consequently be used to the same extent as in a sailing vessel. The advantage possessed by the screw in this respect was clearly proved in the trials between the Archimedes and Her Majesty's steamer Widgeon, where the latter vessel was superior to the former in calm weather or a head wind, but inferior with the wind a-beam." p. 37.

The "slip or recession of the screw," Mr. Galloway considers to be almost entirely a question of magnitude; for "if we increase the diameter without altering the pitch, we reduce the slip without increasing the velocity at which it would be required to drive the screw." The increase of diameter would of course increase the surface friction; but not to such an extent, Mr. Galloway thinks, as to make the total friction greater than in the case of paddle-wheels. But whatever the amount of the "slip" may be, it would seem, from a very curious fact, which has recently come to light in the course of some experiments made at Bristol by the parties concerned in the erection of the Great Britain, (late Mammoth,) that it may be more than compensated by a new source of power derivable from the form given to the

stern.

"A series of trials of the Archimedes have been made at Bristol with different kinds of propellers, in which the form and number of the threads were varied. The writer has

the disposition of the water, according to the first law of motion, to continue in the same state of rest or movement. In the case of a vessel the water must fall in to fill up the cavity which the vessel leaves, and in doing so, flows after the vessel in her.onward course. The velocity of such current is decreased by constructing the after body with finer lines, which would allow the water to fall into the space more gradually, and more from the sides of the cavity than when the form is full; but such a following current must ever exist even in the most delicate form. The propeller, therefore, being placed in a position where such a current exists, has a force acting against its surface, so that, although it really slips through the fluid against which it exerts its power, its motion in relation to the surrounding and inactive water seems to coincide with the action of a screw in a solid body, and, as in the instance alluded to, even to exceed the progress which such a screw would make." p. 33-35.

The number of threads which should be given to the screw, is one of the many points relating to the present subject still remaining to be determined by experiment; but Mr. Galloway mentions that "a sensible improvement was produced in the speed of the Archimedes by substituting the double thread for the original single one."

ments, as they were not considered sufficiently accurate for that purpose.* In one of them the vessel advanced a greater distance than the screw could have propelled her in still water, or, in other words, if the screw had been worming its way through a solid body the vessel would not have advanced so far as she did in the experiment alluded to. This appeared so paradoxical that the author questioned its accuracy, but inquiry has confirmed the fact, while further investigation has furnished a probable reason for the seeming anomaly.

"If we observe the motion of vessels through the water with full after bodies, or, what is the same thing, if we notice the effect of a current acting against the piers of bridges, we find in the immediate wake there is a quantity of fluid which approaches a state of rest, and round which the active current forms an eddy. In such cases, a piece of floating matter will sometimes retain its proximity to the vessel or pier for a considerable time. This obviously arises from

The results as regards speed were decidedly in favour of the original screw of the Archimedes,

wood immediately before the rudder. Instead of this, it has been proposed to make use of two screws, one at each side of the dead wood, or one at each side a-midships. Mr. Galloway thinks rather favourably of the former plan, but entirely condemns the latter.

The means of communicating motion from the engine to the screw are investigated under the three different heads of Toothed Wheels,-Bands or Ropes,and Contact by Pressure of Smooth Surfaces. The "toothed-wheel" mode, which was that adopted in the Archimedes, Mr. Galloway admits to be "not a desirable arrangement," because of the liability of the teeth to get stripped off "by any sudden change in the resistance or from imperceptible decay." "Bands" of all sorts he objects to because of their "liability to stretch," and of the rapid destruction resulting from the employment of tightening pulleys; and "contact by pressure of smooth surfaces" (more com

monly called adhesion) if obtained by a peculiar method of his own, now for the first time submitted to the public, but for a description of which we must refer to the work itself, he seems to consider the most unobjectionable of any.

We entertain a very favourable opinion of Mr. Galloway's "own;" but at the same time we feel bound to remark that he has done any thing but common justice to the "band" system. All bands do not stretch, not at least at any strains to which screw shafts can ordinarily be exposed: the metal bands, for example, of Newall; and if the existing fact were otherwise, it would not, we apprehend, be difficult to devise a band that would be as unstretchable as any solid cylinder ever produced.

Again; bands, when they do break, can be readily repaired or replaced, while wheels stripped of their cogs, or of any number of them, are either beyond repair, or must remain as they are till the vessel returns to port. For these reasons we consider the band system to be indisputably superior to the toothed-wheel one, and one, moreover, which will answer well under all circumstances; though, according to Mr. Galloway, and for the sake (we cannot help suspecting) of better clearing the way for my own," he places both in the same category of inefficiency.

