METHOD OF MAKING THE FINAL SURVEY OF BLOSSOM ROCK.

A scow, 46 feet by 28 feet, was moored on the rock by means of four anchors and lines. Two observers on shore, and distant about one mile from the scow and from each other, took angles simultaneously with theodolites, on a flag staff erected on the SCOW. The direction of its axis at the same instant was determined by compass on board. As soon as the theodolite men had observed its position, two men with lead lines (one on each side and end) would sound all around it. When these sets of soundings were completed, its position would be slightly changed, the theodolite men would again be signaled, would observe its new position, and soundings would be made as before. This operation was repeated many times, until the whole surface of the rock was covered with soundings. We obtained over 1,800 soundings, about 1,300 of which were plotted.

After the soundings were reduced to mean low-water level and plotted, it was found that there were seven places where there was less than 24 feet of water. At these places there appeared to be small masses of loose rock, about the size of a cubic foot or less, rising above its general level. They were so small that it was often difficult to find them a second time. In previous examinations which had been made by us by means of the sounding float or "boom" we had found no place where there was too little water, and therefore the money had been paid to the contractor. As soon as the detailed survey above described was completed, the contractor was informed that these seven lumps still existed on the rock; he therefore, at his own expense, and with as little delay as possible, sent down a diver in armor, who found the lumps. reported that the surface of the rock was uneven, like furrows, and was covered with stones, usually quite small, and seldom as large as a cubic foot. All the lumps on these ridges were scraped off into deeper water. He went down twice, remaining nearly an hour each time. He could only go down about slack water.

He

We made a resurvey of that portion of the rock which had contained the lumps, and found over each of them 24 feet or more of water. Some of these lumps may have existed when that survey was made which resulted in the money being paid to the contractor, because they were so small, and being detached masses were not detected. Some of them, however, were made during the recent survey, when, several times, rocks, used as sinkers to buoys, had been lowered on the rock, and left there by the parting of the lines to which the buoys had been attached. The survey could only be made when the bay was smooth, and only fourteen days suitable for the purpose were found during three months. The soundings over the rock at the final survey are shown on Plate XI.

CONCLUDING REMARKS.

In concluding this report a few words with regard to the manner of conducting the work, the defects in it, &c., may be of value, though they are the natural deductions from the operations already detailed. The main defect was in firing the charges of powder before a sufficient amount of excavation had been made. A glance at the original plan, as partly shown on Plate VI, (the stone pillars were to be replaced by wooden ones,) and the plan actually followed out as shown on Plate VII, shows at once that in the former case the amount of excavated rock was so great that when the shell of the rock was broken and lifted up by the force of the explosion, the greater portion of the débris would fall into the excavation, and there would be at least the required 24 feet of water over the rock or débris; and in the latter case, the amount excavated was not half as much as the amount of rock remaining above the line of 24 feet at mean low water. Consequently, just after the blast there was only 14 feet of water over the débris. In order that the mass of rock over the 24 feet line should equal the mass excavated, about ten feet more of excavation should have been made. The contractor and many others supposed that the tide would sweep away the loose débris, but this idea proved fallacious.

It was supposed that a crust of only 6 feet thick, supported by the pillars of rock or wood, would be admissible; but, in fact, a thickness of from 14 to 18 feet was required to prevent the water from entering the chamber in quantities greater than could be easily controlled; but there was no practical difficulty in excavating more in the central part of the rock, though toward the edges suspicious leaks had developed themselves. The removal of the débris probably cost more than the amount expended up to and including the time of the blast. Had he made this 10 feet of additional excavation, it is probable he would have been perfectly successful, Again, the chamber containing the powder being only about two-thirds full of water at the time of the blast, made the resistance very much less than it would have been had the chamber as well as the shaft been filled, so that the water would have stood on the same level in the shaft as on the outside. To that fact may be attributed some of the contractor's want of perfect success.

Mr. Von Schmidt deserves a great deal of credit for the work he has achieved. His daring character is shown by his accepting a contract in which he was to receive no

money until the completion of an experiment the success of which could only be decided by the United States as the sole arbiter. The energy with which he pushed forward the work until the explosion took place, and the renewed energy with which he pursued his labor under such discouraging circumstances, deserves the success he attained.

W 10.

Report on and plan for the improvement of Wilmington Harbor, California, made under the instructions of the Chief of Engineers, dated the 22d day of March, 1871.

SAN FRANCISCO, CALIFORNIA, June 15, 1871. Congress, at the last session, made an appropriation of $200,000 for a breakwater at Wilmington, California. Wilmington is situated at the head of a small estuary which has its outlet in the bay of San Pedro.

This bay affords good anchorage and shelter for vessels during the greater part of the year. It is well covered from the northerly winds which prevail during the summer months, but it is open to the southerly gales which occur frequently in the winter. At such a time a vessel at anchor in this bay is compelled to run for shelter under the lee of one of the large islands which lie a few miles off the coast. As for a harbor, there is none nearer than San Diego, ninety miles to the southward, and to the north none nearer than San Francisco. This bay is the outlet for the productive plains about Los Angeles, and for a large extent of country to the eastward, which sustain a fleet of steamers and sail vessels of considerable size. Under this condition of things, it is apparent that a breakwater in the bay of San Pedro, which should give security to vessels under its lee at all times, would be a very desirable construction, and it is probable that at some future day the commerce of this bay may attain such dimensions as to justify its construction, which could not be made under an expense of several millions of dollars.

