the following terms:-"Ibi" (Lutetiæ) "vixit ab anno 1666 ad annum 1681. Durante hoc tempore pulcherrima subtilissimaque multa in mathematicis detexit variaque ex iis operibus conscripsit quæ nunc in unum corpus collecta quid in variis Matheseos partibus præstiterit sub oculis ponunt. Præter ipsius jam memorata inventa præclara inter alia duo insigni usu eminent. Libellam telescopio munitam ita construxit ut ipsi præ ceteris fides haberi possit," &c.

The honor of having first applied the air-bubble to the determination of horizontality, seems to be due to that universal genius Dr. Hooke. From all that I can gather, it appears that his invention must have been made subsequent to 25th March, 1674, and prior to the year 1675, as, in his "Attempt to prove the Motion of the Earth by Observations," of date 25th March, 1674, he describes a new method of stilling the plummet, by immersion in water. While in his animadversions,* published also in 1674, after fully describing his invention of the air-bubble confined in a tube, he speaks of its peculiar advantages, and great delicacy of movement, and remarks,"This can hardly be performed by the ordinary way of plummets, without hanging from a vast height, which is not practically to be performed without almost infinite trouble, expense, and difficulty," &c.

Hutton, in his Mathematical Dictionary, remarks, that the application of the air-bubble to the level, "is said to be due to M. Thevenot," but with what justice I cannot say, having been unable to meet with any reference to this instrument in the writings of that author. Thevenot was born in 1621, and he died in 1692.

I have been unable to discover who was the inventor of the circular level, which, I imagined, had been of recent date; but Switzer, at page 91, of his Treatise on Water-Works, which was published in 1734, remarks, that the circular level was then employed in the construction of the surveying instrument called a plane-table.

According to Sir John Herschel, the cross-hair, which gives so much accuracy to all astronomical, as well as leveling, instruments, was the invention of Gascoigne, a young Englishman, who used it in 1640. He was killed, at the age of 23, at the battle of Marston Moor.

M. Le Biont appears to have been the first to conjoin the telescope of Huygens with the air-bubble of Dr. Hooke; and this must have been subsequent to the year 1684, as such an instrument is not shown in De la Hire's edition of Picard's Treatise on Leveling.‡

But it was not till Sisson's improvements that the level could be considered as in any way an accurate, or philosophic, instrument.

• Animadversions on the first part of the Machina cælestis of the Hon., learned, and deservedly famous Astron. Johannes Hevelius, Consul of Dantzick, together with an explication of some instruments made by Rob. Hooke, Prof. Geom. in Gresh. Coll., and F.R.S. Lond. 1674, p. 61, et seq.

Traité de la Construction et des Principaux usages des Instrumens de Mathematique. Par N. Le Bion, Ingenieur du Roi pour les Instrumens de Math. Nouv. Edit. a la Haye, 1723.

+ Traité du Nivellement Par M. Picard, mis en lumière par les soins de M. De la Hire, 12mo. A Paris, 1684.

All that were made previous to this time were coarse instruments, adjusted by a ball and socket, and, in other respects, resembling the common perambulatory survey level, which, from the nature of the construction, can be leveled in only one direction, and cannot be reversed, or moved, even in the slightest degree, without requiring re-adjustment. Sisson may, therefore, be considered as the inventor of the instrument in common use. The main feature in his improvements was the introduction of the four screws called the parallel plate screws, (D, in the diagram.) I have been unable to find out the date of Sisson's improvement; and, indeed, the only notice I can find of him is the following in Switzer's System of Water-Works:-"The invention," alluding to the instrument with parallel plate-screws, "as I take it, (for I am not as yet well acquainted with that gentleman,) of William Sisson, at the corner of Beaufort Buildings, in the Strand."*

Since the time of Sisson, the celebrated Ramsden introduced a tangent screw and clamp, for moving the instrument with accuracy through small distances in an arimuthal direction. Messrs. Troughton and Simms also made several improvements in the arrangement of the various parts of the instrument; and Mr. Gravatt, has, of late years, added a cross-bubble for facilitating the rough-setting of the instrument or that adjustment which is made with the legs of the tripod; and an enlargement of the diameter of the object-glass, so as, by the admission of a greater number of rays of light, to allow of the telescope being shortened, without impairing its optical powers.

