"Professor Brande was unable to give any additional evidence on the observed facts connected with the change suffered by cast iron exposed to the action of salt water, or in mines, and in various other positions. From experiments which he had made, he was led to believe that many of the appearances observed in the changes of cast iron arose rather from a peculiar mechanical combination of the molecules than from a difference in the chemical constitution of the metal; no difference could be detected by analysis in the metal which had undergone change and that which had not. It should be remarked that the contact of two metals was not essential to cause galvanic action; a film of oxide on the surface of the body of metal formed a very active galvanic pile: hence arose the necessity of preventing oxidation by proper paints or varnish before using pieces of cast iron in exposed situations." The usual form which has been adopted, as well in Europe as in this country, for the pile-assemblage which constitutes the main support of the building, is octagonal, according to the drawing No. 1. A modified arrangement has been proposed, according to the drawing No. 2; and last, a square assemblage of the piles, according to the drawing No. 3. In the assemblages of No. 1 and No. 2, it was found that there was a tendency or liability to twist, which is the occasion of the modified form No. 2. But, after a most careful examination of all these assemblages and of their tendencies, the one No. 3 was adopted, as more completely obviating the tendency alluded to, and which, on that account, would probably require a less expensive or less complicated counter-bracing. The plan No. 3 had other advantages. To penetrate the soil of Sand key, it was found necessary to reduce the screw-flanch to about two feet in diameter, and to atfix a cutting bit to its edge. This reduction of the width of the flanch slightly reduced some of the advantages from the usual flanch of the screw pile, and rendered it prudent to increase the number of piles. Plan No. I is an assemblage of 9 piles. The additional number of piles of plan No. 3 compensates for the reduction of the flanch, and gives also a better bearing for the super structure. The Wyre screw-pile light, in England, is sustained on 7 piles, being hexagonal, with centre pile. The Maplin Sands light-house is octagonal, with 9 screw piles-one in the centre. Works of this kind are exposed to accidents chiefly from two causes: 1st. The tendency to upset from lateral pressure. 21. The tendency to sink or yield from perpendicular pressure. Both of these tendencies have been so completely counteracted in the light-house built on the octagonal assemblage No. 1 in Europe, that we would have no fear from the plan No. 2 for our climate, and feel fully assured from the plan No. 3. Another cause of deterioration of these structures is in their liability to vibrate, which is, however, thoroughly counteracted by a judicious sysem of bracing, and of common attention to the braces. Another cause of deterioration is in the liability of the iron piles to rust. I know of no kind of structure which is not exposed to deteriora tion from some cause, and do not consider this as one of any very serious account. But we have experience on that head which will justify positive conclusions. Upon many of the reefs on Long Island sound, it has been the prac. tice for many years to erect wrought-iron spindles of about 4 inches in diameter and from 15 to 25 feet in height. Such spindles last from 15 to 20 years, unless carried away by ice. One spindle has been up about 20 years without being renewed. The wasting takes place principally between high and low water, and has been found to be about one-tenth of an inch in one year. If, however, (says the officer who made the investigation,) the zincing process, or if a precipitate of copper be resorted to, there is every reason for believing that iron thus protected would last twice or three times twenty years. I have remarked that we have proceeded with great caution in these works, which will be readily believed from the fact that, as yet, no more than $12,848 of the appropriation for this work has been drawn from the treasury. Its The principles of the screw-pile are both simple and apparent. first application was a mooring for vessels in storms. It was found to be so extremely efficient, that the idea was adopted of erecting piles upon such screws to sustain light-house structures on sand bars, under water, ex posed to the action of the sea, and where the usual structures of masonry could not be erected, or not without extraordinary cost. The principle of such foundations has already been well tested, both on the east and west coast of England-off the shores of which have been erected screw-pile light houses that have now withstood the storms of several winters without exhibiting the slightest symptom of insecurity or decay. The stability of such structures depends on two causes: first, the firm hold which the broad screw takes of the ground, by being forced far beneath its surface; and secondly, the solid part of the building being placed above the reach of the highest sea, no broad surface is opposed to the free passage of the waves-consequently, the structure is not affected by them. The first foundation of this description was fixed in the Maplin Sands by Mr. Mitchell, in 1838, by order of the Trinity House, at the recommendation of their engineer, Mr. James