Part 5

It was suggested that water might be obtained from the Passaic Falls, at a distance of about eighteen miles from the city, in New-Jersey. The objections to this project were, that it would be going into another state, that an Aqueduct bridge over the Hudson River would obstruct its navigation, and iron pipes laid across the bed of the river would be exposed to injury from the anchors of the shipping. Another plan was proposed which contemplated a permanent dam across the Hudson River extending from the city to the Jersey shore. This dam was proposed to be built about 2 feet above the level of high tide, thereby keeping all the salt water below; and above the dam would be the fresh water for supplying the city, which must be pumped up into a reservoir by means of water-wheels, which would be operated by the overfall of water when the tide was low, but when the tide was up within 2 feet of the top of the dam there would not be sufficient fall to propel the wheels. Locks were to be inserted in the dam, of a sufficient number to accommodate the vessels on the river. The river, at the place where it was proposed to locate the dam, is over a mile in width, and in the channel the depth below the surface to proper foundation for such a structure, would probably be 50 feet. The difference of tides is about 5 feet, which added to the height of dam above high tides, would give 7 feet of the top of the dam exposed to the pressure of the water on the up stream side when the tide is low.

It was suggested that the hydraulic power here obtained, could be used for manufacturing purposes, except that portion of it which would be required for elevating the water to the reservoir. This plan of supplying the city with water was objected to, because it could not be accomplished except by an Act of the Legislature of New-Jersey as well as that of New-York, and it was also questionable whether such obstructions could be placed in navigable rivers without interfering with the powers of Congress to regulate the commerce of the nation. It was feared that in locking vessels through, the salt water would become mingled with the fresh above the dam where a supply would be taken for the city, to such a degree, that it would render it unfit for domestic use. The quantity of land that would be overflowed by the water set back by the dam, presented another objection. The space of time that the tide would be sufficiently low to allow the wheels to work in pumping water into the reservoir, would be entirely too short to insure a supply. This objection was offered by Frederick Graff, Esq., the superintendent of the Philadelphia Water Works, who stated that although the dam on the Schuylkill River is raised 6 feet 6 inches above the highest tides, the delay in pumping, occasioned by the tides, averages seven hours out of the twenty-four; and in full moon tides, from eight to nine hours.

The projector of this plan set forth many advantages which he thought would arise from the construction of the dam, but the obstruction to the navigation of the river, the destruction of the shad fishery, and various objections besides those already mentioned, induced the Water Commissioners to reject the idea of building a dam across the Hudson.

We have now gone over most of the preliminary steps which were taken before deciding upon the source for a supply of water.--Having fixed upon the Croton River as a stream possessing the requisite advantages for a supply, questions naturally arose as to the manner in which it should be conveyed to the city. The distance being about forty miles, over a country extremely broken and uneven, and following a direction, for a portion of this distance, parallel with the Hudson River, encountering the streams which empty into it and form deep valleys in their courses. It will be interesting to notice the different plans which were suggested for forming a channel-way to conduct the water. The following modes were presented:--a plain channel formed of earth, like the ordinary construction of a canal feeder:--an open channel, protected against the action of the current by masonry:--an arched culvert or conduit, composed essentially of masonry; and iron pipes. In deciding which of these modes should be adopted, it was necessary to make a comparison among them as to their efficiency for conducting the water in purity, and in the quantity required, their permanency as structures, and their cost.

The disadvantages attendant upon an open canal were, that by filtration through the banks there would be a heavy loss of water;--the difficulty of preserving the water from receiving the wash of the country, and preventing injurious matter from being thrown into it and rendering it impure, and the impurities which might be contracted by passing through different earths. Evaporation would also occasion a serious loss of water. The banks would be liable to failure in seasons of long-continued rains, and the city depending upon this for a supply, would be cut off, except there should be sufficient in the reservoirs to furnish a supply during the period of repairs. The canal could never be subjected to a _thorough_ repair, because of the necessity of keeping it in a condition for furnishing water constantly during the whole year, so that all repairs would be done under great disadvantages, and the channel would be yearly growing worse until its failure might become a public calamity. In regard to the open channel having the sides protected by masonry, the objections were found to be such as would apply equally to every species of open channel; namely, that it would be exposed in many situations to receive the wash of the country; that it would be unprotected from the frost, and liable to be interrupted thereby, and lastly, that there would be a loss by evaporation. It was supposed that these objections might be obviated by certain precautions; for example, the wash could be avoided by making sufficient side drains; and the interruption liable to occur from frost and snow, and the evaporation, to a certain extent, could be prevented by closing the channel entirely with a roof over the top. The close channel or culvert, composed essentially of masonry seemed to possess all the requisite advantages for conducting the water in a pure state and keeping it beyond the influence of frost or any interruption which would be liable to occur to an open channel. In point of stability this plan had a decided preference over either of the other plans proposed, and the only objection offered was the cost of the work constructed in this way. To avoid too great expense it was proposed to make use of a mixed construction, using the close channel or culvert in situations where deep excavations occurred and it would be desirable to fill in the earth again to the natural form, also where the line of Aqueduct intersected villages, and using the open channel with slope walls for the residue of the distance.

