Office of the Brandywine Light-house,
Philadelphia, November 1, 1839.
Sir: The Following report of the Brandywine light-house for the past season, made in compliance with the general regulations of the bureau, is respectively submitted:
The operations have, in consequence of the failure of Congress to make the additional appropriation called for last season, been limited to such expenditures as could be covered by the balances of former years, and , from the insufficiency of these, were confined to measures preliminary to taking position at the site of the work. Among the most important of these are the building of the caisson, by means of which it is proposed to establish the foundation, and the preparation of the foundation stone. The caisson with unimportant exceptions, was completed some months since. It is elliptical in form, about fifty feet in length by forty feet in width, and twenty-five feet high, constructed entirely of white oak, thoroughly iron-fastened, and in all respects a substantial vessel, capable, it is believed, of resisting the shock to which it will be exposed, and fully equal in other respects to fulfill the objects for which it was designed. It is still on the stocks, where, protected by a roof from the weather, it will remain until required for use. The stone for the foundation, or the foundation rock, as it may very properly be called, is not preparing under the contract of the 9th of January last. Two courses are finished, and two more are in a state of considerable forwardness, and will, with the remaining courses, be ready before the opening of the coming season of operations. The foundation rock is 42 1/2 feet long by 31 1/2 feet broad, and 20 feet high. It is formed of 671 blocks of rough-hammered stone, disposed in ten courses. Of these blocks, 372 weigh each three tons, and are regular and alike in form; and 299 weigh each from one to three tons, and are irregular and unlike. A single course weighs 161.33 tons, and the entire foundation rock, 1,613.3 tons. Each course, as finished, is laid dry at the quarries, where the necessary platforms and cranes are provided, to prevent the delay in building, arising from errors in working the stone, which might prove fatal to the success of the work. The brick pavement bond is the one adopted for the foundation rock; the joints of each course lying at angles of 45 degrees with those of the courses adjacent, in order to resist, in the most effectual manner, the disposition which the mass, from unequal subsidence or other cause, might have to fall of break off. To give still greater security against this tendency, copper dowels and cramps will be used to bind the whole together. Of the latter, nearly 1,700 will be employed.
The original design for this work contemplated a foundation, built on a mole of breakwater stone from the level of low water. The objections to this mode of construction were stated in a communication addressed to the bureau on the 14th of July, 1837, in which, for reasons then given, it was recommended to establish the foundation by means of a caisson. Further reflection has served to confirm these views. Fears were entertained that, by the plan first proposed, the superstructure, being built upon breakwater stone thrown at random on the bottom, would, by unequal settling, be liable to fracture; and it was doubted whether heavy masses of masonry, raised upon such a base, ever proved entirely satisfactory. It was also urged that, as the masonry, until it reached high-water, would necessarily have to be carried on at short intervals of time, and under very great disadvantages at so exposed a position, the cost of construction would thereby be very much increased. These objections are obviated by the use of the caisson, as the work may be carried on at any stage of the tide, and the masonry built from the bottom, saving the thickness of the caisson, which it is presumed will settle in the sand; thus affording a reasonable expectation that the subsidence will be equal, and the superstructure secured from liability to fracture. It will be remarked that the employment of the caisson does not necessarily constitute a modification of the first design, but rather furnishes a means by which that design may be securely carried out. It would fail, however, to yield all the advantages which belong to such a mode of construction, were the foundation not to receive, under the facilities afforded, a more perfect and stable character. It has, therefore, been deemed proper to substitute for this part of the work masonry of wrought stone, instead of the rubble masonry resting on breakwater stone, as was proposed in the first instance. These changes involve, upon the whole, a considerable increase in the cost of the work; an increase, however, which is fully justified by the additional security afforded of prosecuting the operations to a successful termination, and by the greater stability that will be given to the work itself. This increase, as well as that arising from advance in prices since the date of the first estimate, will be indicated under the proper heads. The most prominent among the latter, is in the cost of the breakwater stone, advanced 20 and 11 per cent. respectively for the two sizes, and which is now set down at the contract price of the present season for Delaware breakwater. It will also be seen that the contingencies have been raised to 17 per cent., as that has been the average, nearly, for several years at the above work, and there is no reason to supposed they would be less for the Brandywine light-house.