Walker, for the Maplin lighthouse; and though it stands upon a bank of loose sand, many miles from the nearest coast, and exposed to the swell from the German ocean, yet it is as stable and likely to endure as if based upon a rock. But the first light-house of this description was erected by Mr. Mitchell in 1839, at the entrance of the sea beach leading to the town of Fleetwood-onWyre. A distinction should not be lost sight of, in reference to the screw-pile plan, between a work for a light-house and a work merely for a beacon; the latter requires neither dwelling place, nor lantern, nor lighting apparatus, and, in consequence, will rarely require, in any position, an expenditure exceeding $10,000. The danger of the sea is from the break of the wave; the pile not presenting a sufficient surface, the wave does not break against it, but passes harmlessly by. Structures, therefore, on this principle, experience no shock from the wave. Also, as the wave does not break, and no additional force is produced in it in passing over the shoal, the shoal is not changed or swept away, and the holding-ground of the screw pile is therefore not disturbed. In building a light-house or any work on sand bars under water, exposed to a sea wave, one of two principles must be adopted. Either the work must be on a plan which shall avoid the concussion of the wave, or it must be of sufficient strengh and massiveness to resist that concussion, or, in other words, to resist the blows of a sea-surf. I have no knowledge that any work on the second plan in such positions has ever yet succeeded. I do not say that such works in such positions cannot be made to stand, but I do say the expense would be enormous-far exceeding any estimate that any well informed engineer would like to make, and utterly beyond any advantages in proportion to its cost. Upon the first plan, works have been constructed at a reasonable cost. Moreover, the application of such a plan, because of its reasonable cost, is particularly adapted to our coast; and I have no doubt that beacons and light-houses on this plan can be easily put up, and be made to stand upon any of the many sand bars and shoals which are the sources of so many losses on our coast, from Rhode Island to Florida. This conviction induced me, many years since, to bring the Mitchell screw-pile to the notice of government, in several of the annual reports from this office, and to direct the attention of two officers of the corps, Captain Swift and Captain Hughes, then in Europe, to examine the Eng. lish light-houses on this plan. The opinion of both of these officers, after a thorough personal inspection, as well of the plans as of the works con structed in accordance with them, was decidedly in favor of their use for all such and similar localities. Being, therefore, satisfied that no work of masonry could be put upon Sand key without "inordinate expense," and being also satisfied that a durable light-house, upon the principle of Mitchell's screw-pile, can be put up there at a reasonable expense, the plan of the screw-pile has been adopted for that place, and the work is now being put up. There is a limit of utility in all things. This principle limits the use of the screw-pile for light-house structures to bars and reefs under water, and exposed to the sea wave. When a solid and dry foundation can be ob tained, masonry can be resorted to without fear of inordinate cost or of want of stability. Carysfort reef.-The wording of the law in reference to this work is as follows: "For a light-house on Carysfort reef, the sum of thirty thousand dol lars, heretofore appropriated, and carried to the surplus fund, is hereby reappropriated." It appears there had been in 1837 an appropriation for constructing a light house on this reef of $20,000. In 1838, there was an additional appropriation of $40,000-making a total of $60,000. I do not find any appropriations in favor of that work from 1838 until 1847, when the reappropriation, as above stated, of $30,000, was made. It is understood that no part of the $60,000 previously appropriated had been used, as the estimate by the Treasury Department for the work at this place was for $100,000, and it was, in 1838, considered unsafe to Part ii-26 commence the building before the amount of the whole estimate was ob tained. Our first effort was to have the reef surveyed. It is a coral reef, under water, distant from the shore, and so dangerous to the navigation of that coast that a floating light is maintained to point it out. The examination of the reef indicated generally so hard a surface crust that it was doubted if the screw could be made adequately to penetrate, and, in consequence, the plan was so far modified froin the original intention as to induce a change from the screw-pile to the disk-pile. A second examination, in 1818-49, confirmed these views, and that the iron pile, instead of being inserted by aid of boring, should be driven home, which would add to its power of resistance. The light-house, after being made and framed in Philadelphia, was carried to the locality in December, 1849, and measures were commenced for its erection. As these measures will be better understood by using the words of the superintending engineer, Major T. B. Lennard, the following extracts from his reports are submitted. Extract from report of December 26, 1849: "I had the honor to inform you of my arrival at Indian Key in the barque Robert Morris, with a part of the light-house