1843.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



423 



THE WIER ACROSS THE SHANNON. 



Sir — If your correspondent who furnished the. article on tlie great 

 Weir across the Shannon, in the October number of your Journal, 

 would favour the public with a few more particulars of that work, I 

 think it would be highly interesting to your readers, especially as he 

 appears to make an assertion which in its present form I cannot admit 

 to be true, viz., " that a bar or weir may be constructed across a river 

 in such a manner as to give an increased discharge." At the same 

 time I am aware that the channel of a river may be widened or 

 deepened, so that, notwithstanding the impediment of a weir (of cer- 

 tain dimensions) an increased discharge may be obtained. 



Supposing the Shannon at Killaloe to be in its former condition and 

 flooded to the level of nine feet above the present level of the weir, 

 I apprehend your correspondent does not mean to say that simply 

 placing the impediment of u weir in a certain position would at once 

 reduce the level of the water six feet. Under such circumstances 

 there would not be any perpendicular fall over the weir, merely a 

 slight rapid, the stream preserving iis former direction, which being 

 the case, the cross section must be taken in all places at right angles 

 to the stream, to give the true sectional area of the river, which area 

 would of course be reduced by the weir. If the weir were placed at 

 right angles with the stream, and the river widened till the sectional 

 area of the water passing over the weir was equal to the area of the 

 river above, or if the weir were placed, as shown in the plan accom- 

 panying the communication, and the channel below deepened, so as to 

 give a perpendicular tall over the weir, (in which case the area of 

 discharge must be taken in the line of the weir,) then the advantages 

 mentioned by your correspondent would be obtained : but unless the 

 channel is widened, or deepened so as to give a perpendicular fall 

 over the weir, the discharge must be impeded by any obstruction op- 

 posed to the stream. 



I remain, Sir, &c, 



H. C. 



ON CONTOURING OF MAPS. 



Sir — In your Journal for this month I have read a description by 

 Dr. Livesay of an instrument invented by him in 1839, for copying 

 medals, &c. and in which he speaks of my " important application " of 

 the principle of contouring, which he thinks may be subsequent to 

 his invention. I only trespass on your pages to say, that if Dr. Live- 

 say attributes to me the application of contours to the representation 

 of ground, he gives me a very undue merit. They have been so 

 applied for more than half a century, especially to military plans, 

 where the relative command of ground is of great importance; for 

 which purpose alt officers of engineers are instructed in contouring. 

 In France and Bavaria, and I believe in some of the States of America, 

 they are used in the national surveys. In Ireland, Mr. Bald, the 

 eminent civil engineer, contoured a part of his Map of Mayo. The 

 application of accurate contouring to the representation of ground on 

 the Ordnance survey of Ireland, commenced in 1S33, under the di- 

 rection of Lieut. Bennet, R.E., and the trials have proved that the 

 system may be applied without incurring any such addition of cost as 

 should preclude its general adoption, to which this question of ex- 

 pense had long been the only objection. There are many valuable 

 papers on contouring in the Memorial du Depot de la Guerre ; and a 

 concise chapter on the subject in Williams' Practical Geodesy. 



I have only to add that 1 had nut heard of Dr. Livesay's instrument 

 till I read the account in your Journal fur the present month. 

 I am, Sir, 



Dublin, Your obedient servant, 



November 20, 1S43. T. A. Larcom, Capt. R. Engineers. 



REMARKS ON SHIP'S FASTENINGS AND STEAM BOATS. 



BY J. S. ENTS, ESQ. 



The necessity of great strength in the hulls of steam boats, has long 

 been acknowledged, in consequence of the concentrated weight of the 

 engine and boilers. In recent instances, however, a portion of this 

 weight has been removed by the introduction of wrought iron framing 

 in lieu of cast iron; more especially in large direct action engines. 

 This objection to cast iron framings increases with the size of the 

 engine, since a framing formed of this material, is liable to break when 

 subjected to a variety of cross strains, at the junction of different masses 

 of cast iron, at which points unequal contraction in cooling is apt to 

 produce weakness. 



Engineers have been accustomed to guard against injury from the 



weakness of the ship, by placing the engine on the floors in a framing, 

 as independent as possible of the hull of the vessel, and left the ship- 

 builders to provide a remedy against the sagging, or sinking of the 

 centre part of the hull. 