The following statements will show the
increase which the proposed
and the advance in prices will cause in the cost of the work: The
estimate for the space which will now be occupied by the foundation,
by means of the caisson, was as follows:
|41 tons of breakwater stone, of pieces of two tons and upward, at $2 50 per ton,||....||$102 50|
|1,436 tons of breakwater stone, of pieces of 1/4 to 2 tons, at $1 80 per ton,||.....||2,584 80|
|340 cubic yards of heavy building stone, at $5 20 per cubic yard,||.......||$1,768 00|
|Laying the same, including all expenses, at $17 20 per cubic yard,||......||5,848 00|
|Contingencies, 15 per cent.,||....||1,545 50|
|Total amount by original estimate,||.........||11,848 80|
|The revised estimate, by the modified plan, for establishing the foundation by means of a caisson, and under the advance in prices, is as follows: caisson||..............||$11,011 63|
|962 cubic yards of rough hammered stone, including lewis- holes, and cramp-holes, and channels, at $22 95 per cubic yard,||..........||22,077 90|
|Laying the same, including all expenses, at $8 60 per cubic yard,||...............||8,273 20|
|2,092 copper cramps at $1 25 each,||.................||2,615 00|
|Contingencies, at 17 per cent.,||..................||7,476 21|
|Total amount of the revised estimate by modified plan, &c.,||.....................||51,453 94|
|Total amount of original estimate||.................||11,848 80|
|Total amount of increase,||.................||39,605 14|
How much of this increase is fairly attributable to the new plan, and how much to the advance in prices and increases in contingencies, it is not easy to determine. If, however, the average prices now paid for breakwater stone be taken as a criterion, more than $8,000 is properly assignable to the latter causes.
The exposed situation of the Brandywine Shoal, lying as it does widest part of the bay, and within eight miles of the ocean, renders it absolutely necessary that the protecting work be formed at once on bringing the caisson in position; as on the one hand, were the foundation constructed and no protection provided, the action of the waves, in their recoil from the mass, by washing away the sand composing the shoal, would in a very short time undermine and destroy the work; so on the other, were the protecting work completed in the first instance, deposits of unequal density and irregular form would be induced, where now the bottom is singularly hard and flat. The removal of theses deposits, and of any stone which from carelessness or design may have been thrown within the same space, would be attended with much labor and no inconsiderable expense, and, what is of much more importance in a work of this character, with a delay which might prove fatal to the undertaking. The two operations, indeed should be carried on as nearly simultaneously as their very different characters will permit, and, to be secure against the ice and storms and the succeeding winter, be brought to a close in a single season. At so exposed a position as the Brandywine, this may be set down at barely three months, commencing with the 20th of May, a period certainly very limited to complete a work of the extent contemplated, considering the difficulties and vexations under which it must be prosecuted.
the following estimate for the next
season is based upon the views
given. Admitting their soundness, the necessity of providing at once
means to carry them out, need not be urged. The appropriation of a less
amount would merely take from the Treasury a sum that could not be
profitably to this work. It is also proper that the appropriation be
at least two months before the opening of the season, to afford time
making the necessary contracts and arrangements; otherwise, all
must be postponed until the following year.