Having thus endeavored to describe the successive changes which the level has undergone, I shall now proceed to notice the nature of the present improvements.

The first of these is the substitution of a circular, or, to speak more correctly, a spherical level, G, sluggish in its motions, instead of the small cross-level, which was introduced by Mr. Gravatt. The advantage of the circular level over the common form, is its peculiarity in at once showing the deviation of the instrument from horizontality in both directions, instead of only one.

Before describing the next improvement, it may be proper to state, that the clumsiness of the common level consists in its being at all dependent on the setting of the legs. This arises from the circumstance of the ball and socket motion, K, being controlled in its action by the parallel plate screws of Sisson, D, the consequence of which is, that, in using the common level, care must be taken to set the instrument very nearly level by the eye, so as to be within the range of the parallel plate screws, D, otherwise it is impossible to adjust the instrument. And, although to the practical man, the trouble attending this may be comparatively small, still he will admit that it is one of the most irksome parts of the whole operation of leveling; to say nothing of the time that is lost in adjusting the instrument afterwards, with the parallel plate screws. What appeared to be wanting was a motion for the preliminary, or rough-setting, intermediate in nicety between those of the parallel plate screws, and of the legs. In order

* An Universal System of Water and Water-Works; by Stephen Switzer, 2 vols. 4to. Lond. 1734.

to gain this end, a ball and socket motion, F, having a clamp, N, is introduced in addition to the ball and socket, K, whose action is limited by Sisson's parallel plate screws, D, so that my improved level has two ball and socket movements.

A B, is the telescope; C C, the compass box; M, the screw for adjusting the focus; HH, the tubular spirit-level; G, the spherical, or circular, level; D D D, the parallel plate screws of Sisson; K, the old ball and socket motion; F, the new ball and socket motion; N, clamping screw for socket motion.

With the instrument thus improved, the observer is made quite independent of the level of the ground where he sets the legs of his instrument, and may place them without regard to the inclination of the telescope to the horizon. Looking first to the circular level, G, and releasing the clamp, N, of the ball and socket, F, he, with one hand, moves the head of the instrument till the bubble is in the centre of the circle, an operation which is done almost instantaneously.*

• In the annexed plan the instrument is shown off the level, so that neither the air-bubble of the circular level, G, is in the centre of the circle; nor does the air-bubble in the tube, H H, correspond with the file-marks made on the glass.

The socket screw, N, is then clamped, and the telescope bubble H H, is brought to the absolute level by a slight touch of the parallel plate screws, D. In this way the legs of the tripod never need to be moved after the instrument has been placed on the ground, and the parallel plate screws have almost nothing to do-advantages which all who are accustomed to leveling will fully appreciate.

In leveling over mountainous districts, it very often happens that it is desirable to select a station where the ground is so rugged and precipitous as to render it difficult, if not impossible, to find three points for the extremities of the legs of the instrument to rest on, which shall be on such levels as to bring the telescope within the range of the parallel plate screws; but wherever the instrument can be made to stand with safety, the bubble of the improved level can be adjusted, and adjusted in exactly the same time, and with exactly the same ease, as if the instrument were placed on level ground.

Another advantage of these improvements is the removal of a great practical difficulty which is often experienced on sloping ground. The instrument being set and properly adjusted, the observer, on looking through the telescope, may discover that he is not within the range of the leveling staff; in other words, he has chosen a station. too high, or too low, to admit of his seeing any part of the staff within the field of the object-glass. The only remedy for this is to choose a new station where the instrument must be again set up and levelled, at a great expense of time and trouble. In order to remedy this, it was my intention at one time to have fixed on the telescope a French level, on the principle of the plummet, in order speedily to discover, before making the adjustments, whether the intended station were within the range of the staff, or not. But the instrument can be roughly set with so much quickness, by means of the additional ball and socket, that the French plummet may be considered as being now scarcely necessary.

In my letter to the Secretary of the Institution of Civil Engineers, I pointed out the advantages which would result to the surveyor, were the theodolite provided with a second ball and socket motion; but no opportunity of trying this has as yet occurred.

