324 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[October, 



varnished. On the second trial of the Great Britain, on the 20th January, 

 in the Bristol Channel, in smooth water and during a calm, the engines 

 attained the speed of 18| strokes per minute, when the speed of the vessel 

 through the water, measured by an experienced seaman, with the common 

 log, was 121 knots. 



Feet. 



18-66 X 2-948 x 25 = 1375-242 velocity of the screw. 

 12J knots X 101.2 = 12t7-796 



127-446 slip- 



thus, the speed of the vessel was -907 to 1- of the screw. 



The area of the midship section of the ship, at the time of this experiment, 

 was 408 feet. The diagram, fig. 1, is intended to illustrate this effect, thus : 



the an^le subtended by A, B, C, is an entire revolution of a screw 15 feet 6 

 inches'diameter, and 23 feet pitch, of which B D is the forward effort com- 

 municated to the vessel, and C D is the slip, or welding of water Conse- 

 oaentlv, although the apparent angle of tlie screw is represented hy A B C, 

 the re'al angle is only A C D, since A B D represents the velocity of the 

 vessel. The steam-engine employed to drive this screw, consists of four 

 steam cylinders, each of 88 inches in diameter, hy 6 feet stroke, into which 

 steam is admitted by piston.v.ilves of 20 inches in diameter. As it is very 

 troublesome to lift large piston covers, inanlioles are made in them, and in 

 the pistons, so that the bottoms of the cylinders can he easily examined. 

 The large diameter given to the steam cylinders was purposely with a view 

 to working verv expansively, and on the trial recorded, the steam, being at 

 4 lbs. pressure "in the boiler, was throttled on its passage, and cut oft by the 

 expansion-valve at Jth of the stroke, that is, 1 foot from its commencement. 

 The connecting rods of these engines are applied in pairs to crank pins, at 

 either end of the main shaft, and the same crank pin carries the connecting 

 rod of one air-pump, of the same lenglh of stroke by 45 J inches in diameter. 

 This air-pump is inserted in the wrought iron condenser, which receives the 

 steam from the cylinders. Tlie main shaft is of wrought iron, 1/ feet long, 

 hy 28 inches in diameter, in the centre, and 24 inches in the bearings which 

 are 30 inches long ; through this shaft, as through the cranks and crank 

 pins, a hole is bored, and a stream of cold water is constantly injected, 

 which has an important influence in keeping the bearings cool. Upon this 

 main shaft, is a tocthed drum, of 18 feet in diameter, with a face 38 inches 

 in width, around which, and a lesser drum of 6 feet in diameter, placed below 

 it, four sets of pitched chains work -, the motion of which is remarkaby 

 smooth and noiseless. Each set of these chains consists of two inks, and 

 three links, alternately : the sectional area of the four sets is 24 inches. 1 he 

 best method of giving the requisite speed to the screw shaft, was long under 

 consideration, and the usual means, hy gearing, straps, &c. were not over- 

 looked ; but eacli appeared to have some objectionable quality ; at length, 

 Mr. Brunei suggested the pitched chain, which was finally adopted, these 

 links were very carefully forged ; they were then brought to a dull ret heat, 

 and placed in a proving^maching, where they were stretched one-eighfh of an 

 inch, and, while in tliat state, they were rigidly examined. After boring and 

 planing, they were all finished on one gauging-tool and case hardened. As 

 the engines are intented to work at 18 revolutions per minute, and the speed 

 is got up at tlie rate of nearly 295 to 1, the screw will then make abou 53 

 revolutions per minute. The lower shaft, to which the screw is attached, 

 consists of thiee lengths. On the first, which is 28 feet 3 inches long, by 

 16 inches diameter in tlie journals, is fixed the lesser drum, which is 6 feet 

 in diameter, and at the forword end of this, is the step whicli resists the 

 thrust, or efl-ort of the screw, which will be presently described The second 

 piece is a hollow wrought iron shaft, 61 feet 8 inches long, and 30 imdies in 

 diameter, formed of two courses of plates, each J inch thick, riveted together 

 by countersunk rivets 1§ inch in diameter. The third piece is 2o feet b 

 inches long, and as the screw has no bearing at its outer end, it is 1/ inches 

 in diameter in the journal, just within the stern-post. Tlie .haft does n.t 

 rest in the stern-post, but in another bearing, outside of it, and the water is 

 kept out by a packing, composed of leather and copper. The thrust, or 

 effort of the screw, is received hy a step, composed of a steel plate. 2 f.-et in 

 diameter, against which a gun-metal plate, of similar diameter, afhxed to 

 the heel of the shaft, presses. A stream of water is admitted to a cavity, in 

 the centre of these plates, and very satisfactorily lubricates them. 