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Mr. Galloway gives by way of supplement to "the Archimedean Screw," descriptions of "Ericsson's Propeller," "Mr. Rennie's Spriral Propeller," (i. e. the Conoidal-its paternal and much fitter name,) "Hunt's Propeller,"

Blaxland's Propeller," "Mr. David Napier's Propeller," "Captain CarpenCaptain Carpenter's Propeller," and "Modification by the author," (another design of "my own.")

"Ericsson's Propeller," appears to have been a bit of a puzzler to our author. The average performance of the Archimedes during the trial under Capt. Chappell was about 84 miles an hour; but the Robert Stockton, a vessel fitted with

=

Ericsson's Propeller," "ran (on the Thames, with the tide) 9 miles in 35 minutes," more than 15 miles an hour. To account fairly for this without giving altogether the go-bye to the "Archimedes" was a matter of difficulty; and so, to get rid of the difficulty, Mr. Gallaway places the whole difference to the account of some possible difference in

"the power of the engines." This we must take leave to say is not investigation after the manner of Tredgold, but palpable hap-hazard assertion after the worst manner of quacks and empirics.

a cone.

Mr. (George) Rennie's Conoidal Propeller is treated more amusingly, though we cannot say more justly or philosophically. It consists, Mr. Galloway tells us, of an inclined plane wound round a logarithmic cone or spire"-meaning to produced by a straight line wound round say (we presume) of an inclined plane It exhibits, moreover, according to Mr. Galloway, certain "distinguishing features, which are founded on a close observation of the forms which Nature has given to the impelling organs of those of her creatures which move through the waters;" but though he admits, (so candid is he!) that it is "a safe principle to assume, that the Great Architect has adopted the very best forms in all his works," he cannot bring himself to allow that Mr. Rennie's "close observation" of this very best of all possible models has been of any practical use. It does not suit him, (apparently,) to say that it has, and he does not like to say flatly that it has not, (which, however, is evidently his real opinion). But as it was requisite he should say something on the subject, (this à la Charles Buller,) he treats his reader to the following precious piece of seeming wisdom:

"The difficulty which we have to encounter in our imitations arises generally either from our not understanding all the purposes of the arrangement, or from our inability to apply them in the same way, or give them the same properties. Thus the propelling effort of the fish given out by an alternating action, and a form similar to his tail may not be equally well adapted to rotatory motion. The fish is also endowed with life, and his tail is elastic, so that he suits it to the motion of the fluid in such a manner that the form may lose its best properties if not thus regulated and changed at the will of the animal."-p. 54.

And so, because Mr. Galloway cannot, or affects not to understand how a number of rectilineal movements can be converted into a rotary one, (that is, practically speaking,) and fancies, or affects to fancy, that there is something in the "will" of a fish quite independent of its "tail," Mr. Rennie's most scientific and N 2

most efficient propeller is passed over as a thing of no regard!

Not, however, be it confessed, without more than a due share of the "soft sawder." "For, after all," quoth Mr. Galloway, "there is something extremely pleasing in these modifications, which will secure admiration for their refined elegance and ingenuity, WHATEVER MAY BE THEIR ULTIMATE EFFICIENCY." Faugh! Why does he not say, at once, what he thinks, which is, manifestly, that they are all fudge? Has he any notion how much he insults the objects of his fulsome adulation, by supposing that they can take any pride in elegance" or ingenuity," which has not " ultimate efficiency" for its sole end and object?

66

66

"Hunt's Propeller" is the next which comes under Mr. Galloway's killing kindness. Here he quite omits to state, that the inventor sets no store whatever on the form of his propeller, (See Mech. Mag., No. 880, for June 20, 1840) and he mentions, incidentally only, what is, in fact, the sole distinguishing feature of this very clever invention, namely the combination of the propeller and rudder in one. "Blaxland's Propeller "" fares still worse. It is said to "resolve itself into a method of making the screw by substituting circular blades for the triangular plates described and shown as the mode of constructing the screw of the Archimedes;" and yet we are told, that "it must necessarily be inferior in effect to the screw of the Archimedes "—that is, inferior to what it is said to resolve itself into! Neither is any notice whatever taken, (except in the way of literal description,) of what undoubtedly forms the best feature of the Blaxland Propeller-the mode in which the power of the engine is communicated by bands to the screw-shaft.

Mr. David Napier's plan consists in placing two wheels, only partially immersed, at the stern of the vessel, one a little in advance of the other, so that the blades of one just clear the axis of the other. "An iron steam-vessel," says Mr. Galloway, "has been constructed by Mr. Napier, for carrying this contrivance into effect, which, though obviously defective in form, (her after end terminating nearly in a square, and the propellers, consequently, consuming a large portion of their power in the dead water thus produced,) has attained a speed of 11 miles per hour." Mr. Galloway for

gets that he had just before shown, by the Bristol experiments, that a stern somewhat square is the very best form for a screw propeller.