In regard to iron pipes for conducting the water, it was found that a sufficient number of them to give the same sectional area as would be adopted by either of the other plans would be more expensive, and considering the great distance and the undulating surface over which they would extend, other disadvantages were presented which added to the objections, and the plan was considered inexpedient. Could a line be graded so as to give a regular inclination from the Fountain Reservoir to one at the city, then the expense of laying iron pipes for conducting the proposed quantity of water, would be greater than for constructing a channel-way of masonry; and when laid, the pipes were thought to be less durable. Should the pipes follow the natural undulations of the ground, there would be so much resistance offered to the flow of water that the discharge would be diminished in a very great degree.

The close channel or conduit of masonry was adopted as the plan best calculated to answer all the purposes of conducting the water to the city.

_Sources of the Croton River._

The sources of the Croton River are principally in the county of Putnam, at a distance of fifty miles from the city of New-York; they are mostly springs which in that elevated and uneven country have formed many ponds and lakes never-failing in their supply. There are about twenty of these lakes which constitute the sources of the Croton River, and the aggregate of their surface areas is about three thousand eight hundred acres.

From these sources to the mouth of the Croton at the head of Tappan Bay in the Hudson, the distance is about twenty-five miles. The country bordering upon the Croton is generally elevated and uneven, not sustaining a dense population and cleared sufficiently to prevent injury to the water from decayed vegetable matter. The river has a rapid descent and flows over a bed of gravel and masses of broken rock. From these advantages there is good reason to suppose that the water will receive very little impurity from the wash of the country through which it flows, and there is no doubt that the sources furnish that which is peculiarly adapted to all the purposes of a large city.

The water is of such uncommon purity that in earlier days the native Indian gave a name to the river which signified “_clear water_.”[5]

_Flow of Water in the Croton River, Capacity of the Fountain Reservoir, &c._

The medium flow of water in the Croton, where the fountain reservoir is formed, exceeds fifty millions of gallons in twenty-four hours, and the minimum flow, after a long-continued drought, is about twenty-seven millions of gallons in twenty-four hours.

The dam on the Croton River is about 38 feet above the level which was the surface of the natural flow of water at that place, and sets the water back about six miles, forming the Fountain Reservoir which covers an area of about four hundred acres. The country forming the valley of the River was such as to give bold shores to this reservoir generally, and in cases where there was a gentle slope or a level of the ground near the surface of water, excavations were made so that the water should not be of less depth than four and a half feet.

The great length of this Reservoir is favourable for the purity of the water which enters the Aqueduct: spread over this large surface, it will have an opportunity to settle and part with some of the impurities which it receives, during rainy seasons, from the wash of the country through which it flows.

The available capacity of this Reservoir, down to the level where the water would cease to flow off in the Aqueduct, has been estimated at six hundred millions of gallons.

Could we suppose that the Croton River will ever in any season of drought, fail to furnish a supply greater than would be carried off from this Reservoir and the Reservoirs at the city by evaporation, we have still a supply of water which would be sufficient for one million of inhabitants during the space of thirty days (estimating the amount necessary for each inhabitant to be twenty gallons for every twenty-four hours.)

But we may assume the number of inhabitants at present to be one third of a million, and therefore we have a sufficient store of water in this Fountain Reservoir to supply them for the space of ninety days, in the emergency before supposed. In addition to the quantity in the Fountain Reservoir, we have sufficient in the Reservoirs at the city to supply one third of a million of inhabitants for about twenty-five days, at the rate of supply before mentioned. Thus we find, should such a limit as we have supposed ever happen to the supply from the River, the season of drought cannot certainly be supposed to continue during the length of time (about four months) that would be required for the present population of the city to exhaust the quantity in store when all the Reservoirs are full.

