its vicinity. But it is remarkable that these currents never appear to extend more than twenty leagues beyond the common deep-sea soundings, while their velocity is much decreased when near the shore; from which it may be understood that the depth is much diminished, and the stream broken by projections of bank and sand.

Ships are frequently carried to the westward, quite round the Cape of Good Hope, even against the strongest north-west gales by this current.

Captain Owen then proceeds to state the dangerous nature of the short though high waves produced by the currents and wind being in opposition, and the most effectual course by which the danger may be avoided. It is quite clear that everything here stated is strictly in accordance with the theory here advanced. He bears witness to the constancy of the current from east to west; and in other parts of his work, when treating of the east coasts of Africa, and those of Madagascar, he mentions the rapid nature of the currents passing down from the northward towards the Cape, by which, in one instance, the Leven, in making the point of Mombas, was driven so far to the southward, that it took her six days to regain what she had lost by the failure of the wind for about three hours.-Vol. ii. p. 150.

It is known also that, off the Cape, ships have been driven to the westward, at the rate of sixty or seventy miles per day, even against a strong westerly wind.

The only part of Captain Owen's statement which in any degree stands opposed to what is now advanced, is the allusion to the constant currents at Cape Horn. These are not stated to run to the eastward, or into, instead of out of the Atlantic, but that fact is implied by the theory of evaporation from the Atlantic, which is counterbalanced by entering currents at both capes. This is opposed by the general reports of the navigation of Cape Horn; it is opposed also most distinctly by the much better attested facts of currents out of the South Atlantic towards the north. For if evaporation took place on so great a scale as to produce entering currents at the two great capes, we must admit that an entering current should also flow ̧ from the colder latitudes of the north, which is not the case. As to the fact of the current at the Cape being little felt close in shore, and gradually diminishing in force as it extends to the open ocean a hundred miles or more from land, it is in every way consistent with the whole theory of inland rivers. In the case of a projecting bank or rock in a river, the actual point of contact is exposed to great violence, but every other point of the stream exhibits the phenomena described by Captain Owen off the Cape. Under the most projecting rock or point comparatively smooth water is generally found close to the side; while the main body of the stream drives past, with a distinct and rippling outline, diminishing in force, however, as it spreads out into the expanding pool below*.

We have thus passed in review the great and leading course of this wonderful and most admirable system by which the waters of the ocean are kept in that continued movement so necessary to their purity, and by

Major Rennell gives many interesting instances of bottles and other bodies carried by the currents. In one case a bottle was thrown overboard from the Osprey, of Glasgow, on the 17th January, 1822, in 6° 13′ south latitude, and 15° 35' west longitude, and it was found on the 27th July, of the same year, in Mayard Bay, in the island of Trinidad.

In another case, still more remarkable, a bottle was thrown from the American ship Lady Montague, on the 15th October, 1820, two leagues north-east of the island of Ascension, and was picked up on the west coast of Guernsey, the 6th August, 1821, and notice of it sent to the Admiralty. It is certain that this bottle must have passed, in ten months, over the whole course of the Gulf-stream, and from thence been carried (probably by the coasts of Iceland) into the North Sea, and through the English Channel. We cannot, however, decide from this, or almost any instance of floating bodies, as to the rapidity of the current; for we cannot tell how long it may have been detained at various points, nor how long it may have remained on the spot where it was eventually discovered.

which also it is highly probable that many important ends are effected, in regard to the amelioration of the climates of various parts of the earth. The land and sea-breezes of the hotter climates are now well known and also their causes; we may naturally suppose this wholesome interchange to be powerfully affected by streams of current from the cooler latitudes; and we also may be assured that the heated waters of the Gulf-stream must carry along with them into the Frozen Ocean a degree of warmth which cannot but materially affect the rigidity of those latitudes. Even in our own country we are well aware, from continued experience, of the mild effects of a westerly wind. We have no particular warmth to look for from the lands to the westward of us ; on the contrary, the winters of Labrador and of Canada are well known to be unusually severe. But when we find that a vast reservoir of heated water, and consequently of warm vapours, exists in the Atlantic, we can no longer find a difficulty in naturally accounting for the mild and humid effects of our westerly winds, which, even in winter, produce on Ireland, and the west coast of Britain, the verdant growth of a milder season.

