mination of the friction at high pressure; but for low pressure, depending entirely on the weight of the ram and table, it is easy to obtain approximate results by gauging the pressure with the ram when rising and when falling. The differences so obtained by the Author vary from 2 to 5 per cent., giving a coefficient varying from 1 to 2 per cent. Taking into account the circumstances of the various readings, the Author would place the actual coefficient for low pressures and low speeds at about 1.25, or rather higher than Mr. Tweddell, who gives 1 per cent. as the loss by friction in using water-pressure in well-constructed plant in good order. For high speeds, the friction in the pipes and the difference between cylinder-pressure and gauge-pressure prevent any accurate determination in this way. Steel piping is now always used for the high-pressure supply, not only because of its superior strength, but also for its greater power of withstanding corrosion and furring by oxidation. With accumulator plants, separate piping is used for coupling the accumulator and presses with a simple stop-valve to each press; the higher pump-pressure is regulated by a double-stop or threeway valve, which couples the high-pressure supply to the press or allows the water to pass on to the piping beyond. The same stop, by another handle, couples the press to waste. The Paper is accompanied by three diagrams, from which the Figs. in the text have been prepared. (Paper No. 2414.) "Jetties as applied to Harbour Entrances in the By Professor LEWIS M. HAUPT, M. Am. Soc. C.E. ALTHOUGH nearly four centuries have elapsed since the Latin and Anglo-Saxon races disembarked on the shores of America, yet but little more than a score of years has passed since the inauguration of a systematic method of river and harbour works by the United States Government. In so brief an experience engineers would scarcely expect to find many completed structures or valuable precedents, yet the shores of America are not devoid of instructive lessons. Naturally, however, Americans turn to the old world, so pregnant with great and durable examples of maritime works, for precedents, and for successful applications of general principles, and hence it is that the earlier European methods of jetties and dredging have been adopted. The resources of the profession are stated, in a report on the harbour of Galveston, Texas, to be limited to the two methods(1) "by dredging alone; (2) by using tidal scour between jetties, aided, if necessary, by dredging;" and it is added, "As to the first it has already been tried unsuccessfully." This narrows the choice down, then, to a single system, namely, that of jetties, for effecting tidal scour. Hence it may be inferred that the instances of the successful application of this system, under similar conditions, must be very satisfactory to justify the expenditure of more than $8,000,000 (£1,600,000) in its application at this locality. Is this the case? For a reply, the physical conditions existing at other sites where jetties have been or are now being attempted should be examined. The elements involved are the fresh-water volume, tidal-prism, wave-action, winds, currents, shore-line, and character of the material. The action of these several forces and elements in modifying one another will be greatly influenced by the form of the basin or mould of the submerged ground over or within which they act. They are interdependent and sustain to each other the relations of cause and effect, leading to intricate combinations of many variables, which it is hopeless to attempt to formulate mathematically. The conditions may, however, be readily grouped with reference to the preponderating features. Thus there will be observed a class : 1. In which there are tidal fluctuations of considerable magnitude with but small interior reservoirs and little or no fresh-water drainage; or, 2. The tide may be great and the inner lagoon or sounds large, while the land drainage is insignificant; or, 3. The mean tides may be small, not exceeding a few feet, while the interior bay may be extensive; or, 4. There may be no intermediary settling basin, and the rivers, whether great or small, may debouch directly into tidal waters; or, 5. They may empty into tideless seas or lakes, in which case it will be observed that the mouths are, as a rule, of deltaic formation rather than estuarian. By an examination and classification as above of the conditions prevailing at those places where jetties are found to have been satisfactory, the engineer may discover where their use may be resorted to with reasonable prospect of success, and much time and money may be saved. It may appear presumptuous in the Author to state that he has been unable to find a single instance "at home" where convergent jetties have resulted in securing a permanent improvement. It is true there are a few instances where the incomplete structures have modified the currents and resulted in changing the position of the bars, at the same time reducing their depth several feet during the transition stage; but the conditions of the channels are unstable and formative. Probably the case where the greatest improvements have been effected is that at the mouth of the St. John's River, Florida,1 where the least depth of the channel has changed from 5.8 feet to 13.2 feet in three years; but the crest has advanced seaward about 2,200 feet, and