The minimum flow of water in the river where the dam is constructed, has been stated to be twenty-seven millions of gallons for every twenty-four hours. This would be a sufficient supply for one million of inhabitants, and should the population of the city increase to one million and a half, this supply, together with the quantity in store, will probably be sufficient during any season of drought. There is, therefore, no fear in regard to the supply for the present, and should the time arrive when the city will require more than the present facilities afford during low stages of the river, other streams may be found which can be turned into the upper branches of the Croton, or into the Aqueduct along its course. Other Reservoirs may also be constructed farther up the Croton to draw from in seasons of drought. These suggestions would only be useful to provide a supply during the low stages of the river, for at other seasons the flow of water in the Croton would be equal to the full capacity of the Aqueduct.[6]

_General Design of the Channel-way and Reservoirs._

A description of the general design and purpose of the channel-way in connection with the Reservoirs will serve to give a clear understanding of the operation of the work. Having ascertained the elevation in the city at which it would be desirable to use the water, it was only necessary then, to find a point on the Croton River where a dam could be constructed that would turn the water into a channel having a gradual descent to the required elevation at the city. So that it may easily be conceived, it is only diverting the water into another channel where it will flow on unobstructed. The manner in which water is conducted from its natural channel, for the purpose of propelling the machinery of manufacturing establishments, by a race-way or other channel, is a simple illustration of the operation of this great work.

At the place where it was determined to build the dam across the Croton River, the surface of the natural flow of water was about 38 feet below the elevation required as a head for the water to flow into the Aqueduct leading to the city. By going farther up the river the dam would have been of less height, and a point might have been found where it would be only necessary to build a dam to turn the water, and not form a pond of much extent above it, but for such purpose it would have been necessary to go above where some important tributaries enter the river, and would have required a considerable extension of the Aqueduct. It was perhaps desirable to form this Fountain Reservoir, so that it would afford a supply of water to draw from, should there at any future time, in a season of drought, be more required for the use of the city than would be flowing in the river.

No essential change occurs in the form of the channel-way from the Fountain Reservoir on the Croton, to the Receiving Reservoir on the island of New-York; a distance of thirty-eight miles, except in crossing Harlem River to reach the island, and in passing a deep valley on the island, where iron pipes are used instead of the channel-way of masonry to provide for the pressure consequent upon a depression from the regular plane.

At these points the iron pipes descend and rise again, so that when the water is flowing in the channel-way they will be constantly full. Thus it will be perceived that the channel-way of masonry will never be filled entirely, so as to occasion a pressure on all its interior surface.

The surface of the Fountain Reservoir is 166⅙ feet above the level of mean tide at the city of New-York; and the difference of level between that and the surface of the Receiving Reservoir on the island of New-York, (a distance of thirty-eight miles) is 47⅙ feet, leaving the surface of this reservoir 119 feet above the level of mean tide. From the Receiving Reservoir the water is conducted (a distance of two miles) in iron pipes to the Distributing Reservoir, where the surface of the water is 115 feet above the level of mean tide. This last is the height to which the water may generally be made available in the city.

GENERAL CONSTRUCTION OF THE AQUEDUCT.

Plate I. is a section of the Aqueduct showing the form of the masonry used in earth excavations. The foundation is formed with concrete; the side walls of stone; the bottom and sides of the interior being faced with brick, and the top covered with an arch of brick.

In forming the concrete a mortar is made by mixing three parts of sand with one of hydraulic lime, and then mixing about three parts of stone, broken to a size allowing them to pass through a ring an inch and a half in diameter. Having thoroughly mingled the broken stone and mortar, the concrete is placed in its proper position and form, and brought into a compact state by using a _pounder_; and is then suffered to remain until it set, or become indurated, before any work is commenced upon it. The object should be to mix as many stones or pebbles as will thoroughly bed in the mortar, allowing none of them to come in contact, but all to be enveloped in mortar. This forms a body which becomes indurated and makes a foundation under the whole length of the Aqueduct like one continuous stone. It attains a degree of hardness which gives it the appearance of the conglomerate bearing the name of _Pudding-stone_, and is an article of the greatest importance in forming foundations for walls of great weight; superseding in many instances, where the soil is soft, the use of piles or other timber foundation.

