For comparison with the 18-foot pipe line already considered, to develop the head of 80 feet along the rapids, there may be taken a tunnel, like that now used to convey water from a point above to one a little below the horseshoe falls. This tunnel is of the horseshoe form, with a height of 25 feet and a maximum width of 19 feet; it has a descent of 7 feet in each 1000 feet of length, and is designed to discharge 8900 cubic feet of water per second. To maintain an equal rate of discharge through a tunnel of this cross-section, and 10,000 feet long, the corresponding loss of water head would be 70 feet, but in this tunnel, as in the pipe line 13,000 feet long between the same points, the loss of head is limited to 13 feet.

As the loss of head due to the friction of a pipe or conduit varies with the square of the velocity of the water flowing through it, the velocity in the tunnel 10,000 feet long will be 0.43 of the velocity in the existing tunnel of the same crosssection that descends 7 feet in each 1000 feet of its length. The existing tunnel is designed to discharge 8900 cubic feet of water per second, and on this basis the proposed tunnel of equal cross-section, but with a descent of 13 feet in 10,000 will discharge water at the rate of 8900 x 0.43 = 3827 cubic feet per second. It was found that the 18-foot pipe line 13,000 feet long would discharge 2286 cubic feet of water per second, with a friction loss of 13 feet in head, and it thus seems that the proposed tunnel would discharge 1.67 times as much as the pipe line with the same loss.

The existing tunnel at the falls, with a finished height of 25 feet and

a width of 19 feet, is lined below its center with a layer of concrete faced with vitrified brick having a total thickness of 22 inches, and above the center with timbers and brick to a thickness of 31 inches. From this it appears that the necessary excavation for such a tunnel amounts approximately to 22 cubic yards per foot of length.

It also appears that, at the present prices of tunnel work about Niagara Falls, the cost of the finished tunnel under consideration would be approximately $5 per cubic yard of the necessary excavation. On this basis, the proposed tunnel would require investment of $110 per linear foot, or $1,100,000 for its entire length of 10,000 feet.

an

The 3827 cubic feet of water discharged per second by this tunnel would develop 34,500 H. P. under the head of 80 feet, and with a combined efficiency of 60 per cent. for the tunnel, wheels, and generators, the output of electric power would be 20,700 H. P. With this output, the estimated investment in the tunnel would represent $53.14 per horse-power capacity of electric generators.

As the estimated cost of the 13,000-foot pipe line was $39 per horse-power capacity of the electric plant, it appears that the tunnel would cost 36 per cent. more for the same capacity and efficiency than a line of steel pipe, in spite of the greater length of the latter. Against this lower first cost must be entered the fact that the depreciation would be larger for the steel pipe than for the tunnel. Either pipes or tunnels offer an entirely practicable means. of developing power in Niagara gorge.

C

LOSELY allied to the ladder and dipper dredgers is the grab or grapple dredger. All three varieties possess essentially the form of a scoop or bucket, but

whereas the action in the two former types is entirely that of scraping or scooping, in the last-named type there is also a clutching movement. Accordingly, grab dredgers consist of two or more curved plates or jaws capable of opening and closing in a fashion not dissimilar to the working of the human mouth or of the hands in taking up a quantity of grain. Most frequently, the section of a grab comprises two circular quadrants capable of rotating about a central pivot, the whole forming when closed a semi-cylindrical receptacle. But oftentimes, especially for well-sinking purposes, which, however, lie outside our present purview, grabs possess the form of a hemispherical shell, in three or four segments.

The grab dredger is worked to a very large extent by the aid of gravity. Suspended by a chain or chains, from the head of a crane jib, the bucket is allowed to fall freely by its own weight, with open jaws, until it buries itself in the ground to an extent dependent upon the nature of the material. The jaws are then brought together by means of the tension imparted to the chain. or chains by the winch, and the enclosed mass of earth is lifted and removed in the ordinary way.

The principle of the grab is simple, but there are complex adapta

tions of it in actual practice. Thus, in the first place, the appliance may be operated by one or two chains. Each of these systems admits of several arrangements, so that there are a number of machines on the market possessing distinct characteristics, each class being admirably adapted to a particular purpose. Too much space would be occupied in referring to all these different machines, so we must content ourselves with adverting briefly to the more noticeable of them.

Thus, the Peters grab, shown in Fig. 1, is a single-chain machine, the working of which depends upon the gripping of the lifting chain, prior to the opening process, by a pair of steel arms, which are actuated and controlled by a roller bearing against the chain, and a governing rod attached to the upper edge of the bucket. In excavating, the bucket is closed by the chain which continues taut during lifting. When the chain is slackened, the roller falls and allows the grippers to engage. Then, on hoisting again, the grab is pulled open.

The Wild grab, shown in Fig. 2, also has a single chain fitted with a catch in the form of a half ball, or hemisphere, with the flat surface uppermost. Such a form permits the downward passage of the catch between two small tumblers, but prevents it rising again and the grab from closing until the bottom is reached, when the chain becomes slack and the tumblers are automatically opened by the weight of a

sliding sleeve. The grab can then be closed and drawn up until it reaches a position where a ring in the lifting gear engages two steel hooks, from which the grab is suspended during discharge. A simple mechanical contrivance causes the hooks to be withdrawn again when the grab is slightly lifted prior to

making a fresh descent for reloading.

A very important difference between the two single-chain machines just described and one materially affecting their respective efficiencies is that whereas in case of obstruction or in case of an imperfect or ineffective lift, the Peters machine can be opened at

any stage, the Wild grab, and, indeed all other single-chain grabs within the writer's knowledge, have necessarily to be lifted to the full height of the suspending gear before discharge can take place. If the grab should close upon some immovable object below water, only with the Peters system could it be made to relax its grip without lowering the suspender or without the aid of a diver.

This advantage of the Peters grab is shared by all double-chain grabs, but they, on the other hand, cannot be fitted as can single-chain grabs, to an ordinary crane, requiring as they do a special form of jib head and gear.

Yet

this is but a single drawback, and there are many strong points in favour of double-chain grabs. They possess fewer, less delicate and less intricate working parts, and so do not run as great a risk of getting out of order, nor do they require such skilled attention. They provide a much better safeguard against the loss of the bucket,-a consideration of some importance in submarine work. Where the bucket is upheld by two chains. there is not only less. chance of its being carried away altogether, but risk of breakage to either chain. is materially lessened. Finally a double-chain grab can discharge its load as gradually as may be considered desirable, whereas the action. of a single-chain grab is instantaneous and complete.

ment in the central groove of the frame. From the jib head, one chain is attached to this bar, and any tension on the chain causes the bucket to open; the other chain is wound around a drum on the pivot, and its unwinding, with the assistance of two subsidiary chains con

STRONG TINES

The Priestman grab, shown in Fig. 8, belongs to the double-chain system. The outer corners of the bucket are connected by hinged arms. to a horizontal bar or cross-piece, which is capable of vertical move

nected to the horizontal bar previously mentioned, draws this bar down and causes the bucket to close.

Taking the various classes as a whole, the grab is an excellent implement, but it has limitations. It cannot be employed effectively and expeditiously over large areas, neither can it be counted upon to leave an even surface. The tendency is, rather, to pit the ground with a series of hollows or depressions,