lence, from the assault of waves during storms, owing to the partial removal of atmospheric pressure; a fact which has recently been observed by James Walker, Esq., F. R. S. &c., President of the British Institution of Civil Engineers, and which was mentioned by him at a recent meeting of that Institute, (as reported in the Civil Engineer and Architects' Journal for September 1841,) in the following words: "At the Plymouth Breakwater, during the great storm in the month. of February 1838, several of the largest granite blocks, weighing from three to eight tons each, composing the surface or pavement of the Breakwater, which, although squared and dovetailed into the structure, and embedded in excellent cement to the extent of their whole depth, and thus forming a solid mass, were torn from their positions, and projected over the Breakwater into the Sound."

"Mr. Walker attributed this to the hydrostatic pressure exerted beneath the stones, at the moment when the atmospheric pressure above had been disturbed by the masses of water suddenly and rapidly thrown upon the surface of the Breakwater; blocks of stone were thus often carried to a great distance, not so much by the waves lifting them as by the vacuum created above by the motion of the water, which exerted at the same time its full pressure from below."

And as additional evidence that the formation of a partial vacuum is sometimes a consequence of the envelopment of sea works by high waves, Mr. Walker further stated, "that during a storm in the year 1840, the sea door of the Eddystone Light-house was forced outwards, and its strong iron bolts and hinges broken by the atmospheric pressure from within. In this instance he conceived that the sweep of the vast body of water in motion round the light-honse had created a partial and momentary, though effectual vacuum, and thus enabled the atmospheric pressure within the building to act upon the only yielding part of the structure."

As both the above remarkable instances came professionally under the notice of Mr. Walker, we are bound to place the utmost reliance upon the inferences derivable from his statements; and they indicate conclusively that no lateral connexion by dovetails, or otherwise, will compensate for a want of depth of solid stone; but that we must rely for stability in the coping of sea works, chiefly upon the weight of a large mass of materials, so connected as to gravitate as one body; we must oppose pressure by weight, and counteract by gravity the action of forces resulting from a disturbed equilibrium.

The pressure of the atmosphere is about the same per superficial foot as would be produced by the gravity of 11 feet depth of stone weighing 175 lbs. per cubic foot; therefore, if Breakwaters

were liable to be assailed by waves but twenty feet high,* at the same time that the atmospheric pressure was wholly removed, their summits would require coping with a depth of stone wrought into a solid mass, equal to 114 +7, or 19 feet, measured vertically.

In the nature of things, however, we cannot suppose that the whole pressure of the atmosphere would ever be removed from any point of the summit of a Breakwater whilst a wave of the maximum height was acting beneath; and though this is very much a matter of conjecture, we may probably infer that under no circumstances would more than two-thirds of the atmospheric pressure be removed from any point; this would be countervailed by a depth of eight feet of stone weighing 175 lbs. to the cubic foot, and as we have shown that a depth of near seven and a half feet is necessary to withstand the hydrostatic pressure of a twenty feet wave, we may finally infer that the summits of Breakwaters should never consist of less than fifteen feet average depth of stone firmly bound into a solid mass, by clamps, dowels, and cement, so as to gravitate as one body.t

The fact that continual repairs are rendered necessary, by the blowing up and sweeping away of portions of the pavements of existing

* The maximum height attained by the waves of the most violent storms, at the sites of Breakwaters, is such an important element in forming the plan of the work, that it ought always to be ascertained experimentally, (which would not be difficult,) by attaching a machine upon the principle of a self-registering tide gauge, to a pile well driven at the site, and properly braced against the sea.

This principle of constructing sea works, was adopted in Rudyerd's Light-house, built upon the Eddystone, in 1709, and which, after successfully withstanding the storms of half a century, was destroyed by fire in 1759; regarding it, Smeaton states that Rudyerd "judiciously laid hold of the great principle of engineering," that " weight is the most naturally and effectually resisted by weight," and accordingly formed his light-house, near its foundation, solid, and mainly of stone, for such a height as he conceived would enable the gravity of the mass to resist the upward hydrostatic pressure of the waves, in case the water insinuated itself beneath the building.

This idea was not lost upon the able and sagacious Smeaton, who, in erecting the famous stone light-house which succeeded Rudyerd's, built the first thirty feet high from the foundation, solid, and so proportioned the walls and lantern above the fundamental solid that if their mass were reduced to a cylindrical shape it would add another solid column of about twenty feet in height; so that in opposing the action of the sea, the Eddystone Light-house is equivalent to a solid column of stone fifty feet in altitude.

Now from the above reasoning, 50 × 27-10, or 135 feet, is the altitude of the wave, whose upward hydrostatic action this building is, by its gravity alone, competent to resist; and as the atmospheric pressure is never removed from its summit, whilst the utmost altitude of the jet of water which is sometimes thrown over the lantern is short of 100 feet, its superabundant stability must be manifest.

