138 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[April, 



shorter proportions, and the angles of the abicus are cut oft so much 

 as to render that member octagonal. The centre consists ot tive 

 arches, resting on two ants and four columns, with steps leading up to 

 it, and is crowned by a pediment with a bas-relief by Reich, a young 

 sculptor now rising into celebrity. The merits of the design must, 

 however, be taken entirely upon trust, for there is very little amount- 

 ing to description in what Ernst Forster says of it. Some particulars 

 that might easily have been stated are passed over altogether, not- 

 withstanding that, for want of them, it is impossible to form any dis- 

 tinct idea of the structure. There is, for instance, not a single dimen- 

 sion given by which we might even guess at its size : nur is it even 

 so much as said whether it is an edifice of any magnitude. We are 

 again quite at a loss to understand whether the "seventeen arches" 

 spoken of belong only to the principal elevation, or nut. The 

 " description," in short, is of that kind which, though it may be clear 

 enough to those who have seen either the building itself, or drawings 

 of it, is unintelligible without such additional information, — a iery 

 common, but also a particularly provoking, fault. 



DREDGING AND DREDGING MACHINES. 



[The following communication describes a practice called radius 

 cutting, which has been recently introduced into several large dredg- 

 ing operations. In this new process a lateral sweep, or circular 

 motion, is given to the ladder, or bucket frame, which swings upon 

 its upper extremity as a centre, and in its action imitates that of a 

 scythe in mowing, — with this difference, that the scythe only cuts 

 from right to left, whereas the buckets cut both backwards and for- 

 wards, which is the meaning of our correspondent when he says the 

 machine cuts in both traverses. According to the old system of 

 working, which our correspondent styles trench cutting, the bucket 

 frame bag only a vertical motion, the effect of raising or loweriog its 

 inferior extremity by means of ropes and sheaves fixed at the bows of 

 the vessel. The machine could only cut right forward in straight 

 lines, and had to be hove up to its work or dropped down from it 

 when the buckets had done their work in the axis of the frame. Our 

 coi respondent contends that the zigzag motion from one cutting posi- 

 tion — that is, from one trench to the other— is inconvenient, and occa- 

 sions a loss of power ; that the buckets cutting iu trenches are very 

 liable to cut in holts where the bottom is already sufficiently deep; 

 and that after the dredging has been executed in parallel lines, there 

 are still ridges left between the trenches which must be worked off 

 by a re; etition of the process. At present we are not able to see 

 clearly how the two latter of these objections are obviated by the new 

 >vstem of radius catting. However, we insert the letter of OUT cor- 

 respondent in luil, and shall be happy to receive further information 

 as to the working of ibis system. — Ed.] 



SlB — Having twice had to make out plans and specifications of new 

 ing machines .or works where I have been an assistant, I have 

 bestowed a good deal of attention on this subject; and recent acci- 

 dents to masonry where modes of cutting other than dredging have 

 been adopted, have induced me to send you the following remarks 

 on the new system of "Radius cutting," which has been found so 

 successful on one of the largest navigation improvements at present 

 going on. 



In the ordinary method of "trench" cutting, the power applied to 

 lead the machine ahead into the cutting, has also to resist the reac- 

 tion of the buckets: so that in the Dublin machines it was thought 

 worth while to take the power trom the engine. In the Clyde boats 

 this process requires a great expense of manual labour, but Mr. Bald 

 has taken the power from the engine in the newest machine' (No. 5). 



Now, in radius cutting, the chain from the bow of the machine is 

 not wound up while it is cutting, but is only shortened at each return 

 of the machine, which may be described as swinging on that chain 

 just like the "bob" on a pendulum: the machine being led laterally 

 to the cutting by the side chains. These side chains are compara- 

 tively easy to work, as the reaction of the buckets is mostly against 

 the radius chain. 



There is no more difficulty attending the use of these lateral chains 

 in a harbour, or narrow navigation, than iu "trench" cutting, where 

 corresponding chains, or "guys," are required to keep the machine 



in line, and these are necessarily used on both sides at once ; whereas 

 these lateral chains are only tightened on one side, — namHv, that on 

 which the machine may happen to be traversing ; and where it is 

 required to lower them, to allow vessels to pass, they have only to be 

 tightened up till the buckets fill again. 



In the old system of working the boat ahead, it has to stop when it 

 has cut a trench, come back over the same ground, and be set again 

 to commence cutting another trench; and after the requisite breadth 

 is traversed, the machine must again work over the same breadth, in 

 the same zigzag manner, to cut the "ridges" which were left between 

 the "trenches." This is the most clumsy and unprofitable part of 

 the system, as these ridges yield before the buckets when soft, and 

 stand when hard, causing a continual raising and lowering of the 

 "ladder." Any engineer may satisfy himself of this, by standing to 

 see the soundings at the "well," which are so irregular that half the 

 full work is not performed: and there is no doubt of the immense 

 loss, by the machine cutting in what is termed "holes." 



All these evils are greatly reduced bv working with a radius chain. 

 The machine cuts in both traverses, and carries the work clean before 

 it. The surface of the bank can be pared to any level, the michine 

 passing over the hollows and cutting only the heights, and so never 

 wasting time by dredgiug in holes. 



There is no loss from taking the machine rapidly over the ground, 

 when such might be required to keep the buckets full; but in the old 

 system there would be a great loss in doing so, and paring only to a 

 certain extent at each run, — for the backing of the machine is a dead 

 loss of lime. Yet though this loss is obvious, there might be a 

 greater, from endeavouring to reduce it by keeping the buckets full 

 the whole length of the trencn, for then they might get into holes ; 

 and if a machine contin«e to lift quick stuff from a hole which may 

 happen to be near a hard part of the bark, it can do little good, as the 

 hole, if very quick, fills up of itself, or may soon be expected to 

 collect alluvial matter. 



Although it may be true, that, to cut or trench a bank in the proper 

 current, will, by changing the currents and eddies, remove it by a 

 natural process; yet, as this is a point so difficult to hit upon, it is 

 generally allowed, that, to get lorge stones and rocks taken up, and to 

 cut the surface fair, is the surest way of reducing a bank, and of leav- 

 ing it iu the condition least liable to "silt up." 



It is with these views of the advantages to be derived from the 

 judicious application of machines, that the preceding mode is brought 

 under the consideration of your readers. 



There have been great improvements made in the machinery by 

 the practical engineer; but the civil engineer has not made any cor- 

 responding advancement in the working of the machine, although it 

 is his right hand instrument. By attention to the working of the 

 machine! much time and expense may be saved, which no improve- 

 ment in mere construction of the machinery could ever effect. Dredg- 

 ing machines seem just to be put into the hands of their captains and 

 engine-keepers, to make the best of them. And it cannot be doubted 

 that the performance of these machines might be greatly increased by 

 improved application ; indeed, the improvements here described go 

 far to prove this, as they effect a saving of at least one-fourth of the 

 power. When men of science bestow due attention on the subject, 

 farther advancement may yet be made, and the dredging machine be 

 found not only by far "the most safe and expeditious, but also the 

 cheapest mode of cutting. 



The difference of construction in the machine for working on the 

 radius principle is very little, and need only be made at the lower 

 end of the bucket ladder. As the machine is led side-ways to the 

 stuff, the lower "tumbler" has no flanges, as in the ordinary t.imbler, 

 on which they are necessary to assist m keeping the buckets from 

 swerving as they are pressed forward; but iustead of the flanges 

 there are "snugs" on the tumbler between the chains to keep them 

 on. it would be useless to notice other parts of the construction in 

 this paper, as that is the characteristic difference. The four crab 

 winches usually erected on the deck of a machine serve with a snatch- 

 block to wind the traverse chains. 



3U, Hope Street, Glasgow, W. C, c. e. 



21** March, 1843. 



' No dredging machine is complete unless the work of heaving the vessel 

 ahead be performed by the engine. It is one of the mosi obvious applica- 

 tions of '.he steam power by which the maehi e is worked, and « believe 

 some ot the earliest en-ines— for instance, those on '.he Caledonian Canal- 

 were so contrived, as to be hove forward by the engine when required— El). 



Bell Rock Lit.HTiioisE. — The monthly return from this establishment for 

 February takes notice of a heavy sea upon the rock, from the north-east, on 

 the 14ih and three follow ing days, when the spray rose from 70 to 80 feet on 

 the lighthouse tower. On each of ihese days, says the return, " we felt the 

 building tremble but very little." Various of the travellers or boulder stones 

 upon the rock have been shilted from " Arrnston and Ulbster " ledges to the 

 west end of " Hope's'' Wharf. The boulders connected with the " Royal 

 Burghs " have also been tossed about ; one of these measures nine feet in 

 length. — Calethniun Mercury, 