Edinburgh New Philos. Joura.

Mineral and Metallic Statistics.

By the returns to five several orders made by the House of Commons, which were obtained by the exertions and perseverence of Sir J. J. Guest, Sir C. Lemon, and Mr. Evans, (M. P. for North Derbyshire,) we are enabled to lay before our readers a most correct account of the various exports and imports of iron and iron ore, hardware, cutlery, &c., copper ore, copper, tin, zinc, lead ore, and lead, for the year ending January 5, 1844.

Commencing with iron, it appears there was imported in the year, iron ore 131 tons, chromate of iron 1393 tons, pig-iron 243 tons, un

wrought iron in bars 12,795 tons, bloom 563 tons, rod-iron 12 tons, old, broken, and cast-iron 286 tons, cast-iron only 8 tons, steel unwrought 1697 tons-of these, 97 tons only were entered by weight, the remainder by value, 11,035l. 6d. 9d. Of the several countries from which these importations came, the principal is Sweden, whence we have received of iron 10,909 tons, and steel 1558 tons, leaving but a small portion to divide between twenty other places. Our exports of foreign iron have been, unwrought in bars 3986 tons, rod 10 tons, hoops 2 tons, cast-iron 11 cwt., steel, unwrought, 1456 tons. The total quantity of foreign iron retained for home consumption was 14,782 tons, upon which the net amount of duty was 14,5631. The exportation of that staple produce of our own country, British iron, was as follows:-bar-iron 176,148 tons, bolt and rod 22,625 tons, pigiron 154,770 tons, cast-iron 16,449 tons, iron wire 1508 tons, wroughtiron, consisting of anchors, grapnels, &c., 3058 tons, hoops 14,591 tons, nails 6020 tons, and all other sorts, except ordnance, 44,577 tons, old iron for manufacture 5924 tons, and unwrought steel 3199 tons. Those places which have taken the greatest portions of this produce are Russia 10,963 tons of bar-iron; Denmark 10,447 tons bar, and 7010 tons pig; Prussia 12,009 tons bar, 17,480 tons pig; Germany 13,298 tons bar, 6322 tons pig, 1339 tons cast; Holland 17,509 tons bar, 75,953 tons pig, 4317 tons cast; Belgium 4279 tons cast; France 4237 tons bar, 22,103 tons pig; Italy 21,930 tons bar, 3982 tons bolt and rod, 3005 tons pig; Turkey and Continental Greece 6412 tons bar; East Indies and Ceylon, 20,620 tons bar, 2967 tons bolt; British North American Colonies 6837 tons bar, 1995 tons cast; foreign West Indies 5043 tons bar, 1646 tons cast; and to the United States 21,336 tons bar, and 7148 tons pig. The largest quantity of unwrought steel has been to the latter place, viz., 1336 tons.

Of British hardware and cutlery, we exported, in the year, 17,183 tons, valued at 1,745,5187; the principal of which has been, to Germany 1237 tons, value 159,8897.; East Indies 1402 tous, value 142,6077.; British North American Colonies 1129 tons, value 102,260/.; British West Indies 997 tons, value 80,0407.; foreign West Indies 657 tons, value 48,6097.; United States 4282 tons, value 448,3417.; Brazil 943 tons, value 80,0707.; and divers other places, varying from 100 to

500 tons.

We now come to copper. Of foreign copper ores we have imported 55,720 tons; and of metallic copper, unwrought and wrought plates, and coins 805 tons. Of the ores, the greatest quantities have come from Cuba and Chili; from the former 31,683 tons, and the latter 19,829 tons; 54,370 tons of this ore has been retained for home consumption, the net amount of duty received thereon being 64,4457. 3s. 5d. The total of British copper exported, from twenty-three ports, was 17,515 tons, of which London sent 6469 tons, Swansea 4502 tons, and Liverpool 5587 tons; France took 4891 tons, the East Indies 6125 tons, and the United States 1473 tons. The total quantity of foreign copper exported was 672 tons. Of the above 17,515 tons of British copper, 8463 tons were unwrought, in bricks and pigs; 60 tons of coin; sheets, nails, &c., 8386 tons; wire 6 tons; and other sorts