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steeples of churches, being usually much higher than the surrounding objects, do not require so high a conductor as buildings with extensive flat roofs. For the former, therefore, thin stems, rising from three to six feet above the cross or weathercock, will be sufficient; and, being light, they may easily be fixed to them, without injuring their appearance, or interfering with the motion of the vane.

When difficult to fix, the stem of a paratonnerre for such buildings may even be omitted altogether, and merely the foot of the cross or weathercock be well connected with the ground. This arrangement requires little expense, and is well adapted for country churches.

Paratonnerres for Powder Magazines.

These, of course, require to be constructed with the greatest care; but, in principle, are perfectly similar to the one we have described at length. They should not be placed on the buildings, but on poles, at from six to ten feet distance. The stem should be about seven feet long, and the poles of such a height, that the stem may rise from 15 to 20 feet above the top of the building. It is also advisable to have several paratonnerres round each magazine. If the magazine be in a tower, or other very lofty building, it may be sufficient to defend it by a double copper conductor, without any paratonnerre stem. As the influence of this conductor will not extend beyond the building, it cannot attract the lightning from a distance, and yet will protect the magazine, should it be struck.

Paratonnerres for Ships.

The stem of the paratonnerre for a ship, consists merely of the copper point, already described. It is screwed on a round iron rod, which enters the extremity of the pole of the top gallant mast, and carries a vane. An iron bar, connected with the foot of the round rod, descends down the pole, and is terminated by a crook, or ring, to which the conductor of the paratonnerre is attached, which, in this case, is formed of a metallic rope, and is supported at intervals by rigging; and after having passed through a ring, fixed to the chains, is united to a bar or plate of metal, which is connected to the copper sheathing on the bottom of the vessel. Small vessels

require only one paratonnerre; large ships should have one on the mainmast, and another on the mizenmast.

General Disposition of Paratonnerres on a Building.

It is allowed, from experiments, that the stem of a paratonnerre effectually defends a circle of which it is the centre, and whose radius is twice its own height, from lightning. According to this rule, a building sixty feet long, or square, requires only a single stem of 15 or 18 feet, raised in the centre of the roof. A building of 120 feet by the same rule, would require a stem of 30 feet, and such are sometimes used; but it is better, instead of one stem of that length, to erect two of 15 or 18 feet, one placed at thirty feet from one end of the building the other at the same distance from the other end, and consequently sixty feet apart from each other. The same rule should be followed for three, or any greater number of paratonnerres.

For churches with steeples, although the paratonnerre on the latter must, from its great height, extend its influence to a considerable distance, yet as nothing decisive is at present known from experiment, as to the greatest distance to which it may extend, it will be prudent to consider it as only protecting a space, whose radius is equal to the height of its stem above the ridge of the roof, and to erect other paratonnerres, on the roof itself, according to the rule already given.

General Disposition of the Conductors of Paratonnerres.

Although the necessity of establishing a very intimate communication between the paratonnerre and the soil has already been repeatedly insisted on, its importance is such, that it may be well to revert once more to the subject. If this condition be not rigorously observed, the instrument will not only become much less efficacious, but even dangerous, by attracting the lightning, without being able to convey it to the ground. What other conditions remain to be stated are less important, but nevertheless deserve attention.

The lightning should always be conducted by the shortest possible road from the stem of the paratonnerre to the ground.

Agreeably to this principle, when two paratonnerres are placed on a building, and terminate in one common conductor

(which is quite sufficient), the point from which its branches diverge to the two stems, should lie evenly and at equal distances on the roof between them; the common conductor and its branches may be formed of an iron bar, of the same dimensions as for a single paratonnerre.

If there be three paratonnerres on a building, it will be prudent to give them two conductors. In general each pair of paratonnerres requires one conductor.

Whatever number of paratonnerres be placed on a building, they should all be connected together by establishing an intimate communication between the feet of all their stems, by means of iron bars of the same dimensions as those of the conductors.

When the situation will admit of it, the conductor should be placed on the wall of the building most exposed to the rain, which, by wetting it, renders it, though imperfectly, a conductor, and if the conductor of the paratonnerre be not in intimate communication with the ground, it is possible that the lightning may abandon it for the wet surface of the wall. A further motive for selecting this side of the building is, that the direction of the lightning may be determined by that of the rain; and, moreover, the wet surface, being a conductor, may attract the lightning by preference to the paratonnerre.

Observations on the Efficacy of Paratonnerres.

