These observations are necessary to demonstrate the extreme care that should be observed in securing to the foundations of the bridge, the requisite stability, and the caution with which we should avoid every unnecessary interruption of the current We are left measurably to resources of conjecture in attempting to ascer tain the extreme velocities of the current; but, with us this question is of Little importance, and it is alluded to only with the, view of quieting the reasonable apprehensions of those, whose position in relation to the work gives them a just right to inquire how far their welfare may be eventually affected by our acts. We need but very ordinary powers of observation or reflection, to instruct us that the same quantity of water must pas more rapidly through a narrow, than through a wider channel of the same depth, and that in some proportion to the velocity of the stream is its power to wear away its bed, and that that which is borne away by an accelerated, must ultimately be deposited by a subsiding velocity,--hence the fear that shoals may be formed below the work, is obviously not to be lightly treated, nor the dread of a prosperous commerce being ruined by the failing of under washed piers, or the deflection of a channel from its legitimate course, or the enlarging of its bed by the interposition of broad masses of masonry. The question, therefore, would be important if we were about to diminish the existing water way either in the channels, or shoaler part of the river. But, the direction in which the current struck the piles of the old bridge was such, as to intercept the stream upon upwards of one-fifth of its breadth; and the abutment, at the southern channel, projected so far into the river, that the amount of obstruction at that place, for many years, was at least onethird of the water way. It will be seen, hereafter, that the proposed bridge will give a much freer passage to the current. As to the character of the work, viewed in relation to its dimensions, or the materials of which it shall be built, we find little in the act of Congress that can enlighten us The altitude necessary for the passage of ordinary steamboats near the draws, the position of the draws, and the breadth of the bridge, are all the guides which we find there: the rest is confided to the wisdom of the President. The steamboats which navigate the river above the site of the bridge, may be considered as of two classes-those which ply hourly between George town and Alexandria, and those whose passage is less frequent. The ma chinery of the first class works at a maximum height of about 22 feet from the water, and that of the second class, at 32 feet nearly. Allowing, then that above the extreme low water mark of the river, the water may rise feet, and that 4 feet would be required to admit the safe and free passage of the working machinery of the boats under the key of the arch, (and these a lowances are undoubtedly, abundantly sufficient, and they are taken with that view,) then 32 feet for the ordinary class, and 42 feet for those of the larges dimensions, would be the limits of the height of arches under the key stone In this estimate, the height of the chimneys is not included, inasmuch they may be readily lowered at the time of passing the bridge. Under the belief that the intentions of the act of Congress will be fulfilled in prod ing for the passage of boats of the class first mentioned, I will content mysel with observing, that we could not erect a bridge capable of admitting under its arches the larger class of boats, without violating that provision of th law by which we are enjoined to give all practicable attention to the pres vation of the navigation, if, at the same time, we should conform the termination of the descent from the bridge to the graduation of the city at the corner of square 267. To do so, the draw on this side of the river should be thrown upon the shoals beyond the Washington channel. Whether authority to erect a bridge over a river includes the power to build a road or causeway in the bed of the stream, may be questionable. For my own part, whenever, in the course of the present investigation, I Conceive that the work of which I am treating must encroach upon the water way, I am bound to limit that encroachment as much as may be consis tent with the establishment of sufficient and secure foundations. The masonry, which, on any hypothesis, must be placed in the bed of the river, whether in the ooze of the Washington channel, or deeper among the gravel and stone on the Georgetown side, twenty-six or twenty-eight feet below the lowest water line, becomes for ever fixed there. It is evident, therefore, that our present resolution compromits the future, and that the piers below low water should be arranged, and that they should have the force and dimensions to sustain ultimately the pressure of stone arches. In a navigable stream which carries with it masses of ice, the safest and most suitable form for an arch, is an arc of a circle. Its appearance is light and (sustained above the water-way) it is secured against the impulse of floating bodies; besides which, its piers are susceptible of receiving, temporarily, arches of wood or of iron. I therefore recommend it, in preference to the many other forms of the arch which present themselves, to the consideration of the President. Giving to the piers an elevation above low water of only 8 feet, we would then have for the versed sine, or rise of the arch, but 24 feet, to adapt it to the passage of ordinary steamboats. A span of 96 feet, one which, in the present case, must have to the rise of the arch a ratio of 4 to 1, would be the limit which prudence would prescribe to us in so extensive a series. I have, therefore, limited the opening of the arches between the draw, to that quantity. A less