This construction is not, however, the one contemplated by the act of Congress.

This act looks to the improvement of the harbor or estuary of Wilmington.

The copy of the Coast Survey chart, herewith inclosed, exhibits an estuary about three miles in length, its navigable portion ending at Newtown or Wilmington, and its lower part expanding in a shallow bay divided from the bay of San Pedro by a bar with a depth of water less than 3 feet at low water.

Although the estuary in its navigable part is small, in no part exceeding seven or eight hundred feet in width, it is encompassed on almost all sides by extensive flats, which are covered by water at high tide. These flats extend for more than a mile above Wilmington, and below the village lie on either side of the stream. The area of the part of the estuary and flats lying above the lower end of Rattlesnake Island is about 1,300 acres. If we suppose this tidal area to be covered to an average depth of four feet, which is not far from the depth due to a spring tide, the quantity of water which ebbs and flows in one tide is more than 257,000,000 cubic feet.

The tidal ebb is not re-enforced by fresh water. No streams discharge into this estuary. The bed of San Gabriel river lies close along the upper edge of the estuary, and at nearly the level of the estuary, so that this river is liable to empty its flood-waters into this inlet, but the river bed is dry for the greater part of the year; and it is only for a short period in the winter that any water can be expected from this source, and this is not to be expected except in case of high floods in the river.

Within a few years this river has opened a new channel and outlet into the ocean, 10 miles to the southward. It is not probable that any considerable amount of discharge into the estuary will occur. Moreover, it is scarcely desirable, for the river waters bring with them a mass of sand and detritus, which are deposited on the flats and channel, shoaling the water and reducing the tidal area.

Referring again to the Coast Survey chart, we observe that the upper part of the channel is separated and protected from the ocean by Rattlesnake Island, which is a narrow strip of grassy sand dunes rising 8 to 10 feet above high water. Following along the seaward line of the channel, toward Deadman's Island, we cross a line of low flats, bare at low water, and rising above low water to a variable height of 2 or 3 feet. These flats extend for about two-thirds of a mile, and serve to do, in a partial degree, what is better done above by the higher bulwark of Rattlesnake Island, namely, to cover the channel from the sea, and to confine the ebb current between banks.

We observe that a navigable depth is preserved in the channel so long as these flats exist, and that when we leave them behind we enter upoù a wide shallow bay with no particular channel in any part.

Now, if we conceive Rattlesnake Island, or even the flats just spoken of, to be extended to Deadman's Island, and the channel to be thus confined, so that the ebb-tide shall be compelled to find its exit between Deadman's Island and the shore, instead of escaping as it now does, all the way from Rattlesnake Island to Deadman's Island, and thence to the main land, there seems no reason to doubt that the ebb current would, in time, excavate for itself a much deeper channel than now exists in the lower part of the estuary, provided, of course, that the bottom is of a yielding character. The first half of the ebb now escapes all along the line just referred to, but our observations indicate scarcely any velocity through the two small channels just below Rattlesnake Island, after half ebb. Before this period, however, the ebb escapes here, to a great extent, as well as over the flats.

The object to be attained is to connect Rattlesnake and Deadman's Islands. The survey made for the purposes of this report indicates considerable changes in these flats since the date of the Coast Survey map, which was made in 1859. The flats have been extended toward Deadman's Island, and in some parts have become higher, rising in one place for a small distance above high water.

It is proper to remark here that these flats are composed of fine sand, hard and compact in appearance, but, like most of the coast sands, at times and places quicksands. It has been found that an object of weight placed on the sands sinks, and soon becomes imbedded. Our observations show the existence of a clay-bed in many places below these sands. Any artificial structure placed between Deadman's Island and the flats near Rattlesnake Island must be made strong enough to resist the action of the sea, particularly on the part nearest to Deadman's Island, where the 12-foot curve comes in close to the island. The degree of exposure will diminish as we approach Rattlesnake Island, as we learn from the soundings in the chart. The shoalest water on the upper part of this line is from one-third to one-half a mile outside of the positions we propose to occupy, so that the waves can never reach our construction in their integrity.

Colonel Williamson, in his report published in that of the Chief of Engineers for 1869, estimated for a riprap of stone, on substantially the same line which it is now proposed to occupy, at a cost of $892,000 in coin. There is no reason now to suppose that adequate protection and im

provements could be afforded at a less cost by the use of this material, and this great expense leads to the inquiry whether wood cannot be used, at least in part, to advantage.

