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assigned seems adequate. Many of the movements, however, may be the effect of a change from one reflecting surface to another which presents a different angle; for sometimes there seems to be a succession of different streamers, at other times only a lateral motion to and fro of the same streamer.

The illumined arch which appeared on the 24th September last, and that on 11th September 1814, were evidently a modification of the aurora borealis, because they proceeded from the same quarter of the heavens, and in both cases were resolved into the ordinary appearance assumed by those phænomena. These facts, besides proving the latter position, go far to prove the general theory here advanced. If every circumstance connected with the enlightened arches is duly considered, a doubt can scarcely be entertained concerning their cause. Their form, position, motion, and time of appearing, all concur in pointing it out to be the light of the sun reflected by the spherical surface of the earth, and again reflected back on a different part of it by the atmosphere. From the regular form of the arches, it is probable that the surface from which they were reflected was that of the ocean, which stretches in the direction in which the sun was during their appearance. But later in the evening, when that uniform surface had passed out of the line of direction by the rotation of the earth, and the icy regions of the north pole had intervened, the sun being reflected from a broken unequal surface, the arch was also broken into streamers of the usual appearance. It will be obvious, that without the refractive power of the atmosphere those phænomena could have no existence, because in that case the reflected rays of the sun could fall nowhere except in that space enlightened by his direct rays; but by refraction those rays falling upon the verge of the enlightened hemisphere, must, when reflected, be bent into the dark hemisphere.

It is only to a certain, extent within the latter boundary that streamers can be seen; for beyond that the reflected rays will pass the bounds when the atmosphere has power to reflect them back on the earth: therefore, in our latitude, the streamers generally disappear before midnight; but in higher latitudes they are seen at that time, being nigher the boundary of light; and for the same reason they are seen in the northern regions through a greater part of the year, as before noticed.

If the aurora borealis were of an electric or meteoric nature, as is supposed, their height might be ascertained by the common method of measuring heights at any time when they are stationary; but if the true origin is as above explained, the common method of measuring heights will not apply to them. If they are the reflected rays of the sun, the arch or streamer is a section of those

rays

rays again reflected on the earth by the atmosphere at a certain angle: therefore they will be seen from different places at nearly the same angle, only affected by the angle which the reflected rays make with the horizon in proceeding from the latter towards the zenith; being with that exception similar to the rainbow, which, however the spectator may change his place, preserves its relative situation to him.

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If I am at all right, I should suppose that an approximation to the height of these phænomena might be found by something like the following method, which would apply more particularly to the arch, if such a phænomenon should again make its appearance, because its motion is slow and uniform, and seems only to depend on the motion of the earth relative to the sun. the angle of height and the bearing of the middle or highest point of the arch, noting the time; repeat this two or three times. at intervals till it disappear, and it would be so much the better if these observations were made at two or three different parts of the country as distant as possible from each other. It could then be found, if a line passing through the centre of the sun and the observer coincided with the different bearings at the times they were taken. Find also the boundary of light and darkness and the above-mentioned lines, or rather vertical planes, at the times noted. A little on the light or north side of this boundary must be the place from which the arch is reflected, and which from the regularity of the figure seems to be the ocean in a liquid state, or with a uniform surface of ice, or the latter covered with snow.

From the two times on which this phænomenon was seen so nearly coinciding, it is probable that it cannot be produced except when the earth and sun are in the same relative position.

The times alluded to, it will be recollected, were near the autumnal equinox, from about 8 till 10 o'clock in the evening; and it is scarcely to be supposed that the ocean could be frozen at that time of the year in the direction of the sun between those hours. But the surface of the water seems quite adequate to reflect the rays of the sun with all the splendour displayed by that beautiful phænomenon, when we consider the great obliquity of the incident rays, and their reflection into the dark serene atmosphere which on those evenings favoured their exhibition. The above observations being made, and making allowance for refraction in both the incident and reflected rays, probably increased by the earth's attraction on the latter, (which I am inclined to think is considerable, and principally causes the curve observable in streamers,) it will be seen at once if the phænomena are produced by the cause I have assigned. The reflected ray being traced as above suggested, and being intersected at the

the different stations of observation by the observed angle of ele vation, these intersections will be the height of the arch at such places. The above method would not apply so well to common streamers; but when stationary for a time, circumstances might be determined concerning them by those or similar observa

tions.

It is to be observed, that a circumstance has been noticed in high northern latitudes, which cannot be accounted for otherwise than by supposing an extraordinary degree of refraction to exist in the polar regions. The circumstance is the appearing of the sun above the horizon, many days before that luminary could be expected from the relative position of the earth at the time, with only the quantity of refraction usually allowed. Whatever is the cause of the increased refraction at the north pole, it will greatly affect those phænomena, and must be ascertained at least before their height can be accurately found. I believe the cause commonly, and perhaps truly, assigned for the great re fracting power above noticed, is the dense state of the atmosphere produced by extreme cold: there is however no certainty that the singular state which causes mock suns, landscapes, &c. in the air, may not be concerned here.

