but these did not cut the clay very well. A new cutter was then built, with narrow spiral knives, and proved to be more efficient than the first; but even with this cutter the quantity turned out per hour was never more than 60 per cent. of that contracted for. The clay, which it discharged behind the walls was in the form of nodules, varying in size between that of an egg and that of a Dutch cheese."

66

a

Notwithstanding some disappointing experiences, such as the foregoing, the clay-cutting gear has very strong partisans. Mr. A. W. Robinson * claims for a dredger, the "J. Israel Tarte," designed by himself, and working in blue clay in the channel of the river St. Lawrence below Montreal, world's record for output, measured by the output, of any dredger under any conditions." And Mr. C. W. Darley, in his description of "Dredging in New South Wales," speaks of them as valuable for cutting new channels through "tough or hard clay formations." Any definite pronouncement on the value of the cutter dredger must therefore remain in abeyance, pending the completion of more extensive trials and the determination of the best form of cutting apparatus.

The illustration (figs. 52 and 53) is one of a dredger on the Bates system constructed for the Russian Government. The cutters, of which there are four, are shown at the stern. The forward end is in connection with a discharge pipe.

Ladder Dredgers, or bucket-ladder dredgers (figs. 54 to 58), consist, in principle, of an endless chain connecting a series of buckets which traverse in succession an inclined orbit, approximately elliptical, about two pivots or tumblers, excavating material at the lower tumbler and discharging it into a shoot while passing over the upper tumbler.

Bucket dredgers of this type have either one or two ladders-"ladder ” being the name applied to the frame, with its roller bearings, on which the buckets travel. In single-ladder dredgers the ladder coincides with the longitudinal axis of the vessel. The ladders of double dredgers are situated

at each side of the vessel.

A single-ladder dredger of the same capacity as a double dredger has the advantage of fewer moving parts and, consequently, of less working friction. The central position of the ladder also admits of a more convenient outline for the vessel, from the point of view of propulsion, and affords greater steadiness in a sea way. The broad beam of double-ladder dredgers renders it impossible for them to pass through narrow locks, though this difficulty has been overcome, in one case at least, by constructing a dredger in detachable halves.

On the other hand, a side-ladder dredger can work in greater proximity *A. W. Robinson on "Modern Machinery for Excavating and Dredging," Engineering Magazine, vol. xxv., No. 1, April, 1903.

+ This performance is stated to have consisted in the removal of 1,180,000 cubic yards of material during a period of two months, comprising 52 working days.

Eng. Conf., London, 1903.

to the face of a dock or quay wall than is feasible in the case of a central ladder. But, under these circumstances, the discharge of dredged material has to take place across the whole width of the vessel (unless it be a hopper dredger, which is unlikely, from its unsuitable form for navigation), and either the cross shoot will be too flat to be thoroughly effective, or else the lift of the buckets is excessively high for ordinary purposes. It will generally be found necessary to employ an auxiliary pump to flush the shoot. A central ladder dredger can discharge indifferently to either side, but again, if any mishap occur to a link or bucket, the whole dredger is placed out of action, whereas in a double ladder dredger one ladder may be quite disabled without interfering with the work of the other. In cases where very powerful machines are required, double dredgers have the recommendation of providing greater lifting capacity with buckets of a less unwieldy size.

The bucket dredger is eminently suitable for steady continuous work in hard material. It is the only form of dredger which will excavate rock, and it has proved capable of raising boulders much larger than its own buckets. In stiff clay it is much superior to dredgers of any other type. Altogether, it is an excellent machine, but it cannot be worked in a swell nor in very shallow water.

It is not an economical machine in the matter of power. Owing to the necessity of discharging through a shoot, in cases where an attendant hopper is employed to receive the dredged material, lifting has to be performed by the machinery to the extent of 25 or 30 feet (the writer knows of a case of 35 feet) above the water line, representing a corresponding waste of energy.

The difficulty of dealing with shoals and banks has been solved by a special form of dredger, devised by Messrs. Wm. Simons & Co., of Renfrew, called the traversing bucket dredger. The ladder is supported upon a horizontal longitudinal framing, by means of which it can be projected in advance of the dredger, and thus enabled to cut the flotation of the latter through shallow places. By the same arrangement the ladder can be entirely removed from the water, and less obstruction is, in consequence, offered to its passage, when acting as a carrier hopper or otherwise.

Central ladder dredgers are themselves susceptible of subdivision into two classes, according as the well is situated at the bow or the stern of the vessel. The former is the more general type for simple dredgers, but a stern well hopper dredger derives the advantage of increased speed from a normal stem, with improved manoeuvring qualities and a better shaped hull for encountering heavy seas.

The following are points of practical importance in connection with the utility of bucket dredgers.

Buckets. No object is gained by bringing the lip of the bucket too far forward. The limit of filling will generally be the horizontal line through the inner edge when in the inclined position; hence the bucket is equally

effective with a short face as with a long one, and the former outline is better adapted for discharging. The mouthpieces, or lips, should be of hard steel rivetted to the face of the buckets which, together with the links and pins, are also of steel of special quality. A hole or two in the front is useful for the escape of water. Large buckets free themselves better than

small buckets from adhesive material.

Shoots. The least inclination for the unassisted discharge of miscellaneous material is somewhere about 1 in 4; but this is not always obtainable. With the assistance of continuous and ample flushing, together with some manual appliance, such as a pricker, the limit may be raised to 1 in 10 for mud, 1 in 15 for clay, and 1 in 20 for sand.

Tumblers. The top tumbler actuates the rotary motion of the buckets and should be as small as possible, in order to reduce the amount of intermediate gearing. The ideal form would be the circular, but with straight links and flat backed buckets, a square or pentagonal section must be adopted. The latter is preferable, as it brings all faces of the tumbler equally in contact with the buckets. To achieve this condition with a square tumbler, an additional, or "hunting," link would have to be inserted at some point in the chain. The bottom tumbler does not transmit power and should be made of large diameter to diminish friction, say, with six or more sides. It is suspended from a cross beam on the dredger, and has to be readily adjustable to the depth of water in which the dredger may be working. For the guidance of the buckets, the lower tumbler should be provided with large flanges.

Power.-Mr. J. J. Webster,* from observation of a large number of indicator diagrams, submits the following empirical formulæ for determining the indicated horse-power required to dredge different qualities of material under varying conditions of lift. If H be the height of the upper tumbler shaft from the surface of the ground to be dredged, and W the number of tons per hour to be dredged, then the indicated horse-power required is approximately—

04 W √H for very stiff clay or mud.

034 WH for hard clay and indurated mud.
026 WH for soft mud and light sand.

The illustrations (figs. 54-61) are of the dredger "Cairndhu" and one of her attendant hopper barges, belonging to the Clyde Navigation.

The Dipper Dredger, which is almost exclusively an American type, being much used in connection with the improvement and maintenance of river beds and channels in the United States, is so identical in principle and mode of action with the steam navvy (p. 81 ante), or land excavator, already described, that there is no necessity to make more than a very brief and passing reference to it.

*Webster on "Dredging Operations and Appliances," Min. Proc. Inst. C.E., vol. lxxxix.