The experience of fifty years demonstrates that, when constructed with the requisite care, paratonnerres effectually secure the buildings on which they are placed, from being injured by lightning. In the United States, where thunder storms are much more frequent and formidable than in Europe, their use is become general; a great number of buildings have been struck, and scarcely two are quoted as not having been saved from the danger. The apprehension of the more frequent fall of lightning on buildings armed with paratonnerres, is unfounded, for their influence extends to too small a distance to justify the idea that they determine the lightning of an electric cloud to discharge itself on the spot where they are erected. On the contrary, it appears certain, from observation, that buildings furnished with paratonnerres are not more frequently struck than formerly. Besides, the property of a paratonnerre to attract the lightning more frequently, must also imply that of transmitting it freely to the ground, and hence no mischief can arise, as to the safety of the buildings.

We have recommended the use of sharp points for the paratonnerres, as having an advantage over bars rounded at the extremity, by continually pouring off into the air, whilst under the influence of a thunder cloud, a current of electric matter in a contrary state to that of the cloud, which must probably have some effect towards neutralizing the state of the latter. This advantage must by no means be neglected; for it is sufficient to know the power of points, and the experiments of Charles and Romas with a kite flown under a thunder cloud, to be convinced that if sharp pointed paratonnerres were placed in considerable numbers on lofty places, they would actually diminish the electric matter of the clouds, and the frequency of the fall of lightning on the surface of the earth. However, if the point of a paratonnerre should be blunted by lightning, or any other cause, we are not to suppose, because it has lost the property we have mentioned, that it has also become ineffectual to protect the building it is intended to defend. Dr Rittenhouse relates, that having often examined the points of the paratonnerres in Philadelphia, where they are very frequent, with an excellent telescope, he has observed many whose points have been fused; but that he never found that the houses on which they were erected had been struck by lightning since the fusion of the points.

ART. XLVIII.-On the Aurora Borealis and Polar Fogs. By Professor HANSTEEN.* [Edin. Philos. Journ.]

I. It is well known that, with us, the Aurora Borealis presents itself to our view in the following manner. In the NNW. appears a luminous arch, the convex side of which is turned to the zenith, and the extremities of which bend towards the horizon. That part of the circle of the horizon which lies between its extremities forms a chord of the arch. The segment of the circle between the arch and the horizon is, for the most part, darker than the rest of the sky; sometimes black, sometimes dark-grey. The farther you advance towards the north, this coloured segment becomes less dark, and in the

*From the Norwegian Memoir inserted in the Christiania Journ. of Nat. Hist.

*

highest latitudes it becomes altogether undistinguishable. The highest point of the arch, at least in latitudes not very high, is almost always found in the magnetic meridian, that vertical plane which passes through the magnetic pole. In the North American States, where the westerly variation of the needle is only two degrees, the same luminous arch is seen, but its highest point lies due north. According to Scoresby's observations, on the east coast of Greenland, in Lat. 65° N., the arch of the aurora borealis lies from north to south, in such a manner, that its highest point is either to the east or west, as the arch lies on the one side or the other of the zenith. This agrees with the remarks of the missionary Andrew Ginge on the aurora borealis, in the Colony of Good Hope, in Greenland (Lat. 64° 10′ 5′′); with this difference only, that the arch commonly appears low in the east or south-east, and more seldom approaches near to the zenith. He describes. in the following manner such an arch, as seen on the 12th of December, 1786. "At half-past 4 P. M. the first faint flashes of the aurora borealis arose from the east, which a quarter of an hour after reached the zenith, and from that shot out on all sides. Soon after, these were converted into an arch, which went through the zenith, and almost touched the horizon in the north and south. This arch was white, and so brilliant, that it lighted up Baals River, which is a mile broad. At 7 P. M. the declination had decreased, from mid-day, from 50° 57 to 50 37', that is, 20. At half-past 8 the arch disappeared, and in the south was seen a comb with its teeth upwards. At that time the declination was 50° 20′," " &c.

From the foregoing observations, it is easy to see that this arch must be a part of a whole luminous ring, hovering over the surface of the earth at a considerable height, of which every observer sees his own portion. We may give an explanation of this by means of the hour-circle on a globe. Suppose a little insect creeping round the globe in the 60th parallel, it will only see a small part of the ring, as the largest portion of it will be concealed by the globe which forms the insect's horizon. The highest part of the arch which in this case it can see, will be due north. If it approach nearer to the ring, it will see a larger portion of it; and when it is close under its edge, this will appear to be in the zenith. If it come nearer

* Scoresby's Journal of a voyage to Greenland.

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