It may be asked, If the cause of the aurora borealis is as above explained, why are they not always visible at the two periods before mentioned? The answer is, So they would, were the atmosphere always in that state of purity which is essential to their exhibition. But the complete absence of clouds in so vast an extent of a northern sky must be very rare.

In a more limited space, the atmosphere must be more frequently in such a state of serenity as to favour the exhibition of those phænomena; which, with other causes before mentioned, is no doubt the reason of the frequency of their appearance in more northern latitudes. A conjecture has been offered concerning the cause of the phænomena in question, which being more absurd than that already I think disproved, it is scarcely necessary to take the trouble of refuting. It is, that those lights are produced by the combustion of hydrogen gas that has escaped from the earth, and accumulated in upper regions of the atmosphere. It is evident that such combustion, were its existence not altogether improbable, could neither be periodical nor local, nor exhibit an appearance at all similar to streamers. Were this inflammable gas collected where the oxygen of the atmosphere bad access to it, and there kindled, they would instantly unite with tremendous noise, and the water formed fall to the ground.

But if it ascended above the atmosphere, it would be entirely beyond the means of ignition; neither could combustion there go on, from want of oxygen.

In the case of hydrogen gas ascending in the atmosphere, if it were not combined or mixed in its progress, it would continue to ascend till it surmounted all the other ingredients which compose the latter, because it would expand as the pressure diminished: consequently its specific gravity would at any height bear the same proportion to that of the atinosphere as it does at the surface of the earth: therefore its progress upwards would continue till it was beyond the means of either ignition or combustion.

XXVIII. Description of the Percussion Gun-Lock invented by Mr. COLLINSON HALL, of High-street, Mary-le-bone

THE cock, or hammer, and the touch-hole are the only parts in which Mr. Hall's gun differs from those in ordinary use; and these parts are so simple that a common lock may be converted into a percussion lock on Mr. Hall's plan, at a very small expense.

The detonating powder which is used for the priming is made into the form of a pellet, and is fixed in the centre of a small round piece of paper covered with wax. In this state it is applied to a cavity countersunk in the head of the hammer, to which it adheres by means of the wax, and is thus preserved from the effect of wet. The touch-hole consists of a cylindrical plug screwed into the side of the barrel, and having a pin or nipple projecting from it at right angles: this pin is perforated in the direction of its axis, and thus forms a communication with the powder in the cavity of the plug. When, by the release of the tumbler, the hammer is let go, the countersunk cavity, containing the patch of detonating paste, strikes on the top of the pin of the touch-hole, the paste explodes, and, communicating its percussion through the perforation of the pin, fires the powder in the cavity of the plug, and thus discharges the gun. The corrosive and deliquescent salt resulting from the decomposition of the detonating paste can act only on the hollow of the hammer, where it does no material injury, instead of soiling and occasioning damp in the touch-hole itself. Hence a leck on this construction hardly ever misses fire, and the discharge is remarkably rapid; both of them circumstances which very materially influence the success, and consequently the satisfaction, of the sportsman.

* From the Transactions of the Society for the Encouragement of Arts, Manufactures, and Commerce, vol. xxxvi. The Society's silver medal was voted to Mr. Hall for this communication.

The

The detonation is so powerful, that if a card be laid over the pin, or even if its perforation be stopped with tallow, the gun will, notwithstanding, be discharged. If the hammer is let down gently after priming, the spring presses the pellet close into the cavity, and thus considerably increases the effect.

The paste is made of the several ingredients in the following proportions, viz.

Grains.

Dissolve the gum in as little water as possible; then grind the oxymuriate of potash fine, in a Wedgwood's mortar, by itself, and also the flour of sulphur and charcoal together, with a pestle of the same material. The mixture of all with the gum must then be effected, either in a wooden mortar with a wooden pestle, or, at any rate, in a Wedgwood's mortar with a wooden pestle, taking care to keep it moist during that operation, lest it should explode.

The paste, being of the consistence of soft clay, is then to be formed into pellets, by means of a mould made of a plate of brass or copper, one-sixteenth of an inch thick, and filled with holes of one-eighth of an inch in diameter: this plate being placed upon a table, or other flat surface, over which a sheet of paper is first to be laid, the paste is to be spread evenly over its surface, and then pressed into the holes, either by passing a roller over it, or by beating upon it with a wooden mallet: the paste is then to be removed from the upper face of the mould, with a thin spatula or palette knife; and the mould is next to be slided, for the length of an inch, along the paper, to separate the paste from it; and it may then be lifted up, and the pellets carefully driven out of the holes in it, by striking upon it with a soft brush; they are then to be dried. The round paper patches being cut by a proper punch are covered on one side with beeswax mixed with a little tallow, and coloured red to distinguish the adhesive side from the other: the pellet is then gently pressed on the centre of the waxed side of the patch, to which it adheres, and the priming patch is thus completed.

When used, the patch is to be pressed firmly into the countersunk cavity of the head of the hammer, to which it easily adheres in consequence of its waxed surface being in contact with the metal.

The following recipe for the composition of the pellets has been communicated from another quarter.

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