to be erected on Carysfort reef. The schooner Captain Williams had arrived the day before. The schooner Susan was dismasted off Hatteras, and put back to Philadelphia for repairs. She arrived at Indian Key on the 8th of November. "On the 1st November I began to discharge the Captain Williams at Indian Key, landed the last of the cargo on the 6th, and the same night despatched her to Key West to bring forward the lumber stored there. On the 1st we also put up a forge, and began to build a scow. The labor of unloading the Captain Williams was performed under great disadvantages. There is no wharf at Indian Key, and the ship's boats were the only means of conveyance we possessed to transport the heavy iron to the shore. To move it on shore, I laid down a plank road, and built a truck, with solid oak wheels, banded with iron. "The scow was launched on the 14th November, and on the 15th I began to lighten the barque to enable her to get to the anchorage near the light-ship. This was a work of much labor, as the vessel lay nearly a mile from the shore, and the scow, which received the iron from the ship, had to be towed that distance. On the 17th we had reduced the draught to ten and a half feet, but, the wind being adverse, we did not get under way until the 19th; and on the afternoon of that day the three vessels came to anchor near the light-ship, in the position they now occupy. We are in a secure harbor, about four miles from Cayo Largo, the nearest land. "Immediately after our arrival, Mr. Lewis and myself began a close examination of the points supposed to be eligible for the site of the proposed light-house. The examination of Captain Stansbury was confined, I believe, to the immediate vicinity of the light-ship-to the eastward, on the bank marked CC on the accompanying trace; and to the southwest, on the shoal D, known as the basin bank. If I am correctly informed, the latter position was that to which the Captain gave the preference. Our examination was carried further seaward, and resulted in the selection of the position marked A. This point is on what is termed the outer reef, and is on the edge of the Gulf stream. The outer reef is a coral formation extending from A north 19° east, and south 29° west, and rising near the surface, at low tides, at points, as at AA. The point A has the shoalest water. Within a space comprised within a circle of about 150 feet diameter, the solid surface of the reef is about 4 feet below low water; but above this solid basis there is a rank growth of leaf coral, which in a few places shows some points above the low tide." "The advantages of this position for a light house will be perceived at a glance, and the only objections that can be urged against it are connected with the difficulty of erecting the building at a point so exposed and so remote from our resources. "It is extremely doubtful whether a sufficiently solid foundation could be found on the bank CC. I drove an unshod spruce pile, without ring, eighteen and a half feet into the bank, and the last three blows gave a penetration of fifteen, eighteen, and fifteen inches, respectively. Any position near the light ship would be liable to the objection of having a dangerous reef two or three miles seaward from it, and, besides, the useful range of the light would of course be less at a retired than at an advanced position. "The erection of a forge on tripods, and a timber platform on piles, upon the bank at the point F, occupied us until the 29th November. Everything necessary to commence a platform on the outer reef had been prepared and secured on the scow, awaiting a favorable day to tow her to the reef, a distance of about four miles and a half. The 3d of December being quite calm, four boats took the scow in tow and conveyed it safely to the reef. On the 4th and 5th we could do nothing on account of the unfavorable state of the weather, but on the 6th I drove eight piles, and on the 7th one, completing a square platform of sixteen feet. A foothold was thus obtained on the reef, and we secured the means of working whenever the weather would permit the boats to proceed to the position. The scow could not have been used to drive piles for the entire platform. On the shoalest points it could not have been used on account of the coral points, which would soon have destroyed it; and in deeper water the swell was ordinarily too great, except on rare days, such as that with which I was favored in commencing. "Having secured in a strong manner the small platform, 1 mounted upon it two pieces of timber thirteen inches square and forty feet long, laid parallel to each other, and firmly united by transoms. At one end of these skids I placed the pile-machine, which moved upon a railroad, and at the other end sufficient weight to counterpoise it when projected ten feet beyond the side of the platform. With this arrangement I have overcome the difficulty of erecting the platform, which at first appeared very formidable, as it was not practicable to drive from the scow, and the coral bottom forbade the use of tripods. "The character of the foundation, as exhibited in our pile-driving, is, I think, satisfactory. I have penetrated six feet without encountering cavity or considerable variation consistency in the bottom." I also submit a copy of his report of July 16, 1850: "In my letter from the Carysfort reef of December 26, 1849, I advised you of the progress that had been made up to that time in the preparatory |