They may have their paddles precisely of that breadth which is best.

5. A boat with only a powerful engine on board, may be made to swim in four feet water and under (few steamers draw less than six or seven): while the engine, which is now such an incumbrance to steam-boats that carry passengers, would, by its weight, be positively beneficial to the tug, because she must have substance as well as power (bone as well as blood) to fit her for draught. The engine, it might have been added, can always be placed in a tug, exactly in the centre of the vessel, where its power can be applied with best effect; while, in a boat fitted to receive passengers, this consideration has generally to yield to convenience.

6. Passage vessels that are to be towed may, of course, be constructed of far less draught of water than when they have to carry an engine. Flat in the floor, and buoyant with any cargo, they might, even in the worst weather, be conducted by a tug alongside a shallow pier, and land passengers and goods in comfort and safety, long after approach was impracticable to a loaded steam or sailing boat of even the same draught of water. Flatbottomed boats are so buoyant, that to superficial observers they appear dangerous craft; but in the smooth water of a tug they would be steady; and they are in all circumstances, as every experienced seaman knows, the safest of all boats. The Yarmouth keels, which take stores and provisions out to the men-of-war, are open boats, sunk to the gunwale when their cargoes are on board,-yet no accident ever occurs to them. The Campêche droguers are, in like manner, square boxes, with scarce a sharp end to go foremost, yet they, too, load gunwale deep, take cargoes out through heavy rolling seas to ships four leagues off, and survive all the apparent dangers of their passage.

7. Passengers would be much more comfortable, and safer, in a vessel by themselves, than they can possibly be when embarked with a steam-engine. The heat, smell, smoke, dirt, and jarring, unavoidably caused by an engine, are all serious evils, and aggravate, in no small degree, the pains of sea-sickness. A small neglect of the machinery may, at any time, produce a great calamity: the chances of such neglect are greatly multiplied by the presence of passengers on board, and by their occasional curiosity; while the weight of the engine, in the event of collision with any external object, gives a great additional impetus

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to the shock, and causes the vessel, when a hole happens to be made in its bottom, to go down like a stone. Were tugboats adopted, all these drawbacks on steam navigation would be obviated. At a distance from the tug, and with the means of anchorage on board, the passengers in a vessel in tow would be safe in it whatever happened; and as every corner in the passage vessel might then be given up to accommodation, a thousand conveniences would be introduced, which are at present unthought of.

The exemption from danger would not, however, be so complete as Capt. M. supposes; for a late accident at New Orleans shows that an explosion in a tug may do harm to vessels in tow. But it would be so great, comparatively, as to form a consideration of the first importance.

8. Upon the tug system, high pressure engines might be again introduced into steam navigation.

We are particularly glad to observe the stand which Capt. M. makes in favour of these engines. It is disgraceful to the character of the nation, that a few de structive explosions, occasioned solely by gross neglect and ignorance, should have had the effect of driving them so generally out of use. There is not another people in the civilized world who have been so frightened out of the use of a great mechanical power. Our own stepchildren, the Americans, seem to cry out shame upon us; for there is scarcely on all their rivers a single steam-boat which is not propelled by a high pressure engine.

The advantages of high pressure engines are correctly stated by Capt. M. to consist chiefly in original cheapness of construction, diminished expense of working, superior lightness (nearly as 4 to 5), but, above all, in command of high power, not for current use, but in reserve against occasions when it may be required. In low pressure engines, there is no such reserve; beyond a certain limited point, an increased fire only fatigues the machinery, without adding one jot to the useful effect.

Besides, should tubular boilers be brought to answer in practice, as there seems every reason to hope they will be, every possibility of danger from high pressure engines will be wholly removed. Among the plans for this purpose, Capt. M. notices, with special approbation, that of our correspondent, Mr. Gilman.

9, and lastly. The tug system will be superior in economy to anything yet devised. A large steam-boat, with a powerful engine, cannot be constructed much

under 40001.; but the best steam tugs need not cost above 2000., nor steam vessels (for ferries) above 300/. each.

The saving, we apprehend, would be even greater than Capt. M. here anticipates. An engine of 12 or 14 horse power, if embarked in a tug, would be sufficient for almost any purpose; and a common condensing engine of that dimension, may be obtained for less than 5007., and a boat to embark it in, clinker built and copper-fastened, for little more than 3007. Such at least are the ordinary prices at Newcastle, where tugs are much employed to tow the trade up and down the Tyne; and they cannot differ greatly from the prices elsewhere.

