repeated for fome fucceffive evenings, or while any of this fine effential oil rises to the surface of the water.

N. B. I HAVE been informed that fome few drops of this ef sential oil have been more than once collected by distillation, in the fame manner as the effential oils of other plants here in London. I am,

SIR,

Your most obedient humble fervant,

D. MONRO.

III.

III. DESCRIPTION of a MERCURIAL LEVEL, invented by ALEXANDER KEITH, Efq; F.R. S. & A. S. EDIN.*.

F

BB

IGURE I. is a fection of the inftrument formed of mahogany or boxwood.. AA are two oblong fquare cavities connected together by a narrow clofe channel, running from the bottom of the one to the other. B B are two grooves hollowed out of the wood, in order to contain the fights, &c. They are shut up by a lid, which turns upon a screw-nail at the centre C, as may be feen more diftinctly from fig. 4.

FIG. 2. DD are the two fights, the one with a small hole, the other with a cross-hair. These fights are erected upon two pieces of ivory or hard wood, which are shaped nearly of the dimenfions of the cavities A A, but fo much fmaller as to enter without touching or rubbing on the fides. Mercury is poured into the two holes A A till they are about half full; the two pieces of ivory which support the fights, are put into the cavities, and float on the surface of the mercury.

FIG. 3. is a perspective view of the inftrument when the fights are floating upon the mercury; and fig. 4. is another view of it, when the fights are taken out and the lid is open.

As the two cavities communicate with each other, the furface of mercury in both are always upon the fame line of level; and consequently, if the two fights are once accurately adjusted, they will ever after point out the true level, without requiring any after adjustment.

WHEN this inftrument is to be used, it may be laid on any horizontal surface, and the fights will immediately become an exact

*This Paper was read before the Philofophical Society of Edinburgh in December 1778; and is now printed by order of the Committee for publication of the Transactions of the Royal Society of Edinburgh.

exact level. It may also be fixed on a tripod as the fpirit-level; or it will answer equally well, if it is affixed to the top of a fingle ftake, which is sharpened at the point fo as to be pushed into the ground. If it is to be used as a pocket-instrument, it may be made of feven inches length, being about double the dimenfions of the annexed draught. A common walking cane forms a very convenient fupport. It is affixed to the cane by means of a brafs pin E, which paffes through the hole G, and through the eye or hole of the walking stick; and a brafs nut F, fcrewing to the male-fcrew of the brafs pin, keeps them firm together. The two grooves BB, contain the two fights and brass pin, when not in ufe. Two corks, covered with thin leather, fitted into the holes A A, confine the mercury, when the inftrument is to be tranfported; or, in cafe the mercury is found to escape, it may be poured into a small cafe, made of lignum vitæ, like a tooth-pick cafe; and this may be stopped with a cork, and made to fit into one of the grooves.

༣

THE advantages of this inftrument over the spirit-level are: , It requires no adjustment, confequently two obfervers, though otherwise not equally accurate, must make the same obfervation. 2dly, With this, the level of twenty different places: may be taken during the time required to adjust the spirit-level for one obfervation. 3dly, The nicety of the spirit-level depends upon the small curve of the glass-tube, in the choice of which no rule can be laid down; neither is any thing gained, in point of exactness, by lengthening the spirit-tube above three or four inches. But every instrument of this kind is of one standard; and the further the two fights are removed from one another, the more any error is diminished. 4thly, This inftrument can be made perfectly juft, without taking any observation, or com paring it with another level. In order to do this, let the floats on which the fights reft, be of the fame dimension and weight, and let the cross-hair and eye-hole be of one height, and, without farther adjustment, they will point out the true level.

THE.

THE following is a proof of the exactness of this method. JOHN MILLER, the mathematical instrument-maker, has a line drawn upon the oppofite fide of the Parliament-fquare, fronting his shop, by which he has been in use to adjust his spirit-levels. We placed the mercurial level upon the spot known to be upon an exact level with the line. Both he and I looked through the fights; but could not perceive the line. We fufpected there was fome fault in the fights; but, on making them vibrate, we found that the hair had covered the line; fo foon as they fettled, the line was again covered by the hair.

WHEN there is a strong wind, the fights vibrate too much. In order to remedy this, there is a case of tin'd plate or pasteboard made to inclose the instrument when not in ufe. Vid. fig. 5. When used, the cafe covers only about one half of it, leaving room for the fights to float within the cafe. There are two oval holes at each end of the cafe through which the observations are made.

IV.