Captain Carpenter, as our readers are aware, employs two propellers, consisting of flat blades of a trapezoidal form, placed in the after quarter of the vessel. "The chief defect" which Mr. Galloway finds with this arrangement is, "the making the blades plain surfaces, instead of sections of screws; the result will either be excessive displacement at the outer portions, or in direct opposition to the vessel's progress at the parts near the centre."

Mr. Galloway's "own" plan is thus described.

"The parts of the screw near the centre expend the greatest portion of their effort in turning the water round without aiding considerably in the populsion of the vessel. The object of several of the inventions we have described has partly been to obviate this defect. The evil may be in a great degree remedied without departing considerably from the form of the screw of the Archimedes. To demonstrate this, let us suppose the inner portion of that screw to be removed, and the outer part to be merely attached by arms radiating from the axis to the leading and after end of the screw, and that to the leading arms a number of cords or flexible lines be affixed. Under such circumstances, if the screw be made to propel a vessel at its maximum velocity, these cords or flexible lines will generate helices in their progress through the water. The pitch of these helices, however, will be less than the pitch of the screw, inasmuch as they will coincide with the actual motion of the vessel, while the screw has a pitch equal to the vessel's motion in addition to its own slip.

"If, therefore, we could construct the part of the screw nearest the centre in the form of the helices thus generated, we might make the screw with threads continued from the axis to the periphery, but without diagonal loss near the centre; and to approximate to this as much as possible, the writer proposes to decrease the pitch from the periphery to the axis, so as to make the parts near the middle coincide with the vessel's motion. We consider its chief value to consist in its enabling us to attach the screw to the axis the whole of its length, as in the case in the screw of the Archimedes, but without the same loss of power."—p. 62.

As regards all that part of the preceding plan which consists in removing the inner portion of the screw, Mr. Galloway

has been anticipated by Mr. John C. Haddan, who patented, 22nd January, 1839, a plan for propelling vessels, of which this was the distinguishing characteristic. "I claim," says Mr. Haddan, in his specification, 66 as my invention the forming and using of screws with openings or spaces in their threads."

Mr. Galloway's supplementary list might have included several other varieties of sub-marine propellers, as Woodcroft's, Hale's, &c.; but seeing that all those he has described, with the single exception of the Archimedean,

or

Smith's, are only shown up in order to be dismissed on most frivolous and insufficient grounds, it is scarcely to be regretted that his collection is not more extensive.

Mr. Galloway closes his treatise with an account of some experiments made with the Government steamer Bee, to test the capabilities of the screw, which appear to have come to his knowledge when just on the eve of publication, (for they were made only on the 20th, 21st, and 23rd of June last,) and which he admits square but indifferently with the favourable opinions expressed in the preceding part of the work. "The results," he says, 66 were certainly more unfavourable to the screw than either the experiments with the Archimedes or our calculations would lead us to expect." The Bee was fitted up expressly for the purpose of ascertaining, by a sort of experiment which would be free from every objection, what the real power of the screw was; and accordingly the "machinery was so arranged as to propel her either by paddle-wheels or the screw of Mr. Smith." This we consider even a better test than that of employing "two vessels of the same form and power," such as the Rattler and Polyphemus, the one fitted with the screw, and the other with paddle-wheels; for it is well known that the same lines and same weights do not always in sea affairs produce identical results. Fifteen trials made with the screw applied to the Bee, gave an average speed of only 7.358 miles. Mr. Galloway tells us that "Mr. Smith attributes this result to the screw being too small," and that it may be perfectly ascertained whether this is the cause of the inferior results obtained with the Bee or not, the Admiralty have ordered a larger" one to be substituted.

66

In one way or other there is now a certainty of the question as to the real value of the Archimedean propeller being very speedily set at rest. It appears from the following paragraph that independently of the Bee, there are at the present time no less than six vessels of firstrate magnitude, either just completed, or fitting out on this plan

"The ultimate fate of these methods of populsion is now beyond the influence either of praise or depreciation. Four vessels are already in operation, fitted under the patent of Mr. Smith, and the Great Britain,' now building at Bristol, is to be propelled with the screw driven by engines of 1000 horses' power. A vessel of 1500 tons is also just completed at Londonderry, which is to be propelled in the same way. The British Government are fitting the Rattler, which is of 800 tons burden, and has engines of 200 horses' power, to prove the power of the screw by competing with the Polyphemus, which is of the like power and tonnage; and the French Government are fitting three vessels to be impelled by the same agency. Messrs. Rennie and others are also about carrying out their plans on a large scale. The rapid strides which the system has made in the space of two years, before which it was almost universally unpopular, are the best evidence of its importance; and there can be no doubt that if the machinery for communicating motion from the engine to the screw can be rendered simple and durable, this method of propelling will become useful as an auxiliary power, and in certain classes of war steamers, even if it should not be found equal to paddle wheels under all circumstances."-p. 67.

may

So far as the experiments already made-which have been both very numerous and conducted generally under most favourable circumstances justify us in coming to a conclusion, we should say that there is no likelihood that the Archimedean propeller will ever be able to compete in point of speed with the paddle-wheel, whatever may be the advantages which it possesses in point of position.

