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A DESCRIPTIVE ACCOUNT OF THE

PRINCIPAL BRIDGES ERECTED
OVER THE RIVER THAMES.

BY MR. CHRISTOPHER DAVY,

ARCHITECT.

No. I.

The invention of the arch marked a great era in architecture, it being the only important addition which was made to this science for several centuries after the introduction of the Grecian orders. It formed the chief source of the excellence of the Roman or Italian style, giving birth to those "aspiring arcades" for which the Italian palaces are so distinguished. But though the invention of the arch was thus important, its origin is involved in great obscurity. Almost every writer on the subject gives a different account of the matter; and, after all that has been said, we as yet know nothing certain regarding it. I believe most

parties agree on one point, viz. that the Babylonians were well acquainted with the properties of the arch. Now, there seems to be a great difficulty in reconciling this supposition with the remains of structures said to be of Babylonish origin; for in them no specimen of a correct arch has yet been discovered. The bridge over the Euphrates had the intervals between the piers covered with planks and squared beams, which were removed at night to prevent depredations (this is supposed to be the earliest specimen of a draw-bridge). It is plain, therefore, that arches could not have been turned between the piers. Besides, had they been arched, it is scarcely possible that the arches could have decayed so soon and left the piers standing. An early attempt to increase the height of an opening by the Babylonian and Egyp tian methods is exhibited in fig. 1.

This singular relic of antiquity is called the subterraneous chamber of the treasury of Atreus. The ornaments of this building exhibit also the earliest specimens of Greek sculpture, and may be consulted by students with advantage in the Elgin gallery at the British Museum.

By reference to the sketch, fig. 2, it I will be seen that the chamber was of a parabolic form; but the construction was so deficient in the principles of the arch, that it appears to be one of those straws that antiquarians have been eager to grasp at to set contention at rest. I beg leave to refer the curious for farther particulars to the very interesting description of this building by Donaldson, in Parts 3, 4, and 5 of Stuart's "Athens."

Architecture has benefited not only from the introduction of the arch in its most simple form, but from a great variety of delightful combinations, all emanating from a knowledge of its properties. These combinations may be classed under the following heads:

Varieties of the Italian Style.

1. The archoid, or arch vault. 2. Domoid, or dome.

3. Polygonal ditto.

4. Conoidal ditto, or circular.

5. Pendentoids-when used internally, called a vellar cupola.

6. Groinoids, or groins.
7. Lunoid, or Welsh groins.
8. Lunetted domoid.

Pendant rib ridged vault, upon a square plan.

Rib truncated vault, square plan. 15. Pendant rib truncated vault, oblong plan.

16. Octagonal dome groined vault, with pointed arches.

17.

Circular dome groined rib vault, with pointed arches.

From the preceding enumeration of the varieties of arches and vaulting, it is evident that our ancestors went far before any others in the degree of perfection to which they carried the principles of their construction. In the different varieties of the pendentoid, huge masses of stone were absolutely suspended instead of supported; and in groining, it would appear as if the points of support dwindled into slender shafts, apparently incapable of supporting a tithe of the superincumbent weight.

The more curious of the forms specified in the above will be found exemplified in the following structures:

The polygonal, on a square plan-in the British Museum, Holkham Hall, &c. The conoidal-in St. Paul's, and St. Peter's, Walbrook.

The pendentoids-in the nave and aisles

of St. Paul's, and many public buildings, particularly by Soane. The groins-in Somerset House, lateral passages of the Custom House, Horse Guards, wine-cellar stores at the va rious docks, &c.

The lunoid Fletcher's late chapel,
Finsbury, and many churches.
The pendant rib truncated vault, oblong
plan in the chapel of Henry VII.,
Westminster, and King's College
Chapel, Cambridge.

The octagonal pointed groined vaultin most of the Chapter-houses of the British Cathedrals.

The quadrilateral pointed groined vault

-common in churches and cathedrals in the Gothic style; St. George's Chapel, Windsor; Cloisters, Westminster Abbey.

The particular subject on which I am about to treat will involve the most simple of these forms only-the archway-the extrados and intrados of the same, together with the abutments and roussoirs, &c. &c.

(To be continued.)

A CHEAP AND SPEEDY PLAN FOR INCREASING THE POWER OF ARTILLERY, WITHOUT MAKING ANY CHANGE UPON THE GUNS, AND WITH A GREAT SAVING OF POWDER.

BY MR. JOHN ANDERSON, A.M.

Professor of Nat. Phil. Glasgow.