 (To be continued.) 



ACCOUNT OF EXPERIMENTS UPON THE FORCE OF 

 WAVES. 



By Thomas Stevenson, C.E., Edinburgh. 



In forming designs of marine works, the engineer has always a difficully 

 in estimating Ibe force of the waves with which he has to contend. The- 

 information on sucli a matter, which is derived from local informant-^, who, 

 although intelligent in the departments of trade which they follow, are, 

 nevertheless, more or less prejudiced from being constantly on the spot, is 

 not satisfactory ; and it has, therefore, often occurred to me that it would 

 be most desirable if the engineer could be enabled, lo some extent at least, 

 to disregard the prejudiced statements of otiicrs, and the vague impressions 

 left by them on his own mind, and really to ascertain by direct experiment, 

 what force, expressed in pounds per square foot, the sea actually exerts 

 upon the shores where his buildings are proposed to be erected. 



Before considering the results obtained, however, I shall explain the 

 construction of this simple self registering instrument. The letters D E !• D 



represent a cast-iron cylinder, which is firmly bolted at the projecting 

 flanges G to the rock where the experiments are wanted. This cylinder 

 has a flnn-e at D D. L L is a door, which is opened when the observa- 

 tion is to be read ofl'. A A is of iron, and forms a circular plate or disc, 

 on which the sea impinges. Fastened to the disc are four guide rods 

 B B B B These rods pass through a circular plate C C (which is screwed 

 down lo file flange D D), and also through holes in the uoitoni E F, Within 

 the cylinder there is attached to the plate CC a powerful steel spring, to 

 the other or free end of which is fastened the small circular plate KK, 

 which again is secured to the guide rods B B B B. There are also rings 

 of leather TT, that slide on the guide-rods, and serve as indices for regis- 

 tering how far the rods are pushed through the holes in the bottom ; or, in 

 other words, how much the spring has been drawn out or lengthened by 

 the force of the sea acting upon the plate or disc A A. The object of hav- 

 in-T four leathern rings, where one might have answered the purpose, was 

 merely that they might serve as a check upon each other ; and so perfectly 

 did they answer the purpose intended, that in every instance they were 

 found equidistant from the bottom of the cyliniler ; proving thereby, that, 

 after the recoil of the spring, they had all kept their places. The guide-rods 

 are graduated, so as to enable the observer lo note exactly the quantity 

 that'the spring has yielded.* , ,ir ■ 



This instrument, which may, perhaps, be notimproperly termed a il/ur/«e 

 Dmamometer, is, therefore, a self-ieg.stering apparatus which indicates 

 the maximum force of the waves. In the graduation of the instrument, the 

 power of the spring is ascertained by carefully loading the disc witli weights 

 so Ihat when the quantity that the spring has yielded by the action of the 

 sea is known, the pressure due to the area of the disc exposed is known 

 al«o The discs employed were from 3 to 9 inches diameter, but generally 

 6 inches, and the powers of the springs varied from about 10 lb. lo about 

 50 lb. for every i inch of elongation. Their respective efl-ecls were after- 

 wards reduced to a value per square foot. The instrument was generally 

 placed so as to be immersed at about three-fourth tide, and in such situa- 

 tions as would afl-ord a considerable depth of water. It is not desirable o 

 have the instrument placed at a much lower level, as it has not unfrequenlly 

 happened during a gale, that for days together no one could approach it to 

 read off the result and re-ailjust the indices to zero. It must, however, at 

 the same lime be remarked, that it is in most situations almost impossible 

 to receive the force unimpaired, as the waves are more or less broken by 

 hidden rocks or shoal ground before they reach the instrument. 



In 1842 sevei-id observations were made on the waves of the Irish Sea at 

 the I-land of Little Ross, lying oli the Bay of Kirkcudbright. Since April 



* Ttie plate or disc of llie M 

 stances of its eNp"Sures, stone 

 sMe, «hicli is the case in all di 

 ily composed 



tlie t — 

 disc, the i 

 hably be t 

 But the e 



t that the 



B llynamoiiieter is intended lo represent in the circum- 

 ,rc» =iuur= „. other obstacles to which the sea has access on every 

 se ill all dry stonework, of which liarbours and breakwaters are com- 

 also open ijilework of every kind. It may also be observed that by 

 St of the wave (if unbrok.n) has proceeded so far as lo be above th- 

 . will have registered its maximum, although that 

 lent affected by the statical pressure 

 i liave reteience solely to bod"" ' 



luugii luiiL iua.v....u". willpro- 

 the side of the disc next C C. 

 immersed in water. 