span than even this might be adopted without much sacrifice of effect, if it were not that the great depth of the Georgetown channel, and the velocities of the current there during the floods, are sufficient reasons for avoiding the erection of many piers. The lowness of the shores on both sides of the river, and the graduation established for the streets near the Washington abutment, forbid us to continue this height from abutment to abutment. Nevertheless, for the arches between the draws, and for those at each side of them, it is recommended to keep the crowns in the same horizontal plane; and it is also respectfully suggested, for the consideration of the President, that the opening may be alike for both the draws. Taking the depth of the key and of the road-way to be 5 feet, and the elevation of the soffit 32 feet, we have then 37 feet for the height of roadway, at the crown of the arch, above low water, on the east side of the city channel. The graduation of the street at the S. W. corner of square No. 267, is about 12 feet above the same plane, and the difference 37-12-25 feet, is the ascent from square 267 to the arch next to the draw. An ascent of 1 in 15, or a little more than 4°, is, I conceive, as much as we should alow over the arches: it is the limit nearly of the inclination of such roads as re constructing under the authority of the United States. But 15 x 25-375 eet, would then be the horizontal distance from the corner of square 267 o the crown of the arch; and this would place the draw somewhat beyond ne deepest part of the channel, viz. in 64 feet instead of 9 feet water. I would recommend it to be placed there, rather than to increase the inclination of the road-way beyond that which I have stated. But the draw might be placed in the same condition with that of the former bridge, were the United States to compensate individuals for the damage to be sustained thereby, provided that a change of the graduation, in that vicinity, were authorized. On the Virginia side of the river, although the shore at Alexander's island is very low, there will be no difficulty to be overcome, in conducting the road-way to it, by reason of the great distance of the deep channel from the shore. Should it, however, be thought essential or advisable to preserve an air of harmony throughout the work, the same angle of inclination might be given to the road-way there that it has on the Washington side of the river, and there would then be four arches between the inclination of the road-way and the Georgetown draw. There would be, under the inclined road on each side of the river, three arches, decreasing in span and elevation symetrically towards each shore, or from each of the abutments to the outer pier of each series of inclined arches, 300 feet. We would then have, agreeably to this arrangement, the following series, viz. Proceeding from the Maryland abutment: For three arches and three piers to the draw opening For the Georgetown draw and pier = 292 feet. = 88 feet. 3734 feet. 88 feet. 452 feet. = 270 feet. 4924 I conceive that the point in which the axis of the bridge should unite with the termination of the Maryland avenue, should be near the middle of the carriage way of the avenue, and I have acted upon that supposition. That the works of the former Bridge Company should be suffered to remain an encumbrance to the public highways, is out of the question. Their con nection, therefore, with the contemplated bridge can be viewed only in the light of a temporary accommodation to the work, and for this purpose only can they be permitted to remain. The entrance to the old bridge was at the intersection of the centre of the avenue with the centre of 14th street. But the wharfage having extended the shores of the city far into the river, the prolongation of the water frost or line of the Corporation wharf, taken in the direction of the S. W. corner of 15th street, places the termination of the Maryland avenue 95 feet in a vance of the entrance to the bridge. Water street, then, is blocked up by the old abutment, nearly the whole breadth of the avenue is occupied by and by the toll-house and its dependencies, and of the whole of the breadth of 14th street, (110 feet) there is left open, for a communication with the avenue, not more than forty feet, including the pavement. It is, therefort, apparent, that these can form no part of the new work, nor can they rea after its completion. It may not be here out of place to remark, that the old abutment, which may constitute a part of the wharfage for the protection of the new work, is a very slight structure, formed by two perallel walls of dry rubble work, intended to sustain only the filling, which is of earth and gravel; that they are but 6 or 7 feet above the low water mark, and are sûrmounted by low and light parapets of brick. It may be well, also, to remark here, that, in consequence of the oblique direction of the bridge, the face of the existing abutment lies close to the lower side of the avenue, which is 160 feet broad. Taking, then, for the point of departure at the Washington abutment, the intersection of the centre of the avenue, by the prolongation of the line of the Corporation wharf, and crossing the river on a line nearly parallel with the old work, assuming the half of 39 feet-194 (for the half breadth of the bridge between the exterior sides of the parapets, 36 feet road and footways,) we will have 704-40-30 feet, nearly, clear space between the parapets of the old and new work, which will permit us to profit by the temporary use of the remains of the old bridge, to maintain freely, at all times, the communication which is so essential along the line, the extent of which advantage can scarcely be estimated in the erection of a work so extensive and so exposed as this is. The springing line of the piers, if 