the exterior slopes have changed from convex to concave. The officer in charge states that "until the very rapid changes, caused by the opening of the channel between the jetties, have ceased, and a somewhat normal condition shall have been re-established, deductions as to the stability of the deep channel across the bar cannot be fairly made. These results will not be permanent until the jetties have been extended to their full length and given a permanent cross-section." Annual Report of the Chief of Engineers, U.S. Army. 1888. Part II. p. 1080 et seq. "The approved project is to obtain a least mid-channel depth across the bar of 15 feet at mean low water by the contraction of the stream by two long jetties, starting from the opposite shores of the entrance and converging until, near their outer extremities on the bar, they shall be 1,600 feet apart. The jetties are to be built of brush or log mattresses and rip-rap stone, and suitably capped. The estimated cost of this improvement is $1,306,409 (£261,282). The usual low-water depth on the bar before the work was begun varied from 5 to 7 feet, with a mean rise of tide of about 5 feet. The channel across the bar shifted from time to time, north and south, through a distance of about a mile. Since the adoption of the present project, in 1879, five appropriations have been made by Congress for the work, aggregating $675,000." The total amount expended to the 30th of June, 1888, was $670,957.13. The total lengths of the unfinished jetties at this date were, for the south jetty, 6,667 feet, of which 4,100 feet out from the shore end were built to and above the level of mean low-water. The remainder had its crest about 6 feet below low-water. For the north jetty, 6,585 feet, of which 553 feet were at full height and capped, and 5,079 feet were at the height of mean low-water. The rest was from 0 to 10 feet below. The physical conditions at the mouth of the St. John's would place it in group (4), having no interior basin but discharging directly into tidal waters, and hence a favourable position for improved results, although, as stated, the increased depth cannot be regarded as permanent. There are other less successful experiences with convergent jetties in the United States, and one in which a modified plan of submerged convergent jetties was tried, which contains some important suggestions. The deepest channel across the bar at Charleston, South Carolina, is about 7 miles south of the gorge between the islands, and its greatest available depth is only 12 feet. By the project, approved in 1878 and estimated to cost $3,000,000 (£600,000), it was intended to secure 21 feet of water, at a point directly in front of the gorge, by two jetties, designed to admit the flood-tide freely by submerging their shore ends, and to train the ebb currents across the bar by raising their outer extremities to or above the surface. The initial report on the project states :—1 Annual Report of the Chief of Engineers, U.S. Army. 1878. Part I. p. 563 et seq. 1. "The north jetty will reduce the half-tide area of the waterway from its present area of 78,880 square feet to 41,593 square feet (47 per cent.)." 2. "The south jetty half-tide water-way will be reduced from the present area of 201,365 square feet to an area of 94,684 square feet (53 per cent.)." 3. "In the gap," between the jetties, "where alone erosion can take place, the present mean half-tide water-way is 29,572 square feet, and the mean low-tide area 22,840 square feet." 4. "After the jetties shall have received their maximum scour, aided by dredging or other artificial appliances wherever clay-beds are encountered and the equilibrium of flow is resumed, the original average slope S = 0.000,002,498 will be restored." 5. "The aggregate average discharge per second before the jetties were built will also be restored." On these assertions the hydraulic radius between the jetties is computed to" be 42 22 feet at mean half-tide, or 39.71 feet at mean low-water." 6. "In the new channel between the jetty-heads, where the hydraulic radius is 39.71, it may be expected that the area of depths of more than 24 feet will constitute a very large proportion of the total area of the gap, and that maximum depths of 75 feet and upwards would be maintained in mid-channel." 7. "The sectional area of the gorge profile between Cumming's Point and Sullivan's Island is :- ... Mean ebb-tide area 176,600 square feet." From extracts Nos. 4 and 5 it would appear that there will result the same surface slope (S), and the same aggregate average discharge (Q) after the construction of the jetties as before. In other words, the jetties would not affect the slope or aggregate volume. In the Chezy formula for the velocity of discharge, which was used in this case, V C/RS, and, solving with reference to S, it V2 = becomes S = but R is equal to the area (A) divided by the C2 R' wetted perimeter (p), which latter may be regarded as a constant. V2 Hence, if S is constant, or "remains the same as before," must R be a constant ratio, or V2 must vary as R varies; but R being a function of A, is reduced nearly one-half by construction, hence V2 must also be reduced nearly one-half. As the quantity is a product of the area into the velocity, and as both of these factors are reduced by the construction of the jetties, it follows that the "aggregate average discharge" cannot be the same as before. |