letters by its lateral spread, at the same time that the coin receives the blow of the screw-press.

Coins are generally completed by one blow of the coiningpress. These presses are worked in the Royal Mint by machinery, so contrived that they shall strike, upon an average, sixty blows in a minute; the blank piece, previously properly prepared and annealed, being placed between the dies by part of the same mechanism. Of these presses, and of the other machinery and operations used and carried on in the Mint, an excellent account, with illustrative engravings, has been given by Mr. Mushet, in the supplement to the Encyclopædia Britannica.

The number of pieces which may be struck by a single die of good steel, properly hardened and duly tempered, not unfrequently amounts at the Mint to between three and four hundred thousand, but the average consumption of dies is of course much greater, owing to the different qualities of steel, and to the casualties to which the dies are liable:-thus, the upper and lower die are often violently struck together, owing to a fault in the layer-on, or that part of the machinery which ought to put the blank into its place, but which now and then fails so to do. This accident very commonly arises from the boy who superintends the press neglecting to feed the hopper of the layer-on with blank pieces. If a die is too hard, it is apt to break or split, and especially subject to fissures, which run from letter to letter upon the edge. If too soft, it swells, and the collar will not rise and fall upon it, or it sinks in the centre, and the work becomes distorted and faulty. He, therefore, who supplies the dies for an extensive coinage has many casualties and difficulties to encounter. There are eight presses at the Mint, frequently at work for ten hours each day, and I consider that the destruction of eight pair of dies per day (one pair for each press), is a fair average result, though we much more frequently fall short of than exceed this proportion. It must be remembered, that each press produces 3600 pieces per hour, but, making allowance for occasional stoppages, we may reckon the daily produce of each press at 30,000 pieces; the eight presses therefore will furnish a diurnal average of 240,000 pieces.

122

ASTRONOMICAL AND NAUTICAL

COLLECTIONS.

i. Table of the DEPRESSION of MERCURY in the TUBE of a Barometer, including that of Mr. BOUVARD (Conn, des

The numbers inclosed within parentheses are derived from an empirical interpolation.

If we compare this table with that which was published in the Connaissance des Tems for 1812, and thence in the Philosophical Magazine for February 1816, we shall be disposed to admire the perseverance of Mr. Laplace and Mr. Bouvard, in still adhering to the very inartificial mode of calculation which was first employed by Dr. Young in this country, but rejected by him as soon as he had discovered a more correct, though an equally laborious method of solving the problem.

The calculations of Dr. Young and of Mr. Ivory, as published in the Supplement of the Encylopædia, articles COHESION, and FLUIDS, are completely independent of each other, and their errors appear in general to be of an opposite character; the greatest difference of the results, for the depression, is .00031, or less than the three-thousandth of an inch, while the approximation of Mr. Bouvard differs from both

of them in one case as much as .00218, and in almost every case much more than they differ from each other.

After all, however, it is only in one or two of the cases computed, that any of the differences amount to one thousandth of an inch, and either of the tables may therefore be considered as sufficiently accurate for the practical correction of the height of the barometer. The error of Lord Charles Cavendish's experiments appears, in one or two cases, to have been ten times as great as this, which may have happened without any particular negligence in the observer.

ii. Remarks on the CHANGE of Inclination of the DIPPING NEEDLE. By Mr. ARAGO. Annuaire, 1827. p. 207.

At Paris the declination of the compass disappeared altogether in 1666, and, becoming westerly instead of easterly, continued to increase until 1819, when it was 221°, and has ever since that time been slowly diminishing.

The dip has continually diminished, from the date of the most ancient observations to the present time: it is now about 68°.

When the horizontal needle was found to be stationary, it "was natural to suppose," that the dipping needle would become stationary also, and that its inclination would afterwards be increased. But this conjecture has not been confirmed by observation; and the dipping needle, on the contrary, continues, as in times past, to approach nearer to the horizontal position. This is a circumstance which must be explained by those who attempt to investigate the cause, hitherto entirely unknown, of these singular changes.

[With all due deference to the opinion of so cautious and accurate a natural philosopher as Mr. Arago, the law, if not the cause, of these singular changes, may very easily be accommodated to the facts in question. For the simplest possible theory of the change of magnetism, is to suppose that the magnetic pole performs a certain orbit round the pole of the earth; and whether regular or irregular in its form, this orbit must, at some one point on each side, have for its tangent a great circle passing through the point of

observation. At the moment when the magnetic pole passes through this point, the declination or variation will obviously be a maximum, while the change of inclination will as obviously be greater than in any other part of the orbit, other things being supposed equal: since the pole will be moving directly to or from the place of observation. On the other hand, the change of declination will probably be the most rapid when the dip is the greatest, as it probably was in London and Paris, towards the middle of the 17th century, or a little later, the declination itself being inconsiderable, or somewhat westerly, because the nearest point of the orbit seems to be on that side of the true meridian of London.

It is singular that this useful little almanac should annually present to the public, among the "Epochs of the principal discoveries in Geography," that of the Canary Islands, by the Genoese and Catalonian navigators in 1345; while there seems to be no reason for doubting that these islands were well known to the ancients, one of them being described by Pliny under the name of Canaria. (vi. 32.)]

iii. Experiments on the effect of the direct WHITE LIGHT of the Sun in making Steel MAGNETIC. By Mr. A. BAUMGARTNER, of Vienna. Ann. Chim. Nov. 1826.

When I was repeating, in the course of last summer, the experiments of Mrs. Somerville, on the effect of the sun's coloured light in making steel magnetic, I discovered a process which appeared to succeed more rapidly and more certainly than that which Mr. Morichini and Mrs. Somerville employed. It led me to this conclusion, that a piece of steel, of the size of a common knitting needle, of which one or several parts are polished, and the remainder unpolished, when exposed to the direct white light of the sun, acquires the properties of a north pole at each polished part, and a south pole at each unpolished part. The process that I have employed is the following:

I took a piece of English steel, of the size of a common knitting needle, and heated it till it became covered with black oxyd; I then removed the oxyd at one or more parts, by rubbing it on a hone with oil, and finished the polishing