Captain M. professes himself unable to imagine what objection can be stated to the tug system, in opposition to so many advantages, except it be some supposed difficulty in managing two boats together in certain circumstances of tide, current, weather, &c." But to this he gives the following satisfactory answers. "1st. The thing has been already done on the American rivers, at least as rapid and as stormy as any of ours.2dly. Where there are thus great advantages to be obtained, and only one small difficulty to be overcome, with common talent and energy, if there is a will there will be found many ways."

A method of making the two boats act with ease simultaneously, is suggested by Captain M.; but as he allows the Amecan mode to be "still simpler," and it has, besides, the advantage of having been tried, and fourd to answer, we shall content ourselves with quoting his account of the latter. "Two iron rods are secured, one to each bow of the passagevessel, so as easily to play up and down; and their other extremities are brought together, as in a triangle, and are jointed and pivoted on the taffrail of the tug." "As general principles," Captain M. adds, the nearer the two boats are kept together, the smoother, the lighter, and the more manageable, will be the draught; and inflexible rods, besides their convenience for backing and keeping the boats apart, transmit the impulse undiminished; whereas, ropes act like springs, and a considerable portion of the power is expended in merely stretching them."

If the preceding views of Captain M. be correct, as we are well convinced they are, he seems fully justified in the more general conclusion which he draws, that tugs may be employed with equal advantage in towing vessels across seas, as well as across and up and down

3

rivers. "Should a gale of wind arise, the tug, relieved from its incumbrance, would probably be always able to command its own safety; while the passagevessel, lying to under jury masts, with the means of erecting and rigging which she might be furnished, or at anchor, would be much more lively without the engine."

The Americans, with their usual acuteness, have already begun to employ the tug system under all circumstances and we have little doubt that, ere long, it will be universally followed.

NEW PATENTS.

Professor Farish, of Cambridge, for improved methods of clearing out water courses.-4 Sept.6 months.

Thomas Robinson Williams, of Norfolk-street, Strand, for certain improvements in the making of hats, bonnets, and caps, and in the covering of them with silk and other materials, with the assistance of machinery.11 Sept.-6 months.

John Melville, of Upper Harley-street, Esq., for certain improvements in the propelling of vessels.-18th Sept.-6 months.

John Jones, of Leeds, brushmaker, for improve ments in machinery, for pressing and finishing woollen cloth.-25 Sept.-6 months.

INTERIM NOTICES.

We have to regret the unavoidable postponement, from different causes, of several communications which have special claims to early insertion; particularly a reply by Lieut. Green to Amicus, on Lightning Conductors; a vindication by "An Observer" of Mr. Scott's Patent for preventing deposits in Steam Boilers from the criticism in our 271st Number; and an explanatory letter from Mr. Gutteridge, on the subject of Gas and Liquid Meters, all of which shall, however, have a place in our next Number.

We give, this week, two additional papers relating to Fires and Fire Engines; but have yet many more to lay before our readers. Mr. Rough shall have a page next week, We are glad, in the meanwhile, to learn that the formation of the Society for the Prevention of Loss of Life by Fire, goes on prosperously.

Communications received from Mr. BaddeleyMr. Squire-Henry D---Mr. Utting - Mr. Brew-J-n D-y.-Mr. Payne-Z.-Mr. Brown -Mr. Shires-Mr. Woollgar-C. R., T.-Mr. Greenall.

Communications (post paid) to be addressed to the Editor, at the Publishers, KNIGHT and LACEY, 55, Paternoster Row, London

Printed by G. Duckworth, 76, Fleet-street,

226

ELLIOTT'S DOUBLE-ACTING

ROTARY AIR-PUMP.

The principle on which this apparatus is constructed (that of rotary motion), cannot be said to be of very modern discovery; but our surprise is only therefore the greater, that a mode of operation so equable and harmonious, and which, with the aid of the fly-wheel, yields so uniform a power, has not long ere now led to the total abandonment of the air-pumps of the common construction. The air-plates of Mr. Elliott's machine are supported on neat scroll irons, and the train of wheels by a triangular frame of iron, lightly constructed. On the axis of the toothed wheel d are two cranks working the pistons of the barrels, which are two in number, and placed directly under.

