dition that it be stopped when effective scour ceases, but does not definitely fix the position of the eastern jetty. The adopted project referred to in the above indorsement is, therefore, understood to be the project for the west jetty. Since experience has shown that improvement cannot be obtained from the west jetty alone, the Board deems the suspension of work on it proper for the present, and that if the work be continued an eastern jetty should now be begun. (2) Before discussing the position of this jetty some data from the reports of Lieutenant-Colonel Elliot and General Warren may be recapitulated. The area of Nantucket Harbor having a depth at low water greater than 9 feet is 750 acres, and greater than 12 feet is 48 acres. The mean rise and fall of tide is 3.07 feet, the tidal prism is 529,000,000 cubic feet, the cross-section at the throat of the harbor, which is 2,200 feet wide, is 24,500 square feet, giving an average discharge during the whole of a flood or of an ebb of 25,000 cubic feet per second, with an average velocity of 1 foot per second. In the throat of the harbor lies a shoal 970 feet wide, having less than 6 feet on it at low water, and with a channel in its Brant Point side having maximum depths of 33 feet, and another on the Coatue Point side with a maximum depth of 12 feet. The maximum observed bottom velocities in the Brant Point channel are 2.1 feet; but as this channel is practically a concave bend, and as "swirls and eddies" were observed, it is not to be assumed that the observed velocity would maintain so great depths elsewhere. Indeed if a curved section 1,400 feet west of the throat of the harbor be taken from Brant Point to the 6-foot curve, nearly the same mass of water passes through a section but little larger, and hence with mean velocities but little less, but it maintains depths of but 16 feet. The maximum of the mean velocities here apparently does not exceed about 2 feet. Lieutenant-Colonel Elliot adopts average velocities during the whole of a tide of 1.67 feet per second as adequate to scour out a deep channel. This would give maximum mean velocities of about 3 feet, which should be large enough; but it will be well to watch carefully the results produced by the work during its construction, reserving the power of increasing or diminishing the channel section rather than to trust to estimates of velocities made in advance. The distance from the 15-foot curve under Brant's Point to the 15-foot curve outside of the bar is about 6,800 feet, and the navigable depth over this bar has in many years varied little from 6 feet at low water. West of the harbor entrance a jetty of stone 3,955 feet long has been carried out at right angles to Brant's Point. It has produced no sensible effect in improving navigation, and an eastern jetty should now be begun. General Warren's report contemplated an ultimate navigable depth between the jetties of 12 or 14 feet; Lieutenant-Colonel Elliot assumes a depth of 15 feet. (3) Lieutenant Colonel Elliot submits two projects for an eastern jetty, shown on tracings marked "Plat A" and "Plat B." Plat A shows a jetty which with Brant's Point and the west jetty limits a channel varying generally from about 1,600 to 1,092 feet in width. Near the shore end of the west jetty a training-wall is proposed, to limit the otherwise great width here between the jetties. This project depends mainly on tidal scour for obtaining the needed depths. Plat B shows an eastern jetty, which is parallel to the prolongation of the western jetty at its outer end for about 1,500 feet and 1,222 feet from it, then runs direct to Coatue Point. As this, except at the outer end, gives little contraction to the water-way, dredging is to be used to procure a channel. Plan A takes the eastern jetty across the present channel, and so would require the dredging of a new channel before this portion could be built, and to obtain contraction the jetty is placed in deeper water than that of Plat B. The estimated cost of completing the two jetties, including dredging to 12 feet for the second plan, is about the same, or about $280,000. Plan B gives a wide water-way, except at the outer ends of the jetties, and requires much dredging. Even with the aid of a dredged channel, since the water way would be in places over 4,000 feet in width, the Board does not feel certain that the dredged channel would be unchang ing in position and permanent in form. While adopting the second of Lieutenant Colonel Elliot's plans, it proposes some modifications. It prefers a site intermediate between the two, in the expectation that somewhat greater certainty in the permanence of a dredged channel will result, and, by a greater contraction over greater lengths than is given by Plan B, that less dredging will be necessary. Such a location is shown by the broken red line C D E on Plat A, returned herewith. The interval between the outer ends of the jetties is only to be definitely fixed when construction of that part begins. (4) The prolongation of the present west jetty strikes the 15-foot curve at an angle of about 45 degrees. By giving the prolongation curvature to the westward, as shown by the broken red line F G on Plat A, this jetty will meet the 15-foot curve more nearly at right angles, will enable this jetty to be shortened 1,000 feet, and, if from advance of shoreline the jetties should in future need prolongation, this prolongation would be less if the jetties are perpendicular to the curves of equal depth than if oblique to them. (5) Of the water which passes through the throat of the harbor, a large quantity will be lost by escape through the jetties before it reaches their outer ends. Near the shore the west jetty is exceedingly open. Later work by Lieutenant-Colonel Elliot is more compact; but in the opinion of the Board a free use of small material, down to stones of small size, should be made in order to render the jetties as compact as possible. Besides retaining the water to scour the bottom this compactness will have another use. With jetties over a mile long and tolerably tight there would be differences of level of water on the two sides at times that might amount to 2 or 3 inches. Such heads would, with large openings, give velocities through the jetties which might lead to considerable settling. Even with the jetties made as tight as practicable, the Board fears the escape will much exceed 10 percent., since the longitudinal section of the two jetties below mean tide will be about 90,000 square feet. Even if the voids were reduced so low as one-fifth, this would leave a water section through the jetties about equal to that of the main channel. With the uncertainty in the amount of water which will be flowing at any section of the jettied channel, it is important that the proper widths between the jetties shall, as far as possible, be found by experience. In the location submitted by the Board the jetties at the outer end are 1,000 feet apart, and 3,000 feet inside are about 1,600 feet apart. As the jetties approach each other, it is hoped, about the time the works reach this latter point, that experience will indicate whether the outer width of 1,000 feet is too great or too small. If the former, it can be reduced by making the prolongation of the west jetty more nearly on the present line; if the latter, the opening can be increased by swinging the straight outer end of the east jetty a little |