8 feet above low water be adopted for the height of that line, should be the height of the wharfing for the whole breadth of the avenue on that side of the river, and for the wharfage or protection wall, also, at Alexander's island, to the extent that may be found necessary there to secure the abutment against the force of the current and the pressure of the ice. The position and form of the Washington shore leaves no serious ground of apprehension that the abutment shall ever require unusual precautions to protect it against the action of the current: we need not therefore lay it much below the surface of the ground. But the soil on which it must be erected is yielding and compressible, and will therefore require either piling or grillage to sustain it. It would also be highly advisable to extend the wharfing on that side well up the river, (towards the wall) in order to give to the current the same direction with the sides of the piers. The road-way must necessarily be much above the present surface of the ground at the abutment, and it will require a considerable embankment of earth to form it. This embankment will extend to 14th street, and, as it must also cross Water street, the necessity of extending the filling along that street, both above and below the bridge, with the view of permitting an easy assent thence to the avenue, as well as to the bridge, is apparent. From the abutment to Water street, the embankment must be sustained by a stone rivetment at least, and that direction should be given to the face of the rivetment which should allow abundant space near the bridge, and an easy access to it; whatever counterposts may be required should be erected on the inside of the walling, On the Virginia side of the river, although the banks slope very gradually towards the deep water, and although the most rapid current is on that side, yet, from its great distance from the shore, I am confident that it will not be necessary to give to the foundation of the abutment there a greater depth than that which will be required for the Washington abutment. To give to the current under the arches a direction parallel with the sides of the piers, it will be indispensably necessary to drive a range of piles hav ing sufficient backing, or to build a low wall from the upper projecting point of Alexander's island, towards Mr. Mason's fish house. The deep bend below that point presenting its concave surface to the stream, reflects it obliquely to the general course of the river, as well as to the axis of the bridge, and it affects injuriously, also, the navigation of the river, and in reference to that object alone, the expediency of the course I am recommending cannot well be questioned. The law, in conformity with which the angle of reflection always equals the angle of incidence, assures us that, in presenting to the current a plane lying more nearly in the direction of its own course, we straighten it; and, consequently, the channel also. If a bridge crossed a stream at right angles with the current, the extent of obstruction would be measured by the breadth of the piers, if these were always of the same form and were perpendicular to the axis of the road; but, were the piers oblique to the current, the sine of the angle of obliquity would be a constant quantity to be added to the obstruction occasioned by every pier that stood obliquely in the stream. Nor would this constitute the extent of the objection to this condition of the stream: the sides of the piers presented to it, receiving upon their broad surface the shock of floating bodies, must necessarily be liable to be injured. The danger to vessels attempting the passage of the bridge, struck by an oblique current, is equally apparent. It is for these reasons that the Virginia shore above the bridge should be straightened, and I need scarcely add, that the beneficial effects of such an improvement of the shores of navigable streams are of almost daily occurrence. In connection with the direction which ought to be given to the current in its passage under the arches, is the condition of the ancient abutment on the side of the river now under view. This abutment, like that on the Washington side, is a low causeway of the breadth of the bridge, and it is carried beyond the shore. It is sustained by dry walling, surmounted by slender parapets of brick work, on the upper side of the bridge in its whole length, and on the lower side for about half its length. It encroaches upon the water way 250 feet, nearly; and, in narrowing the river that much, it contributes to the evil of which we have just spoken. Immediately above the abutment, there is a deep cove into which heavy masses of ice sometimes drift, their passage being impeded by the projecting abutment. Part of the up stream parapet has been overturned by the drift ice, which had attempted, with some success, to force a passage over it. To guard against this evil, and to relieve the current, it is propos ed to erect the abutment at Alexander's island, near the high water mark, and to prevent the accumulation behind the abutment, by closing the entrance to the cove. That all this may be accomplished, the consent of Mr. Mason, the proprietor of the island, becomes necessary. In throwing back the face of the abutment to the position which it should have, in reference to the safety of the work and the interest of the navigation, some slight disturbance of the position of Mr. Mason's dwelling may be necessary. This, however, the building being a low frame, could be ef fected at a very little cost, and I doubt not with the free approbation of Mr. Mason. Whatever were the position of the ancient abutments, they could by no means constitute the abutments of the proposed bridge. It is only as por tions of the wharves or wing walls, that they could be brought within the |