The courses of this celebrated construction being dovetailed and joggled together, so as to prevent movement laterally; as long as its materials are proof against decay, the immutable laws of gravitation will retain it in position, and enable it to defy, as it has for eighty-two years defied, the utmost force of the Atlantic storms; unless, indeed, it should be assaulted by waves more than 135 feet high, which is not within the range of probability.

Breakwaters, during violent storms, strongly sustains the views taken in this paper, and shows but too clearly, that the mass of stone usually combined upon their summits, is deficient in the requisite stability.

The form in which the stone ought to be laid and connected together, should (the writer believes,) be that of a semi-cylinder, the axis lengthwise of the work, and the base laid upon such an inclination seaward as may counteract sliding, and prevent the possibility of its overturning upon the harbour angle, for we know that on a level plane, one half the amount of force will overturn a body which is necessary to lift it.

In the case, then, of a Breakwater similarly situated to that of the Delaware, stone weighing 175 lbs. to the cubic foot, and exposed to the assault of waves not exceeding twenty feet high, it would seem that the section shown in the following Figure 5, would be a proper

a, the high water line of spring tides, above which storm tides rise three feet or more; b, the plane of low water; c, semi-cylindrical mass of stone at least thirty two feet in diameter, thoroughly cemented and bonded together; d, fore shore securing the angle of the sea slope, to be mainly formed of cubical blocks of rough stone, weighing at least ten tons each; e, the general foundation raised to the low water plane by rubble stone thrown promiscuously into the water from the decks of vessels;* f, cemented foundation prepared for the semi-cylinder, and having a top slope seaward of about six feet base to one foot rise.

The advantage of a semi-cylindrical solid (which must average fifteen feet deep vertically) is that if in the construction it is properly bonded on the diameter, with thorough courses of dressed stone, whilst the blocks which form the curved surface are cut like arch stone, no one stone could be by any means extracted from its place if properly doweled laterally, and the whole would resist motion by its gravity as one solid mass; the interior backing or hearting of the semi-cylinder would be composed of rubble stone well set in cement mortar, and grouted full in low courses, divided into sections for the purpose.

It is a general idea that the forces acting against a Breakwater are augmented by a great rise and fall of the tide, but from the above reasoning it would appear that such a rise and fail as will allow a wave of the maximum height of twenty feet to exert its greatest energy; or a difference of ten feet between the top water of storm tides and the low water plane, (as exists at the Delaware Break water,) will enable such waves to act with as much power upon sea works as any other variation of tidal surface, and hence a greater rise and

* Experience at the Delaware Breakwater proved that by this process alone a rough stone work could be brought up with precision to a plane of two feet under the high water of neap tides.

fall than ten feet will not increase, whilst a less one would certainly diminish, the effects of the assailing waves.

The section of the Delaware Breakwater, as planned by the Commissioners appointed by the President of the United States, in 1829, under the act of Congress of May 24th, 1824, was trapezoidal in its general outline, the sea slope having a base of 1054 feet to a height of thirty-nine feet, and being profiled after the curvilinear figure to which the waves of storms had reduced that of the Breakwater at Cherbourg; the top was fixed at twenty-two feet, and the internal or harbour slope at one to one, or thirty-nine feet base, the entire base being 1663 feet to a height of thirty-nine feet; the base of the section which we have proposed as sufficient for a similar work, is 160 feet, and the transverse area would be nearly the same as that of the Breakwater in the Bay of Delaware.

Philadelphia, Nov. 1st, 1841.

FOR THE JOURNAL OF THE FRANKLIN INSTITUTE.

Cast Iron Rail for the Hiwassee Railroad. Designed by JOHN C. TRAUTWINE, Civil Engineer.

TO THE COMMITTEE ON PUBLICATION.

GENTLEMEN.—I send you a drawing of a rail which I conceive to possess some advantages over those hitherto employed, and which I shall introduce upon the railroad now under my charge. The great distance of our line from the nearest available shipping port, would have increased the cost of European iron to so serious an extent that our Board of Directors resolved to manufacture their own rails. About $110 per ton would have been the cost of English rolled rails, delivered along the line; nor could we obtain any lower bids from our Tennessee iron masters, although advertisements were published for some time, calling upon them for proposals. The principal cause of this was the limited scale of their establishments, which would not admit of their embarking in an enterprize so much more extensive than any of them had been accustomed to.

In furtherance of the object of the Board, the President and myself were deputized to visit the East, with instructions to procure such information, machinery, and workmen, as were essential for the manufacture of our iron, in every department, from the ore to the finished rail.

* With the design of increasing it to thirty feet if subsequently found necessary.

This road runs from Knoxville, in East Tennessee, to the dividing line between the States of Tennessee and Georgia, where it unites with other lines now under construction, extending to Charleston and Savannah. Its length is 94 miles.