Communi

cating with these barrels are the
bent tubes or exhausting-pipes a a,
connected with the air-plates bb,
and furnished with the necessary
stop-cocks. The connecting-rods
of the cranks are fixed to anti-fric-
tion wheels, working in the guide
frames c c. Attached to this are
other connecting-rods, one on each
side of the cylinders, and connected
together by cross heads; forming,
in this way, an effective parallel
motion to the pistons. The fly-wheel
is fixed on the same axis that sup-
ports the pinion e.
The apparatus
is put into action by turning the
handle f, and the exhaustion com-
mences either in one or both vessels
g, by opening or closing the com-
munication through the bent tubes,
by stop-cocks attached to each. h
shows the situation for a gauge.
C. DAVY.

Lond. Mech. Inst.
Oct. 3, 1828.

I

mine "F.'s" first definition, to which a reference has been made. suppose it will now stand thus:Any curve, in rolling upon another, engenders a roulette."

Now this definition, although it is so indefinite as to comprehend curves of every description, but being confined to curves, necessarily excludes the cycloid, which requires a right line. If, however, we substitute the 66 curve," word "line" for the word both the cycloid and the evolute will be included; the definition then being, Any line, in rolling upon another, engenders a 'roulette.'

66

999

Although the definition, thus altered, will include the cycloid, &c., yet, as it still comprehends every kind of line that can be rolled upon another, it becomes, as I have stated, "almost as indefinite as the word curve."

If the word "simple" be prefixed to "roulette," it would, perhaps, be sufficient to distinguish those lines which are engendered by the rolling of right lines and circles, from all others. It would, however, be still more desirable, as the greatest proportion of the terms for curved lines are derived from the Greek, if a word from that language, with such a signification, could be found.

Such a term would admit of two general divisions :

The first, when one line is a right line, and the other a circular line;

The second, when both lines are circular lines.

The first of these divisions* admits of two cases of rolling motion; viz.

1st case, when the circle rolls upon the right line;

2d case, when the right line rolls upon the circle.

The first case produces the lines known by the name of cycloids; the second, the line called an evolute. Professor Robinson, thinking, itwould appear, that it is as necessary to have terms for these opposite prin

*It may be necessary to say that these divisions are not those which I have called the first and second divisions in the Septenary System:

In

The limits of this division are, when the circle is infinitely small and infinitely great.

The second division (here so called), to which the term epicycloid is commonly applied, admits of six cases, for which distinguishing terms for both motions and lines are yet wanting.

1st case, equal circles, one moving within the other.

2d case, unequal circles, the smaller rolling within the larger.

3d case, unequal circles, the larger rolling upon the smaller within it.

4th case, equal circles, the exterior of the one rolling upon the exterior of the other.

[This is the case to which "F." has referred, in his demonstration.] 5th case, unequal circles, the exterior of the smaller rolling upon the exterior of the larger.

6th case, unequal circles, the exterior of the larger rolling upon the exterior of the smaller.

I have, indeed, given terms to these cases, as well as for every other case of motion in the Septenary System; but I must repeat,-if terms from the Greek can be found for all, they will, probably, supersede the terms given by others, as

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well as those I have formed, which I candidly admit are, in some cases, too circumlocutory, although not more, if so much so, as many terms in botany and chemistry. In connexion, however, with the diagrams which I have invented to exhibit the several kinds of motion, my terms may, I think, in general, be easily understood, the diagrams conveying as comprehensive an idea of the several kinds of motion, as, perhaps, it is possible to have, especially when explained with an instrument.

We have five terms for distinguishing the sections of a cone; yet these sections do not differ more from each other than the lines produced by the various motions of which this division is susceptible. The term epicycloid may be very proper as a general term; but surely it is as necessary to have terms to distinguish the peculiar motion which produces the ellipse, from that which produces the cardioid, as it is to distinguish parabolas from hyperbolas, or the cycloids from the evolutes.

The extremes of variation in epicycloidal motions having a given base, are four, three of which are infinite, the other finite; two are internal, and two external.

With any given generant, the extremes are also four, and, as before, three are infinite, and one finite; two internal, and two external.

Some of the terms for curved lines are descriptive of the particular motions by which the lines are generated; others describe the appearance of the lines produced.

In "Rees's Cyclopedia," it is concluded, "that there is something arbitrary in all the classifications of curve lines hitherto proposed;" but that this is not always the case, I need only again mention the conic sections. No one can, I think, say that a correct classification of these sections can be arbitrary. If we begin with the circle, the finite limit of which is a point-then the ellipse is the next section, whose finite limits are a point, a right line, and a circle-the parabola, whose finite limit is a right line, follows: