Captain Beechey, in the Blossom, in Kotzerne Sound, 250 miles to the southward of the ice, it was always observed in a northern direction. It would appear, therefore, from this fact, that the margin of the region of packed ice is most favourable to the production of the meteor. The reports of the Greenland ships confirm this idea; for, according to their concurrent testimony, the meteoric display has a more brilliant aspect to vessels passing near the situation of the compact ice, than to others entered far within it. Instances, however, are not wanting, of the aurora appearing to the south of the zenith in comparatively low latitudes. Lieutenant Chappell, in his voyage to Hudson's

66

Aurora Borealis.

Bay, speaks of its forming in the zenith, in a shape resembling that of an umbrella, pouring down streams of light from all parts of its periphery, which fell vertically over the hemisphere in every direction. As we retire from the Pole, the phenomenon becomes a rarer occurrence, and is less perfectly and distinctly developed. In September 1828, it was observed in England as a vast arch of silvery light, extending over nearly the whole of the heavens, transient gleams of light separating from the main body of the luminosity: but in September 1827 its hues were red and brilliant. Dr. Dalton has furnished the following account of an aurora, as observed by him on the 15th of October, 1792:"Attention," he remarks, was first excited by a remarkably red appearance of the clouds to the south, which afforded sufficient light to read by at 8 o'clock in the evening, though there was no moon nor light in the north. From half-past nine to ten there was a large, luminous, horizontal arch to the southward, and several faint concentric arches northward. It was particularly noticed that all the arches seemed exactly bisected by the plain of the magnetic meridian. At half-past ten o'clock streamers appeared, very low in the south-east, running to and fro from west to east. They increased in number, and began to approach the zenith apparently with an accelerated velocity, when all on a sudden the whole hemisphere was covered with them, and exhibited such an appearance as surpasses all description. The intensity of the light, the prodigious number and volatility of the beams, the grand intermixture of all the prismatic colours in their utmost splendour, variegating the glowing canopy with the most luxuriant and enchanting scenery, afforded an awful, but at the same time the most pleasing and sublime spectacle in nature. Every one gazed with astonishment, but the uncommon grandeur of the scene only lasted one minute. The variety of colours disappeared, and the beams lost their lateral motion, and were converted into the flashing radiations. The aurora continued for several hours." A copious deposition of dew-hard gales in the English Channel-and a sudden thaw after great cold in northern regions, are circumstances which have been frequently noticed in connection with auroral displays.

The sky of the southern hemisphere occasionally exhibits this strange and mysterious light, contrary to an old opinion upon the subject; and here it must be called Aurora