[The following interesting paper on gunnery, by the celebrated Professor Anderson, of Glasgow, is printed from the original manuscript, in the handwriting of the author, and is now published, we believe, for the first time. It bears the date of 1782, and appears to have been submitted to the Board of Ordnance, which was at that time under the direction of the Duke of Richmond. The professor is known to have considered himself ill-used by the duke; and it is not improbable that inattention to this memoir may have been one of his causes of complaint. It is but fair, however, to add, that with regard to the chief improvement which it is the purpose of this paper to recommend, namely, the adoption of shot of a spheroidal form that subsequent experiments have shewn that it would be no improvement.

In an unpublished work by Colonel Macerone, from which we lately made some interesting extracts on rifles, there is the following remark on this subject: "It has been often imagined to construct bullets of the shape of an egg or a pear, on the supposition that the bigger or heavier end would steadily preserve the foremost position during the flight; and that, as the whole bullet, although one-third or one-fourth heavier, would afford no more superficies to the resistance of the air than a sphere of the same calibre, its range would be considerably greater. Experience, however, proves this to be erroneous; for this egg-shaped bullet, upon its exit from the barrel, whirls over and over during so great a portion of its flight, as greatly to increase instead of diminishing the surface of resistance, by which its range is not only curtailed but considerably disordered."-ED.]

After all the books that have been written by men of science concerning projectiles, and all the experiments that have been made by artillerists of the greatest merit, the gunner's art is still very imperfect: for if discharges are made from the same gun, and with the powder, ball, wadding, heat, weather, and all circumstances as much alike as they can be made by the ablest practitioners, yet the balls are thrown to distances that differ from each other by hundreds of yards; and their deviations from the line of direction are many degrees, if we reckon the muzzle of the gun the centre of a circle.

In support of this assertion, I need not bring the many experiments which I have lately made. It is proved, in the clearest manner, by the writings on artillery of the best practitioners in every country of Europe. A twentyfour pounder, says Robins, vol. i. p. 300, when elevated to seven degrees and a quarter, and charged each time with the same quantity of powder, threw one shot 2300 yards, and the very next shot to more than 3000 yards. And I have seen, says he, page 251, a piece fired with great care in an invariable position, and yet two successive shots of it have flown

in directions which, on the horizontal plane, made an angle with each other of above fifteen degrees.

The experiments of the ablest foreign artillerists prove the same thing; but they need not be quoted in this place, as many of them are to be found in the Posthumous Works of Robins, who surpassed every writer on this subject in force of genius and accuracy of practice.

These vast irregularities in the paths of military projectiles are produced by four causes, the windage, the wadding, the rotation of the ball in the gun, and the resistance of the air through which it moves in its flight.

If the ball fits the bore of the gun with perfect accuracy, its centre will coincide with the axis of the gun, and the direction of the ball will be that of the gun. Such accuracy does not take place in common practice, on account of the inequalities which the balls get when they are formed in the mould, and which both the balls and the bore contract from rust and filth. This difference between the diameter of the ball and the bore of the gun is called the windage; and it is not less sometimes than four-tenths of an inch. If the ball always rubbed against the lowest part of the bore, it would be discharged lower than the direction of the gun; and this, though an error, would be invariably the same, and, therefore, would cease to be an error, because it would be known. As, however, the ball may, on account of the windage, rub a different part of the bore at every discharge, hence the path of its centre cannot be known. This is an inconsiderable circumstance: were balls always globes, as the word imports, but it is well known that they are not so, the ball, improperly so called, has a flat part upon its surface, and touches the edge or outmost circle of the bore when it leaves the gun, it will deviate to that side; or if it has a protuberance, it will deviate to the contrary side. Now every one knows that an error of the tenth part of an inch at the muzzle of the gun will make a deviation of many yards at the distance of a mile.

and if

Wadding is necessary in order to compress the powder when put into the gun, and to hinder it from escaping

by the windage, when, by fire, it is converted into an elastic fluid, like steam. It is commonly made of the yarn or threads of old ropes, wound into balls, which differ in the closeness of their texture and the firmness with which they are rammed; in consequence of which, the same quantity of the fired powder does not enter into their vacuities, and the balls are not always equally impelled by it upon the same parts; and hence the line of their deviations will not always be the same.

The rotatory motion of the ball, or its whirling round its axis, by rubbing against the bore of the gun, makes it strike against the bore and the air differently from what it would do if it had no other but a progressive motion: and as this must be different in different balls, for the reasons that have been mentioned under windage and wadding, hence the deviations arising from this cause will scarcely ever be the same.

