MINERAL WEALTH OF SOUTH AUSTRALIA. When Mr. Mengé published, some years ago, his catalogue of South Australian minerals, various degrees of credence were vouchsafed to the extraordinary list, a credence, in short, proportioned to the readers' acquaintance or otherwise with the talented and worthy, but somewhat eccentric author. Not only, however, has all that Mr. Mengé predicted respecting the metalliferous minerals of this colony been fully realised, but there appears to be no lack of precious stones, the existence of which here, was at one time, and that not long ago, looked upon as apocryphal. We have to-day to announce the gratifying intelligence (gratifying to every colonist and most fortunate for the worthy proprietor) that upon one of the sections in the centre of Flaxman Valley, the property of George Fife Angas, Esq., there has been discovered an astonishing combination of metalliferous minerals and precious stones. The metallic substances comprise veins of rich copper ore intersecting the land, and lead in several parts; whilst, in a large formation of hornstone, lic embedded, in considerable masses, the kind of opal called

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precious," as well as white and red cornelian, agate, and jasper. Asbestos, which is so frequently and variously met with in the district, is also found upon the property referred to, as well as white marble of considerable purity, and large quantities of excellent freestone. Neither is the great mineral feature of the province wanting in this locality. Iron in almost every variety is there, and in such manifest abundance that its quantity seems to be almost without limit. In short, after what we have seen, and been assured of, we may say in perfect truth and soberness, that if future iron-works, as vast as those which now operate and find their raw material in Britain, should one day be established here, or draw their supplies of ore from this province, the " iron-masters" will find inexhaustible stores beneath the millions of tons of surface mineral, which so frequently meet the eye of the mountain explorer.-Adelaide Observer.

SCREW-PROPELLING, NEW FORM OF SCREW.

Sir, I beg to hand to you a plan and description of a propeller, which has not yet been published, and which I conceive rossesses decided advantages over all those now in use.

It is a well-known fact, that to all vessels in motion there is a certain amount of after-current, proportioned to the speed at which they are driven, and to the fulness of their water lines abaft; this after current being greatest directly behind the sterr-post, and decreasing as

the distance from it increases. Such being the case, it follows that a propeller placed at the after-end of a vessel must, when the vessel is under way, be travelling through water which is passing it with a different velocity at different points in its diameter; for example, in a vessel having a very full run aft, the water might, at the centre of the propeller, be leaving the vessel at the rate of 5 miles an hour, and that, at the extremity of the blades, at 10 miles an hour. To meet this difference of velocity in the water, and to obtain an equal amount of end pressure on every square inch of its surface, the propeller ought to be made with a pitch gradually and uniformly increasing from the centre to the extremities of the blades. This difference of pitch would, of course, vary with every variety of form of vessel; being for very bluff vessels, perhaps as much haps only 5 per cent. as 50 per cent., and for fine vessels, per

Such an arrangement of propeller as this would, I am certain, produce a more useful effect than can be obtained by a propeller with a uniform pitch from the centre to the outside, as the centre parts of the blades of a screw of an uniform pitch lie so much in a line with the direction of the vessel, that they can only absorb the power of the engines by thrashing the water round.

In the annexed sketch, I have shown a common screw of an uniform pitch (fig. 1); and, for the purpose of showing more clearly the difference between it and the improved one, I have shown an extreme case of the latter, and made the pitch at the centre, half that at the outside. The diameters are supposed to be 20 ft., and the pitch of the common screw 20 ft. throughout; that of the improved propeller, 20 ft. at the outside, and 10 ft. at the centre part. The figures on the radial line show the pitches at those points. You will perceive that, in the improved propeller, the blade is radial only in the centre of its length, that is, on the line a b, and that at any other point it is a tangent to a circle whose diameter is proportional to the distance of that point from the radial or centre line. I am, Sir,

Your obedient servant, P. P. St. John's-wood, Oct. 2, 1845.

P.S. The reader will find a sketch in Tredgold's Appendix on Screw-propelling, which shows the after-current I have alluded to.

had, would be equivalent to the supposition that the forces applied to the body, being in equilibrium were capable of producing a dynamical effect; which would be a contradiction in terms. This, though not always clearly understood by practical men, or by persons superficially informed, is, in fact, among the fundamental principles of mechanical science.'

"Now, is it not strange that, though this sentence is extracted from 'Lardner upon Steam,' that the word 'momentum' should never be encountered, and that in all his arguments in his sixth edition against the possibility of crossing the Atlantic to New York, the advantage to be derived from increased 'momentum' should be entirely overlooked, as if it were not capable of producing a higher average velocity?

"Let me now quote the Doctor against himself, by citing a few lines from a work, in which he acknowledges to have had some share in the composition.

"From the preceding details it appears that, motion is not adequately estimated by speed or velocity. For example, a certain mass (A) moving at a determinate rate has a certain quantity of motion. If another

equal mass (B) be added to (A) and a similar velocity be given to it, as much more will evidently be called into existence. In other words, the two equal masses A and B united have twice as much motion as the single mass, A, had when moving alone, and with the same speed; the same reasoning will show that three equal masses with the same speed have three times the motion of any one of them.

"In general, therefore, the velocity being the same, the quantity of motion will always be increased, or diminished, in the same proportion as the mass moved is increased or diminished. '-Kater and Lardner on Mechanics, page 39.

"I will now illustrate the advantages possessed by the larger vessel over the smaller, by taking some case in point. Suppose therefore the two vessels be that beautiful little steamer Locust, and the Great Liverpool, the tonnage of the former is about 291, that of the latter 1560. Now it is asserted of the Locust, that in perfectly smooth water they have obtained a velocity of 10; and that is equally affirmed of the Great Liverpool: but two vessels that steam at the same rate cannot show the one to possess any advantage over the other; consequently, as long as circumstances remain the same, they must pass over an equal quantity of ground in the same time, directly; however any undulation shall retard their velocity, the loss of the one will be in proportion to the loss of the other, as their different momenta bear to each other; for, the

sea being an obstructing force, the vessel with the greatest momentum will be best able to overcome it. The same is evident when we wish to select a vessel to tow another to the greatest advantage: multiply then the tonnage by the velocity and we find the relative momenta in the cases cited.

"This law was beautifully exemplified a short time since by the Great Liverpool and Megara; but, singular enough, none traced the effect to its true cause; the gain being solely attributed to the different powering' of the two vessels. Captain

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Engledue himself assigned that reason to me in these words: I expected we should beat her, as no Government vessel has sufficient horse-power.' I will now relate the particulars. In consequence of the Great Liverpool having broken one of her pistons, and therefore working but one engine, Admiral Sir John Louis sent some dispatches of importance by the Magara, taking it as matter of course she would arrive first at Gibraltar; but the Great Liverpool, as Captain Engledue had anticipated, beat the Megara in this instance by even as much as 10 hours..

"Now how is it possible to assign the cause with any show of reason to the superior power of the Great Liverpool? for both engines have equal power; therefore if she had lost the use of one, her power was reduced to one-half, viz. 232-horse, and which power had to impart motion to a double quantity of matter, as their tonnage is as 1,540 to 730.

"It is true, other causes might possibly be assigned by a close inspection of both logs; but it must be granted me, that, if I have not clearly established my own point, I have most satisfactorily demolished that of my friends; let the cause be what it may, the case is a remarkable one, and well worthy of consideration.

"As I have occasionally used the words maximum velocity, I think it advisable that I should define its meaning; for however strange it may appear, the want of having this clearly explained has led to much error. This, then, is what I would wish understood; it is the highest velocity obtainable from any vessel, at her average or mean draught, with the greatest beneficial use of power; this, in my opinion, can only be found in perfectly calm weather and smooth water, free from the effects of tide and current; any higher velocity than this, such as that shown by running in a gale of wind, I term 'casual,' as it can never form data for any calculations. Were this carefully attended to, much useless argument would be saved; and much labour in reconciling apparent contradictions.