How the case may be, if Ericsson's propeller, or any of the others proposed is adopted, we have not as yet the means of judging. Ericsson's plan, though neglected in

this country,

appears to be making great way in America. Besides the Robert Stockton, of which we have before spoken, and the Clarion, which was lost on the coast

of Florida, there have been six other American vessels fitted with Captain Ericsson's double wheels-two which ply on Lakes Ontario and Erie, passing through the Welland canal, and four (of iron) which trade between Philadelphia and New York. A newspaper account of two of the latter will be found among the Notes and Notices of our present Number.

Although Mr. Galloway has by no means treated his subject in a way to be generally commended, it is not to be denied that he has collected together a great deal of most valuable information upon it. It may be even admitted that he has, by his mathematical, or quasimathematical investigations, advanced to some extent our knowledge of its scientific principles. But to qualify a person to wear, with any credit to himself or his employers, the mantle of a Tredgold, he must possess something more than mere industry and a mere smattering of learning; he should be able to sound the depths as well as the shallows of science -have eyes to see the both sides which every question has, and the courage and honesty to pronounce which in his judgment is the right.

THE CASE IN LIFE

ASSURANCE-MR.

SCOTT IN CONCLUSION.

Sir, We have calculated from the Northampton Table, where the rate of interest is 3 per cent., that the annual premium is £4. 13s. 54d., or the whole in one premium £38. 78. 84d., and that when the interest is 4 per cent., the like premiums, are £4. Os. 14d., and £31. 128. 93d. The annual premiums when the rate of interest is 3 and 4 per cent., by the Swiss Table are £5. 68. 9 d., and £4. 11s. 74d. The single premiums being £45. 5s. 13d., and £37. 6s. 1d.

Supposing the proprietary of such a scheme are to have 3rd of the profits arising from deaths, but that the living subscribers are still to receive £100 each at the expiration of 20 years, it is required to determine what the annual and single premiums will be on this supposition?

1st, By the Swiss Table, when the interest is 3 per cent.

Each living subscriber will have a profit of (100-8174) £18. 5s. 24d.; but,

if rd of this profit, viz., £6. 18. 84d., go to the proprietary, the profits of each surviving subscriber will only be £12. 38. 54d. In this case, instead of £100, each living subscriber will only be entitled to (100—3rd of 18:26) £93·913; hence, by proportion, 93-913: 100 :: 5-339 5685 = £5. 13s. 84d. That is, the annual premium will be £5. 13s. 84d., and 81.74 + 3rd of 18:26 = 87·827, and 87-8271806111= 48 622 = £48, 12s. 54d. the single premium.

By a like process, when the rate of interest is 4 per cent., the annual premium will be £4 178. 94d., and the single premium £40. 1s. 94d. The like premiums by the Northampton Table will be £5. 5s. 04d, and £4. 9s. 24d.; the single premiums being £44. 08. 104d., and £36. 6s. 14d.

There is a curious circumstance respecting the waste of human life deduced from the Northampton and Swiss Tables of mortality. Thus, between the ages of 23 and 43 the mortality of 100 persons by the Northampton Table is 31; and by the Swiss Table, for the same ages and number of persons and period of time, the deaths are only 18; and these numbers are in the proportion of 7 to 4 (almost exactly). Now contrast this with the following extract from Mr. Morgan, vol. ii. page 443.

"During 33 years, from January 1768, to January 1801, the number of assurances (in the Equitable) on single lives had been 83,201, of which number 60,597 were on the lives of persons under 50 years of age, among whom the deaths were fewer than those in the Northampton Table in the proportion of 4 to 7."

And now, Mr. Editor, having answered, as well as I can, the conditions required by your correspondent, Iver McIver, I shall only in conclusion add, that if any Assurance Company were to adopt the scheme mentioned by Iver McIver, and to calculate their premiums from the Northampton Tables, their loss would be most certain, unless they wished to play the same kind of game as the Western Assurance Company did about two or three years ago.

I am, Mr. Editor, yours, &c.,
GEORGE SCOTT.

Cochrane Terrace, St. John's Wood.