It is scarcely necessary, in conclusion, again to revert to the theory of the winds being the prime movers of the currents; for besides the arguments already adduced, by which, we trust, it has been shown that oceancurrents could not but exist, even if there were no winds whatever, we have only to examine the numerous instances mentioned even by Major Rennell, of ships being drifted far to windward, in the very teeth, not of transient breezes alone, but of settled and heavy gales. One ship," says he, was carried 10° of longitude (equal to 570 miles) to the westward, between Cape Verde and the Cape of Good Hope, and yet had been subjected to the south-east trade-wind. Another was driven 220 miles, between the Canaries and the coast of Brazil. Another, in the equatorial current, in June and July, was set 297 miles to the westward, in five following days, between 30 north and 41° south, and yet had entered the south-east trade-wind." Such, and numerous other instances, well known to all seamen, are sufficient to show that the currents must be set in motion by some much more powerful and less superficial cause than the mere friction of the winds, however fixed or severe. That the winds agitate the surface of the waters no one will attempt to question; but that this agitation can extend to the vast depths, at which the law of fluids above explained must operate, we have not the slightest reason to suppose. Major Rennell brings forward, in proof of his theory, the well-known fact, that the surface of a canal, or of a lake, is always higher at the leeward than at the windward side. This fact is at once admitted, but it is one of very small effect, and merely superficial, being occasioned by waves, and instantly subsiding with these waves. But in order to prove the point, it must be shown, that in a straight canal of several miles in length, with a strong breeze right on end, the force of the winds, near the middle of the distance, (where they must have acquired their full force,) can affect an object of no great weight at the bottom of the canal, and at a depth of four or five feet. If this effect takes place in canals, or in large inland lakes, such as those of North America, and also at considerable depths, the theory might be supposed to derive some support from it. But this is not the case; and in inland lakes, of whatever extent, although the surface may be raised on the leeward side, in violent winds, objects deposited at a few feet of depth lie perfectly secure and unmoved.

The winds would not, therefore, effect the end for which the great circulation of the waters of the ocean is obviously intended; and any theory of the currents, which is mainly founded on so false a ground, however ably it may be treated, cannot but mislead the mind, and in many instances prove injurious, not only in a scientific, but also in a practical point of view.

RANGES OF ORDNANCE.

A PERFECT knowledge of the power of projection, peculiar to each variety of ordnance, is much required; for it cannot reasonably be expected that gunnery will ever arrive at the precision it should attain, unless practical gunners are aided by correct experimental ranges of the different pieces they may be appointed to serve.

This want of information may be ascribed to the difficulty of obtaining it; since, for the purpose of determining the ranges of large ordnance, a horizontal plane of considerable extent, and of easy access, is necessary. There is not, probably, any place inland that is so available, or that affords so good a means to obtain the object in question, as some parts of the seacoast; for at low water, an extensive horizontal plane can be readily found; and should there be any obliquity in the plane, it can be easily detected by the level of the sea; which also furnishes the best guide in adjusting the elevation of the piece under trial. These matters considered, the following method is proposed for the determining of ranges of ordnance :

Let a vessel of large scantling be strongly-built, of a tonnage sufficiently great to float with the heaviest description of ordnance; the floor to be flat, upon which let a platform be fairly laid; breadth of beam and length adapted to a draft of water at 3 or 4 feet.

Make choice of a part of the sea-coast that is suited to the purpose, (taking care to choose such as will afford a soft bed for the vessel to ground upon,) measure the plane at low water, and mark it at every hundred yards, or any convenient distance, by driving in iron rods, each with a distinguishing mark, so that they can be easily discerned. In some following tide, at slack water, and at the same instant of time, mark the level of the sea at the extreme and middle stations, by which the obliquity of the plane, should there be any, can be ascertained *. Thus prepared, let the vessel, with the piece for trial, be brought, at a convenient time of tide, to the spot chosen for her position; and when she grounds, bed her so as to bring the platform horizontal. The trial may then commence, and be continued till interrupted by the rising water.

If the piece be correctly bored and perfectly symmetrical, having the same thickness of metal between the bore and outer surface throughout any section directly across its axis, the correct method of ascertaining the direction of the axis would perhaps be, to erect, perpendicularly, a graduated scale before the piece. The difference of the semi-diameters at the base and muzzle-rings of the piece being known, and the distance of the scale from the notch on the base-ring being also known, it will be an easy matter to point the piece horizontally, or at any angle of elevation that may be required; since it will be necessary only to compute the height on the scale at which the aim should be directed t. Horizontal lines having been already determined by the level of the sea, to which the necessary adjustments can be referred.

The assistance of several persons, besides those who serve the piece, will be required to note the grazes of the projectile, time of flight, and any other incident.

I. H.

*Perhaps there is not any coast more convenient than that lying between Deal and Ramsgate, known by the name of the Sandwich Flats or Pegwell Bay.

When the piece is pointed, the scale may be taken down till the piece be discharged and ready again to be pointed.

NEW INFANTRY EXERCISE-COMMANDING GROUND, &c.

As the Infantry Exercise Regulations, just published, have with much propriety directed commanding officers to practise their young officers and non-commissioned officers in the important duties of selecting ground or positions on which they and their detachments are to defend themselves; as well as the most prudent and expeditious methods of attacking posts so chosen, &c., it may not be amiss to give young officers some idea of what, in a military point of view, may be considered as properly falling under the denomination of commanding ground.