|11,711 tones of breakwater stone, of pieces two
tons and upward, at $3 per ton,
|21,438 tones of breakwater stone, of pieces 1/4
to two tons, at $2 00 per ton,
|Cost of breakwater,||........||$78,009 00|
|775,24 cubic yards of rough-hammered stone, including
cramp-holes, and channels, at $22 95 per cubic pard
|Laying the same, including all expenses, at $8 60 per cubic yard,||......||6,667 06|
|1,686 copper cramps at $1 25 each,||.......||2,107 50|
|Contingencies, 17 per cent,||......||19,649 78|
|Total amount,||.........||135,236 72|
|Amount already appropriated,||.........||45,000 00|
|Additional appropration required for next season,||.........||90,236 72|
It is proposed to carry on the operations under the foregoing estimate, in the following manner:
The proper position for the light-house is at the point where a line drawn on the usual sailing course of vessels, proceeding up the bay, bisects the mouth of the channel between the Brandywine and Brown shoals, and strikes the former at the assumed depth. To determine this point, it will be necessary to lay down the lower half of the channel between the Brandywine and Brown, including the seaward points of those shoals, and the western side of the former, with their relation to the meridian and the shore. This operation will consist, in the first instance, in fixing the position of not less than four permanent stations, well selected on the above-named shoals, from which the detailed hydrology, so essential to a correct determination of the point in question, will be carried on. These permanent stations will, each, consist of a single tripod of timber, 35 to 40 feet in height, properly strutted out and weighed at the foot, with a fourth spar rising from the apex, and surmounted by usual tin cone to ensure its being seen distinctly from the shore. To one of more of these will be attached the tide registers, necessary to a correct reduction of the sounding to any given plane. Of the permanency of these tripods under all circumstances, except against ice and the worm, no doubt is entertained.
The next operation in order is to mark out the site with piles, to guide in placing the caisson and depositing the stone which will surround it. The small number of these piles will allow of their being driven in a few days, by a pile engine placed on a platform, resting on timber tripods of the kind already described, but smaller in size. To ensure the completion of these operations in proper season they should be commenced as early in the spring as the boisterous character of the locality will permit.
The placing of the caisson in position is an operation of great delicacy; and on giving, at once, the necessary protection to the bottom around it, depends the success of the work. It is in fact the turning point in the undertaking, and, in comparison with which, the subsequent risks and difficulties are of small moment. No means should, therefore, be neglected to ensure it against failure; and a shortsighted economy would prove fatal to it. These means have been the subject of much anxious reflection, and being predicated on the truth of the proposition that the soil composing the shoals capable of sustaining the work, are all directed to the single object of retaining this soil in its natural position. In what manner it is proposed to effect this object will now be explained.
The caisson, provided with the necessary moorings and machinery for hoisting stone, having had laid on board as much of the foundation as will cause it to draw about 15 feet water, will be towed by one of more steam-boats to some convenient harbour in the immediate neighbourhood of the Brandywine shoal. This point will likewise be the rendezvous for vessels carrying Breakwater stone, and that portion of the foundation coming next in order in the construction. Here the final arrangements will be made, and taking advantage of a settled state of the weather, the whole will move down to the scene of operations. This site, it will be remembered, had already been marked out. It will, also, be borne in mind, that the caisson is supposed to be loaded to a draught of fifteen feet, or 3 feet more than the depth at the proposed sire, at the lowest spring tides. It will be evident, therefore, that the caisson cannot be placed in position at less than half tide; and, to allow sufficient time for securing it over the selected spot, this should be done on the flood. At half ebb the caisson will fall on the bottom, when the work of loading it with additional foundation stone will be prosecuted with great diligence, in order to prevent, if possible, its floating again on the rise of the tide. To effect so desirable an object, it will be necessary to take on board about 200 tons in the time that will elapse between half ebb and high water, or about 8 1/2 hours. For this purpose two boom-cranes attached to the caisson, will be employed. These, fully manned, and unloading from separate vessels, will be able, in a favorable state of the weather, to take on board in the time stated, 85 stones weighing 255 tons; affording a large excess of weight as a set-off against the difficulties and delays incident to so exposed a position, over and above the quantity required to retain the caisson on the bottom at any stage of the tide. The stone thus transferred to the caisson, will be placed conveniently on the decks for being laid by the cranes. The eight mooring piles, attached to the caisson will now be driven, by engine provided for the purpose, in order to prevent any lateral motion to which it may be liable from currents or waves, until the further loading shall make it perfectly secure. If from stress of weather, or other cause, the amount of labour calculated upon above be not accomplished, arrangements will be provided, for flooding the caisson to ensure its safety until such time as the work may be resumed.