Though much has been done to strengthen steamers, yet enough 

 remains undone, to render it a legitimate object of inquiry, whether the 

 limit of strength has been reached in ship-building, while the severe 

 and rapidly increasing competition of iron vessels, renders it of im- 

 portance to the builders of timber ships, to consider everv practicable 

 means of improvement and reduction of cost. Conceiving the iron 

 strap used by miners in connecting together the main rods in shafts, 

 and common in all framing designed by engineers, stronger than the 

 ship-builder's knee fastening, a frame was submitted to the society 

 for the midship section of a steamer, in which a strap wes used for the 

 purpose of connecting the deck beams to the side ; this strap passes 

 round one or more timbers, and is then bolted to the deck beams. 



A similar method was shown for strapping an internal series of 

 timbers to the floor heads and deck beams (forming two internal sides 

 at the position of the bunkers). The arrangement of the coal boxes 

 of large steamers on each side of the engine and boilers, would faci- 

 litate the adoption of a plan of this nature, without the loss of space 

 exceeding one foot in the internal breadth of the vessel. The plan of 

 four sides in this portion of the vessel, could be adopted in iron ves- 

 sels with the greatest ease, in consequence of the facility with which 

 the fastenings could be effected. 



As regards the strength of the strap, it may be remarked that the 

 straps connecting together the main rod employed in pumping water 

 from deep shafts in the Cornish mines, have been known to bear the 

 sudden repetition, for thirty million times, of a steam strain exceeding 

 eighty tons (six strokes per minute average during ten years.) 



The difficulty of effecting repairs will be made an objection to the 

 employment of the iron strap in shipbuilding. Important as such se- 

 condary objections are, yet they are too often brought forward as the 

 prominent features, in opinions that are given against the success of 

 proposed alterations in shipbuilding. Time would alone prove whe- 

 ther the less tendency to require repair, where the iron strap is used, 

 would equalize their average cost in repairs. — Report of the Cornwall 

 Polytechnic Institution. 



REMARKS ON FLOATING LIGHT VESSELS. 



BY J. S. ENYS, ESQ. 



The comfort of the light keepers on board floating light vessels will 

 be increased, I apprehend, by the same means that would tend to 

 produce greater safety in the vessel itself. It has been proposed to 

 employ a larger vessel constructed of iron, to enable it to ride more 

 easily at anchor — and such would be the result, if the same breadth 

 was allowed, from the lessened draught of water. The form in which 

 I am disposed to consider the subject is the relative proportion of the 

 depth and breadth (outside measure), for the sake of comparison, of 

 iron and the common system of timber vessels, and that of three series 

 of planks fastening together. Taking the least possible depth at 7£ 

 or 8 feet, the breadth should not be less than IS or 20 feet, and at 

 3-3 to 4 times the breadth for the length, from GO to 80 feet long. The 

 form of the midship department should be similar to that of a water 

 fowl or of a light Dutchman. The former, however, does not ride at 

 anchor as the latter does in bad weather, and we should look for the 

 conditions of safety to some modification of these forms. 



The floating light vessels I have seen, resemble the Dutchman only 

 in their heavy appearance, and seem constructed to resist as an oak, 

 rather than to bend as a willow. It is essential, I conceive, that a 

 vessel suited for this purpose, should possess a large proportional 

 surface department; in other words, that a light vessel should be 

 lightly loaded. To enable her to rise with greater facility to the sea, 

 she might be anchored by the keel by iron straps, passing round it and 

 one or more of the deck beams. If the double plan was used, by 

 changing the length of the chains more or less ease could be given to 

 the bow in rising over waves. Perhaps a keel deepening aft would 

 prove advantageous in some positions, though perhaps objectionable 

 in cross tides and seas, and sharp bows might be used u ider some con- 

 ditions. Ihe species of knowledge required for mooring small craft in 

 rough water, is seldom acquired by parties accustomed to large ships 

 in the Atlantic or other open seas, and an apprenticeship on board a 

 Dutchman, or experiments on board a trial vessel with tanks, by means 

 of which a variable displacement could be obtained, seems to offer the 

 best means of procuring the data required for a complete solution of 

 the question, of the best form for a floating light vessel.— Ibid. 