"I will now explain what steps are necessary to be taken to prove the law of the squares. Suppose ten miles an hour be the maximum from four boilers, and it be required to know what half the quantity of steam will give; close your throttle valve to one-half, the consequence will then be, that the half steam not drawn from the boilers will press on the safety-valve and escape; the velocity obtained will be 8 knots on the principles previously explained: as soon as you have satisfied yourself that half the steam entering the cylinder is attended with this effect, draw half the fires; for, as yet, you are generating steam to waste; but still take care to keep the throttle valve closed to one-half, for otherwise you would soon exhaust all the steam from your boilers: this is often resorted to, when the boilers do not generate steam fast enough to meet the demand of the vessel. Boilers, I cannot help conceiving, ought to be constructed to supply the full demand of cylinders with the most inferior coal, leaving it to the discretion of the commander to reduce his fires as may be necessary. Following this rule, from the above maximum the relative velo. city from each boiler stands thus: one boiler, 5 knots 6 fathoms; two boilers, 8 knots; three boilers, 9 knots 2 fathoms; four boilers, 10 knots.

"I wish in this place clearly to define the difference of steam used thus, and that from the expansive system. Captain Otway has fallen into error in conceiving both to be attended with a mechanical effect; this is certainly a great mistake; 'steam* wire drawn,' as the former mode is expressed, has no mechanical properties; all the advantage to be derived from it is, that you obtain a higher proportionate velocity for the quantiy of fuel expended; not through any mechanical property in itself, but on the principle that the resistance increases according to the square of the velocity; the pressure is constant and uniform and not expansive.

"It is otherwise, however, with the beautiful law that governs the expansive system; here the pressure is not constant and uniform throughout the cylinder. I cannot help conceiving that the great advantage to be derived from expansive steam is not so clearly understood as it ought to be; for it involves a contradiction of terms to suppose parties to be insensible to their own interests, whe

I have denied any mechanic properties to "wire drawn steam," to keep the advantage to be derived from "expansive steam" in the foreground; but all power exerted through an agent is strictly speaking mechanical but this is not the sense in which it is used by Otway (see page 10); he confounds things which in their cause and effect are different.

ther they be individuals or communities. Can it be accounted for by stating, that the few fully conversant with the subject will not condescend to make their definitions sufficiently intelligible to meet the understandings of the many, who, after all, as practical men, will have to carry it into execution? Be it as it may, in all the works I have read, much fault is to be found in the manner of bringing this subject under consideration, than which, short of the invention of the engine, there cannot be found one more important. Many are content with giving you the results of their calculations, without troubling themselves to explain how these results were obtained; others furnish us with an illustration in lieu of a definition, as if such would convey any distinct idea; but an illustration neither shows a perfect knowledge of the matter, nor will serve to make your ideas intelligible to another; even when most happy, they convey but imperfect notions; however, it is easier to illustrate than define. If you doubt the correctness of these remarks, only peruse what Captain Williams has written in his lectures on Steam, pages 14 and 32.

"I also gather from Captain Otway's writings that he has no distinct notions on the subject; for he fairly contradicts himself.

"Doctor Lardner seems to have a knowledge of the matter as far as the mechanical effect gained; though it is poorly explained in the 6th edition, in the 7th and last edition his definitions are certainly more intelligible; but is it not surprising that he doubts the applicability of expansive steam to steamers in his 6th edition, and in his 7th, though admitting it is now in use, forms all his calculations with regard to the expenditure of coal as if no such invention had ever been called into existence? Hugo Reid hazards little, and consequently is safe from criticism. How can any man by reading these, arrive at a satisfactory conclusion on a naturally abstruse question, when he finds it engulphed in such a sea of contradiction?

"But the Doctor, in denying the law of the squares, robs himself of the landmark that would safely guide him through these troubled waters; he loses his third term in a simple rule of proportion, which third term enabled me to speak with confidence as to the amount of the loss of velocity in proportion to the saving of fuel.

"I shall now try to define the expansive system: it is a mechanical property peculiar to steam thus used, by which, though you save one-half that would be required completely to fill the cylinders, you lose less than one-sixth of the effective pressure on the piston. A cubic inch of water will produce a cubic foot of steam: it has then ex