Every one may understand this motion by looking at that of the marbles with which boys play. Sometimes a marble, when projected, has only a progressive motion; and at other times it has likewise a revolution round its axis by rubbing on the ground. This second motion is sometimes in the direction of the first, sometimes in lines that make different angles with it, and sometimes in a line that is directly contrary. Hence it is that sometimes when the marble has moved forward in a straight line it suddenly returns directly backward, because the second motion has overcome the first. And hence, if cannon balls receive a rotatory motion from their friction in the gun, and if that friction is different in different balls, for the reasons already given, this will be a third cause that will vary the angle of their deviation. The resistance of the air is the greatest cause of uncertainty in the flight of military projectiles, and yet it was considered, by the ablest philosophers and most accurate artillerists, as altogether inconsiderable, till about forty years ago, when Robins first shewed that if a twenty-four pound ball is impelled by its usual charge, the opposition of the air to its motion is equal to a weight of at least 400 lbs.; that this resistance continually dimi

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nishes its velocity; that if it is impelled at an elevation of forty-five degrees, its range, or the distance to which it will go, is not a fifth part of what it would be, could the air be entirely removed; that the swifter it moves, the greater will be the resistance of the air: and, according to this law, with a double velocity it will be four times greater; with a triple, nine times; with a quadruple, sixteen times, and so on; but when the velocity is greater than 1200 feet in a second of time, then a new law takes place, for it is three times greater; that, in consequence of this immense resistance of the air, an increase of the initial velocity of a ball does not make a proportional increase of its range; that, as there is a great resistance in the front of the ball, less at the sides, and a vacuum behind, it will deviate to a side where the opposition is least, till it condenses the air in its new path as before, and thus it goes zig-zag in its flight; that, in consequence of this prodigious resistance, and which is very different with different degrees of velocity, many of the irregularities in the flight of balls may be explained, though the causes in particular cases cannot be ascertained.

Did we not know from history the slowness of man in making discoveries in every science, it would appear astonishing how both theorists and practitioners should have supposed that the resistance of the air to military projectiles was so inconsiderable as to

Dr. Hutton has denied the correctness of this statement. He "Insays: stead of leaping at once from the law of the square of the velocities, and ever after being about three times as much, my experiments prove that the increase of the resistance above the law of the square of the velocity takes place at first in the smallest motions, and increases gradually more and more to a certain point, but never rises so high as to be three times that quantity, after which it decreases again." Dr. H. adds a table of the actual quantities of resistances, deduced from accurate experiments with a ball of 1965 inches diameter. From this it appears, that at a velocity of 1000 feet, the resistance was = 350 ounces, at a velocity of 1200 = 546, at 1400 785, at 1800 = 1319, &c.—ED.

=

deserve no attention; when every boy knows, that if he runs forward with his windmill, in the calmest day, it will turn round as if there were a strong wind; or if he gallops his horse, he feels a resistance from the air that is very great, though the velocity with which he moves is not the twentieth part of that of a cannonball.

When I considered how to remove, or to diminish, these disturbing causes of the regular flight of military projectiles, and how to increase their power in such a manner as to be useful in war, I saw, after many theories, and a variety of experiments which would fill a volume, that nothing of importance could be done with balls, and therefore I fixed upon a shot of a different kind. It is a spheroid, but a part of it over the lesser axis is a cylinder; it is double the weight of a ball of the same matter that fits the same gun; it is impelled by common powder, a bladder called an addition, and a wadding that is uniform and hard. This cheap and simple contrivance seems to be a great improvement in artillery; and for the following

reasons:

1st, As the cylindrical part of this shot is made to fit the bore very nearly, its centre will coincide almost with the axis of the gun, by which means there will hardly be any windage, and, consequently, none of the causes of deviation that arise from it.

2dly, The wadding will have three properties: it will be impenetrable to the fired powder-it will allow very little or none of it to escape-and it will always impel the shot in the same way. Consequently, it will remove, or it will diminish, the irregularities which common waddings produce.

3dly, As part of this shot is a true cylinder, and nearly as large as the bore, it is impossible that it can revolve like a ball in the direction of the gun; and, therefore, if this is a cause of irregularity in the flight of balls, it is totally removed. If it should revolve lengthwise, that is, upon the axis of the gun, this will be an advantage, like that which arises from rifled musket-barrels, or the feathers upon arrows and shuttlecocks, by which their deviations are greatly diminished,