The caisson being now secured, the next object is to prevent the sand of the shoal from being carried away by any new action, given by its presence to the current or the waves. This will be effected by paving the shoal with breakwater stone. To this end about 75 vessels will be provided, which, estimating their average load at 60 tons, will carry 4,50 tons, a quantity sufficient to cover the bottom to a depth of 3 feet for 60 feet round the caisson: allowing that this space will accommodate ten vessels at the same time, and that two hours would be required to unload a vessel, this quantity may be deposited, in moderate weather, in sixteen hours. The early completion of this measure is deemed so important to the success of the undertaking, that a large force will be employed to prosecute the work as rapidly as possible; and the insure, as far as practicable, a uniform distribution of the stone over the designated space, the place of each vessel and of her deposits will be represented on a diagram. No importance is attached to any minor irregularities which may occur in the paving, as the tides in flowing over the general surface of the stone, so far from removing the sand, will cause deposits in the spaces between the stone. And, again, with a view to compensate for any loss which may have occurred before the paving is completed, a result not anticipated, but principally to prevent the great sill-piece from the attacks of the worm, clean sharp sand will be deposited in large quantities alongside the caisson. The vessels, as they successively deposit their loads, will return to the quarries for more stone to form so much of the final mole as is considered necessary to place the work in a condition of safety against the storms of the approaching winter. This quantity is set down in the estimate at about 83,000 tons (can't read the first number - might be 33,000 tons) and may be deposited in eleven weeks, or at the rate of 3,000 tons a week, the average quantity received at the Delaware breakwater. The laying of the stone of the foundation rock will be resumed immediately, on the caisson becoming fixed upon the shoal, and the work vigorously prosecuted until completed. As two and a half courses will be laid before proceeding down the bay, but seven and a half of the ten courses will remain to lay after arriving at the shoal. These consists of 500 blocks, and as they will be furnished with lewis and cramp holes, and lettered and numbered, agreeably to the diagrams in the hands of the workmen, the hope is entertained that even the time stated above may be sufficient to finish this part of the work.
In thus laying down a plan of operations, it is not for a moment supposed that it will be expedient, or at al time practicable, to adhere to it. The object is more to elucidate general views in regard to the principles which should govern the mode of preceding, than to point out the details, which, in a work of so novel a character, must depend upon circumstances which cannot always be anticipated, and must be provided on the spur of the occasion.
It now remains to give a revised estimate for the entire work, under the modifications and increases already noticed. No revision has been made in the plan of the light house proper, or lantern, nor is it probable that nay changes in either will be found necessary, that will materially affect the cost of the work. The present object is the successful establishment of the foundation, in which consists the only real difficulty in the construction.
|14,734 tons of breakwater stone of pieces of
tons and upward, at $3 00 per ton
|25,037 tons of breakwater stone, of pieces of
1/4 to 2 tons, at $2 00 per ton
|Cost of artificial island, or protecting mole,||...................||$94,276 00|
|Cost as already stated||...................||$43,977 00|
|The original amount under this head was||21,787 73|
|From which subtract (now estimated
for the foundation) the cost of 340 cubic yards of heavy building stone at $5 20 per cubic yard
|And for laying the same at $17 20 per cubic yard||......||$5,848 00|
|Also for laying 148 cubic yards of superstructure, reduced from $17 20 to 8 60||.......||$1,272 80|
|Subtotal to subtract from original cost
|| $8,888 80
|New Cost of the light house proper||............||12,898 95|
|Cost, the same as originally estimated||............||1,340 39|
|Contingencies, 17 per cent||..............||25,923 82|
|Total amount ........||178,416 89|
It is not improbable that complete security may be given to the structure short of the fulfilment of the profile on which the above estimate is founded, and that the protection given in the first instance may prove amply sufficient, reducing, correspondingly, the aggregate expense. The grounds for this hope may be found in the fact that the proportion of two to one given to the exterior slope of the latter work, is the same, up to the 30th September, 1836, as that of the Delaware breakwater, a work, certainly exposed to greater shocks, though, on the other hand, more secure from the greater depth of water in which it is founded. The result may likewise show that the mason's work is set down at too high a rate. Nevertheless, it is deemed safest in a work constructed under such novel circumstances as the Brandywine light-house, to retain both the items at the highest rates, to meet any unforeseen contingencies to which the operation may be liable.
I have the honour to be, sir, very
respectively, your obedient
Major Topographical Engineers, &c.
Col. J.J. Abert, Topographical Bureau.