For a great part of the line cribs of wood would be more expensive, and, I think, a less secure construction than sheet-piling. The shifting sands are so easily moved, either by the powerful agency of storms or the more quiet and constant action of the currents, that cribs, unless founded on the underlying rock or clay, would be subject to the danger of being undermined; while at the same time they would not give the best kind of resistance to a change of direction in the channel. For instance, it is a supposable case that the outlet should have a tendency to make out through one of the channels opening to the eastward just below Rattlesnake Island, and in this event the crib-bulwark might be undermined and overthrown, and the current would have its course more or less free direct to the ocean; but if we sheet-pile this part, the current is obliged to eat its way to the foot of the piles before its course is open.

On the accompanying drawing entitled "Proposed construction for the Wilmington breakwater," will be found plans and sections of structures, five in number, with which it is proposed to occupy the line between Rattlesnake Island and Deadman's Island.

Construction No. 1 is the simplest and cheapest, and with this it is proposed to cover the portion of the line from Rattlesnake Island to the end of the flats, from X to Y, 3,630 feet; with this reservation, that in crossing the channels near the island, for a distance of 550 feet, construction No. 5 shall be used.

From Y to Z, Z being about 500 feet from Deadman's Island, it is proposed to build construction No. 3, which consists of two rows of heavy piles, driven in each row close together, the rows 10 feet apart, well braced and filled with stones. The sheet-piles in every case to reach below the probable depth of the adjacent channel.

From Z to Deadman's Island, a distance of 500 feet, where the bottom is rock in place, it is proposed to use cribs filled with stone, as shown in construction No. 4.

Now it is possible, as we build toward Deadman's Island, that the increased velocity of the current may wash out the line ahead, so that it would not be safe to use construction No. 3, so far as is now proposed, and in this case it may be necessary to use cribs over a larger portion than now anticipated.

Stone is not found about the bay of San Pedro in any great quantity, and what there is has no good qualities. Stone filling of cribs will, for this reason, be expensive. This circumstance accounts for construction No. 2, which it may be practicable to use on the less exposed portion of the line. This construction resembles No. 3, but differs in some details of the timber-work, and in the filling, which is of sand instead of stone. The line to be occupied is indicated on the tracings of the harbor in a heavy black line, so located as to afford easy flow to the current and to leave sufficient width for the probable channel.

The largest section of the estuary now is opposite the lower end of Rattlesnake Island, where the depth is as much as 20 feet. The area of this cross-section is nearly 7,900 square feet at low tide. All the tidal water to fill the flats above, ebbs and flows through this section. It is questionable, perhaps, whether so much water will ebb and flow in the estuary after our case construction is completed, as is now the case; but opportunity is left for the excavation of a cross-section at least equal to that above named.

It will be observed that these constructions are not carried far above high water.

It is to be presumed that the flats where construction No. 1 is to be placed, will, by virtue of the construction, be elevated to the top of the timber-work. It is supposed that the obstruction will intercept and hold the sand, and that after a time, longer or shorter, a strip of sand dunes, similar to those on Rattlesnake Island, will spring up on these flats, imbedding the timber-work, which when imbedded will have done its work; and henceforth it remains a matter of indifference, comparative at least, whether the bulwark remains sound or decays. Here, as on the other part of the line, the object of the work is less to cover the interior waters from the wind and waves, than to give proper direction to the ebb current.

Timber used in these circumstances is not only subject to usual causes of decay, but that portion exposed to constant immersion in the water is liable to the attacks of the teredo navalis, or ship-worm.

The first part of the air-line from X to Y, with the exception of the water-way of the little channels, may, I think, be constructed of unprepared timber.

The only process for treating timber by hydro-carbon oils, used on this coast, is the vapor, or Robbins process. The Seely process is not used here.

So far as I can learn, there is some reason to suppose that the Robbins process is likely to afford some protection against the teredo navalis. Experiments have been made in this harbor, extending over two years, which give promise of beneficial results, but the only real test, which is time, is wanting.

There is evidence, apparently conclusive, that the Bethell process, when applied to the extent of 10 pounds of oil to the cubic foot, afforded security; but to effect this requires expensive machinery, and involves a pressure of 150 to 200 pounds per square inch. The wood was saturated with oil. Neither the Seely nor the Robbins process pretend to accomplish so much, and if I am correctly informed as to the probable cost of effecting this result, it would be out of the question to think of doing it. The Robbins company, however, claim to saturate the wood to the depth of an inch, and to expel the albuminous matter from the whole of the timber.

Now it costs $10 per thousand feet, board measure, to treat timber here by the Robbins process-price in gold. I have received an offer from this company to establish works at Wilmington for the purpose of treating this timber, for $15 per thousand in gold.

This seems to be the state of the case: either the vapor process at $10 per thousand here, or no treatment. It is quite certain that in three or four years, if we do not treat this timber, the worms will have destroyed it. It is not certain, but it is probable, if we do treat it by this process, that it will last longer, how much longer cannot be said.

It is certain that the reputation of the company will be at stake in this case, if their treatment fails and the worms destroy the timber in the usual time, or they can never expect to treat any more timber for such purposes after the fact becomes known. I think, then, that they will try to make good work, and I am of the opinion that the experi ment ought to be made.

It is proposed to collect the materials, so far as the available money will permit, during the coming fall and winter, and to this end adver tisement ought to be made at once for lumber. I inclose with this report a copy of an advertisement for bids for lumber: