lines drawn upon them, which are brought to the points of the needle when the observation is made. A light brass box, dd, with a glass cover, is fitted over the plate CC, to preserve the needle from disturbance from the wind; it also supports a small double microscope, M, intended to assist in examining when the index line points exactly to the end of the needle; the stem of the microscope is fitted in a dovetail groove, and can be removed to the opposite end of the box at pleasure.

The centre pin of the plates E and C terminates in an extremely fine point, on which the needle is suspended, having an agate cap, B, to diminish the friction as much as possible. The needle is of a cylindrical figure, about the five-hundredth of an inch in diameter, and ten inches long, and weighs 65 grains, and is terminated by two conical points; the circular enlargement of the centre has a hole through it, for the reception of the brass socket, B, which has the agate cap fixed in it; the needle is, of course, provided with the usual apparatus for lifting it off the point when not in use, to avoid wearing the point of suspension. a, a, are two arms, screwed down upon the plate, carrying the vernier, D; they support a brass frame, b, which has the usual clamp screw, L, and tangent screw, R, the former to fasten it to the arch, m, and the latter to give a slow motion, and adjust the box for the observation.

The transit telescope, O P, is supported over the instrument by two pillars, N, N, fixed on the brass plate, E E, and having small frames or boxes, f, at the top, for the reception of the Y's, in which the pivots of the axis, Q, of the telescope are supported; this axis is conical, and is fixed exactly at right angles to the tube, Q P, in the same manner as other transit instruments.

On the extremity of one of the pivots of the axis, a small divided circle, R, is fixed, and has an arm at the opposite side turning about on the centre, and provided with verniers to read the divisions on the circle; to this arm a small level, S, is attached; the whole forming a flying index to set the telescope at any required altitude, by setting the index at the proper division, and then moving the telescope till the bubble of the level, S, shews the index to be horizontal; the eye-piece, h, of the telescope has a small dovetail in it to admit a dark glass for observation of the sun; g, are the screws for adjusting the wires in the eye-piece as usual; the separate figure, TV, is a level to adjust the instrument.

Manner of adjusting the instrument previous to observation.— First place the feet, k, l, of the level, upon the brass plate, EE, in different directions, and bring it level by the screws, H, I, K; then apply the level upon the two pivots of the transit, the covers VOL. II. N° II,

G

of the boxes, f, opening with hinges for this purpose, and the bottom of the feet, k, l, have notches to rest upon the pivots; if the axis does not prove level, one of the Y's must be elevated or depressed, by a screw in the farther pillar, N, until the level stands horizontally, and, reversing it end for end, proves every thing to be correct.

Now set the vernier, D, at zero, and put an additional object glass, over the glass, P, of the telescope; and in this state the marks near A A, against which the needle reads, can be seen through it when directed to them; this proves the zero of the division to be exactly in the plane of the telescope motion; a small screw near Q will rectify it, if necessary, by moving the Y a small quantity in the box, f; the transit may be reversed in its Y's to rectify every thing.

The telescope is now to be adjusted to the meridian by the transit of the stars in the usual manner, the screws, W, X, being used to turn the whole instrument round when requisite; but after these observations have been once made in a satisfactory manner, distant marks should be set up, both north and south, and these will give the means of adjusting it at once, and in the day time.

When the instrument is properly placed, the taking of the observation is extremely simple. The needle being suffered to settle, the box is turned about on its centre till the mark comes very near the point of the needle; the clamp screw, L, is then fastened, and the screw, k, is employed to make the coincidence exact, in which the microscope, M, greatly assists the eye. The vernier, D, now shows the angle of variation. It is proper,

after the needle has been once observed, to attract it with a piece of iron, and cause it to make a slight vibration. When it settles again, it will not in all cases come to the same point, because of the friction of the suspending point; it is therefore advisable to make this trial three or four times, and take the mean of the whole.

The instrument is placed on a pillar, in the open air, free from any iron, and is adjusted before every observation; the vernier is divided to half minutes, and if the meridian be not at a considerable distance, an advantage will be gained by diminishing the aperture of the object-glass by a pasteboard cap. I have found the needles should not exceed the weight mentioned, 65 grains.

It may be worth while to remark, that this instrument will serve as a portable transit instrument for naval officers to examine the rate of their chronometers while on shore.

ARTICLE IV.

On an easy Method of procuring a very intense Heat. By A. Marcet, M.D. F.R.S. Physician to Guy's Hospital.

(To Dr. Thomson.)

DEAR SIR, Russel-square, July 8, 1813. AMONGST the various improvements which have assisted the progress of natural science within the last twenty years, that of carrying on experimental researches upon very small quantities of matter, ought, perhaps, to occupy the first place. The advantages which often result from a minute scale of operations are too obvious to require any comment; and indeed the philosophical records of this country have, of late years, afforded innumerable instances of the utility of this mode of investigation.

You may, perhaps, think it worth while to point out to the readers of your valuable Journal an easy and convenient method, which occurred to me about a year ago, for producing, upon a small scale, a degree of heat which has, I believe, never been exceeded in intensity, unless it be by the magnificent voltaic battery lately exhibited by Mr. Children. The process in question simply consists in urging the flame of a lamp of spirit of wine by a current of oxygen gas. The apparatus most convenient for this purpose is a tin vessel, or gas holder, such as represented in Plate IX. Fig. 2, from which a small jet of oxygen gas is forced out, with some degree of violence, by the introduction of water, through a funnel of one or two feet in length. If a diamond be exposed to the flame of a lamp, thus acted upon by the jet of gas, it burns, and disappears in a few minutes. Platina wire of moderate thickness is instantly melted, and globules of this metal, weighing as much as 4 or 5 grains, can, thus be obtained in quick succession.* During this process of fusion, a scintillation of the metal is observed, as if it was undergoing combustion; but this appears to be owing to minute particles of melted platina, which are simply dispersed by the intensity of the heat. Iron wire is burnt by this means with a degree of rapidity and brilliancy which even exceeds that of Ingenhouz's striking mode of burning iron wire in oxygen gas; and small needles of quartz are readily melted and vitrified by the same means.

I am not aware of this mode of excitement having been employed before. Intense degrees of heat had long been pro

*It was from globules obtained by this process, that Dr. Wollaston succeeded in drawing platina wire of extreme minuteness, as stated in the paper which he has lately given to the Royal Society on the subject.