It must be very obvious, that to have our works or position overlooked from any point in possession of an enemy, however distant it may be, is a positive evil. It enables him, by seeing our measures, of either attack or defence, to shape his counter-operations accordingly. It is a maxim in war, to keep the enemy at all times in the dark.

The operations carried on for our reception within the castle of Badajoz, were, during our second attack, seen from Fort La Lippe, at Elvas, by means of a powerful glass. Elvas is twelve or fourteen miles from Badajoz. Had we continued our attack of this front to completion, there cannot be a doubt that our knowledge of what we had to encounter within the place, in the shape of retrenchments, &c., would have been of the greatest advantage to us. If it be bad to be overlooked, therefore, it is evidently much more so to be both overlooked and commanded at the same time. The distinction is this:-in the former case we are merely seen into; but, in the latter, we are not only seen into, but we are also posted within the range of the enemy's shot. This is a very critical situation to be placed in. The castle of Badajoz was seen into, but it was not commanded, from Fort La Lippe. It was, however, both seen into and commanded from Fort Christoval, which was within 500 yards of it, on the other side of the Guadiana. There is not, it should be kept in mind, in any fortress or position, a point so weak as that which is overlooked within the reach of shot.

"Since the time of Vauban, the battering artillery," says Sir T. May, "has been improved fourfold, and the gunpowder fully double." We have been hitherto in the habit of allowing from 250 to 300 yards as the range of a soldier's common musket in action; and 600 yards to the point-blank range of a 24-pounder, with a full charge of powder.-It has been found that a 24-pounder will not batter, with effect, at a greater distance than 800 or 1000 yards.

After this, therefore, in so far as regards battering, all command, from whatever quarter, may almost be considered harmless. It is stated by Colonel Jones, however, (in his Journal of Sieges, vol. i. p. 481,) that the fire of the iron 24-pounders, in No. 11 battery, placed on a commanding height against the castle of St. Sebastian, and which took the high curtain of the land front, en écharpe, at the distance of 1500 yards, repeatedly struck the terreplein with effect. From this, then, it would appear that 1500 yards should be regarded as the maximum of the effect of shot.

There exists, in fact, but little or no difference between the force of shot fired from a practicable elevation, and that fired from a field-piece on a dead level. It is well known to military men, that artillery, firing from an elevated situation on bodies of troops, is less destructive than when firing on the same level. In the former case, the shot can hardly hit more than one or two men; whereas it has been ascertained, that one single horizontal or rezant shot has killed forty-two men in a close column.

Height of situation, we must notice, often affords great facilities to the operations of the miners, where the guns cannot be sufficiently depressed to see their base, and there is not any contiguous point to establish flanking works:-otherwise a fortress or position so situated is desirable, as not being liable to surprise. If not too precipitous, the enemy's movements and operations may be seen from it; and it invariably gives a most destructive U. S. JOURN., No. 61, DEC. 1833.

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effect to the fire of musketry on the "approaches" of an enemy. Works constructed on heights have, in a peculiar degree, also the excellent defensive property of having their scarps, palisades, and defenders more effectually covered than works that are on a plain; and when the rise of the height is rapid, it precludes the application of ricochet firing.

Command is far more prejudicial to the defence of those works or positions whose prolongation it intersects than to those to which it is parallel.—Whoever is master of Mont Feron, for example, may be considered as in possession of Ath.

The effects of being commanded from high ground in our immediate vicinity, however, may be greatly counteracted by a judicious disposition of field-works, &c., and their defilement. But this cannot well be explained without a diagram. Young gentlemen should therefore TAKE ADVICE in regard to these very essential matters. WM. TAIT,

Late Royal Staff Corps, and Teacher of Fortification.

Egerton's Library, Charing Cross.

PARABOLA FROM A CIRCLE.

MR. EDITOR,-The following simple method of producing a parabola from a circle, may perhaps amuse some of your readers.

I have the honour to remain, &c. &c.

m.

ALFRED BURTON, Captain R. M.

e

с

a

On any diameter a e, describe a semicircle; and draw at any distance from each other, any number of ordinates bf, c g, dh, &c. In the present instance they may be supposed equidistant.

Produce bf to i, making bi-a f.

In like manner produce c g to k, making e ka g.

&c.

Then will a, i, k, l, m, be points in the curve of a parabola. Because as abac af2: a g3, &c.,

by the circle. And as

by the parabola.

abac

bi: ck2, &c.,

And b, is the focus, because a b=} bi.

This circle and parabola are both sections of an equilateral cone: their common vertex a, being in the centre of its side.

The triangles a fb, a g c, &c., are to the triangles a ib, a kc, &c., respectively, in the sub-duplicate ratio of their corresponding exterior abscissæ be, ce, &c., to the entire axis a e.

That is to say, as

And as

be: Nae:: ▲ afb: ▲ a ib. √ce: √ae:: Aagc: A akc. &c. &c. &c. &c.