Though we have evidence that concrete was used by the Ancient Romans in the foundations of some of their structures and even in the formation of their roads--such as the Appian-Way, and though we find it used in the foundations of the feudal castles of the Norman Barons of England, still it has not been introduced into the general practice of architecture until quite a modern date, and even at the present time is not widely appreciated in this country as a material of so much importance in foundations.

The side walls are laid up in a character of workmanship styled “_rough-hammered work_;” the stone required to be of sound and durable quality and laid in a manner to render the work water-tight. Though attention is given in some degree to insure a proper bond to the wall, yet the point more particularly attended to, is to make it compact and impervious to water. The bonding of the wall is not by any means disregarded, in all situations where it is required, yet the position of the work generally, where it is in excavation below the natural surface of the ground, renders such precaution of less importance than that of making it compact. The mortar used in these side walls is formed by mixing clean sharp sand with hydraulic lime, using the proportions of three parts of the sand to one of the lime; and these are thoroughly mixed and incorporated before they are wet; when this mixture is wet and thoroughly worked, it is used immediately and always kept properly tempered so as to render it plastic, and to prevent any disposition to become hardened before it is in the wall. After the side walls are finished and the concrete between them has received its proper form, a coating of plastering, about three eighths of an inch in thickness, is put on over the surface of the concrete and on the face of the walls before the interior facing of brick is commenced. The proportions of this plastering are two parts of sand to one of the hydraulic lime.

The bricks used in this work are generally of quite a different character from those used in ordinary house-building; being harder burnt and of a superior quality of material. They are required to be burnt to such a degree of hardness that they present a cherry red, or brownish color, and give a clear ringing sound when struck; and when broken, must present a compact and uniform texture. All bricks brought upon the work which are soft and of a pale color, such as are usually denominated _salmon brick_, are rejected. Those which are used, possess nearly the hardness and durability of ordinary building stone, and are calculated to resist the action of the water, to which they will be exposed.

The advantage of using brick is, that a smooth channel offering little resistance to the flow of water can be formed with less expense than with stone, and greater security can be obtained against any leakage; for besides the coat of plastering which covers the face of the walls and the top of the concrete, there is also a mortar joint between this plastering and the brick work. The bricks being of good form and easily handled, can be more expeditiously and closely laid than the face of a wall of stone, and afford a smooth and uniform face to the wall with less expense. They are required to be bedded full and flush with mortar, so that on lifting one from its position in the work, no imperfections be discovered, but the impress of the brick be found distinct throughout.

The proportions of the mortar for the brick work, are two parts of sand to one of hydraulic lime.

The inverted arch of brick, as well as the brick facing on the sides, is four inches thick, and the roofing arch of brick is eight inches thick.

After the masonry is finished the excavation which was done to receive it, is filled up around it, and over the top of the roofing arch generally to the height of 3 to 4 feet, and in some instances of deep excavation, up to the natural surface. If the natural surface be not of sufficient height for the top of the earth covering, the earth is raised to the requisite height with proper width on the top and slopes on the sides for protection to the Aqueduct masonry.

Plate II. is a section of the Aqueduct in open cutting in rock.

After the rock has been excavated to the required depth and width, the bottom is levelled up with concrete to the proper height and form for the inverted arch of brick, which is laid in the manner before described for earth excavation. The side walls of stone and brick are bonded together by headers of brick entering the stone walls as shown in the drawing, and the walls of stone are built closely against the sides of the rock and forming a junction with it. On the exterior of the roofing arch a heavy spandrel of stone masonry (of the same character as the stone walls beneath it) is built, filling the space between the arch and the rock. After the masonry is finished, the rock cut above it is filled with earth to the same height above the roofing arch as mentioned for earth excavation.

Plate III. is a section of the Aqueduct in tunnel cutting in rock.

The width of the tunnel excavation in rock is the same as that of open excavation in rock; and the manner of building the masonry to form the channel-way is the same, with the exception that the rock roof of the tunnel serves as the roof of the channel-way, where it is sound, but in cases where the rock is soft and liable to fall, a brick arch is built over the channel-way, and the space between its extrados, or outer surface, and the rock roof is filled with earth closely rammed in. In some instances where the tunnel perforated rock which was at first quite hard, the roofing has by exposure to the air, become soft and insecure, so as to render it necessary to turn an arch for its support. This is attended with inconvenience and some difficulty after the channel-way has been completed and closed through the tunnel.

Plate IV. is a section of the Aqueduct in tunnel cutting in earth.