1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



357 



lish, in some form applicable to the intended purpose, instructions on 

 this subject, combining the improvements which science h^s suggested 

 since the period of M. Gay-Lussnc's report. It is by no means impro- 

 bable that many parishes might be induced to erect conductors on their 

 churches, but the subject is so little understood that no one ventures to 

 propose it ; and unless these conductors are properly erected they are 

 more likely to do harm than good. 



The old cathedral of St. Paul's was twice struck with lightning, and 

 both times seriously damaged. In 1444 the lightning destroyed the 

 tower of the cliurch, and in 15til it set the church on fire and nearly 

 burned it to the ground. In 1769 the Dean and Chapter applied to 

 the Royal Society for their advice as to the best means of securing the 

 present cathedral from the effects of lightning, and a short description 

 of the mode adopted in this immense edifice may be interesting. 



The ball and cross are supported by seven iron rods; these rods are 

 then connected with other rods (used merely as conductors) which 

 iniite them with several large bars descending obliquely to the stone 

 work of the lantern. A ring made of iron, one inch square, connects 

 all these bars, and four iron bars, one inch square, connect this ring 

 with the lead covering of the great cupola, a distance of 48 feet; and 

 from thence the communication is continued by means of the pipes 

 which discharge the water on to the floor of the stone gallery. 

 From these pipes large strips of lead connect them with the pipes 

 which discharge the water on to the roof of the building. This roof 

 is entirely covered with lead, and from thence the communication is 

 continued to the ground by means of the lead water pipes, which pass 

 into the earth, tbus completing the entire commuiiicntion from the 

 cross to the ground, partly through iron, and partly through lead. On 

 the clock tower a bar of iron, one inch and a quarter square, connects 

 the pine apple at the top with the iron staircase, and from thence with 

 the lead on the roof of the church. The other tower has a bar of iron 

 of the same size extending 88 feet from the pine apple to the roof of 

 the church. By these means the metal used in the building is rendered 

 available for the purpose of the conductors, the metal employed 

 merely for the conductors being exceedingly small in quantity. 



The general neglect of the use of lightning conductors probably 

 arises from the belief of their insufficiency and ignorance of their pro- 

 per construction. But it can be satisfactorily shown that wherever 

 they have failed to afford protection it has arisen from either defective 

 construction or subsequent derangement. 



Earl-sinet, Sept. G. 



H. C. 



GREAT BRITAIN STEAM SHIP (LATE MAMMOTH). 



The Mechanic's Magazine, No. 99G, gives an account of the con- 

 struction and dimensions of this ship, together vi'ith drawings 

 of her machinery and its arrangement, explanatory, ulthongh necessa- 

 rily drawn to a very small scale ; and useful, could we depend upon 

 their accuracy; but the author (Mr. Hill) tells us " his rough dimen- 

 sions were obtained by pacing, otliers by a graduated walking stick," 

 a system of mensuration somewhat unusual among engineers of the 

 present day. From the materials there furnished we form the follow- 

 ing calculations: — 



The diameter of each cylinder is said to be 88 inches, the stroke G 

 feet, which at 19 strokes per minute, or 228 feet, is equal to 295 

 horses each, or of four cylinders to 1180 horses, and not of lOUO, as 



88 X 7x 2*^8 

 stated by the author. Th\is, area — — — _ " i=294.15 horses or 



^ 33,UUU 



117G.G0 for _/bur e)!^/nes, doubtless intended to be of the collective 

 power of 1200 liorses, which in fact an additional 1 of a stroke per 

 minute would produce. The air pumps at 3' 9 "diameter and G'U" stroke 

 have a content of GG.24 cubic feet; one pump and condenser being 

 apportioned to ttvo cylinders. The cylinders have a content of 253.38 



cubic feet; both =506. 7G cubic feet, we have therefore — =-1 .1 



G6.24 

 times, or about the proportions of our best engineers. The condensers 



are 12.0x8.6X5.0. content = 510 cubic feet, liere we liave = 



66.24 

 7.65 times, or nearly thrice as much as experience shows to be neces- 

 sary. If any dependence is to be placed on the boiler sketches, the 

 Bristolians have yet much to learn in that department of marine 

 engineering. We are inclined to doubt their authenticity, for having 

 so good an example as the boilers of the Great Western before them they 

 ought to have certainly produced something nearer perfection, but of 

 course we can only reason on what is before us. For example, we have 24 

 fires, each 6 feet long by 2 feet wide, a total grate surface of 288 feet 

 for 1200 horse |iower, or about 1 of a foot per horse power (nominal) 



0): le»s than half the proper quanliiy. This fact, coupled with the in- 

 different arrangement of the flues, are plain indications that the con- 

 sumption of coals will be the reverse of economical. 



We make this analysis with reference to an article on "The Great . 

 Britain, the largest vessel in the world," published in the Tinus 

 last month, wherein it isolated she has stowage for 1000 tons of coals 

 and 1200 tons of measurement goods. The former amount seems in- 

 adequate for the purpose intended, viz., a voyage from Bristol or 

 Liverpool to New York. 



If we take the consumption at the moderate computation of S lb. per 

 H.p. nominal per hour, we have 1200 x 8 z= OlJOO, or per day = 103 

 tons ; and supposing an average passage 12 days, the consumption 

 would in that time be 1236 tons of coals without any surplus for the 

 contingency of bad weather and prolongation of voyage. We think 

 the consumption of coals will be nearer 10 than S lb. per horse per 

 hour ; and if we are correct in this opinion, we have 1200 lb. per hour, 

 or, 129 tons per day, or 1548 for a voyage of 12 days — and mind, 

 no surplus. 



But, will she make the passage in less than 12 days ? Tiie same 

 authority informs us that her load line is at 16 feet; at this, her im- 

 mersed section is about 600 feet. Taking the powers as the cubes of 

 the velocities, and using the factor 1400 (which experience shews to 



be correct for this class of vessel) we have i- - L-' :=2S00= 



600 section. 

 14 miles per hour in still water, presuming her to be propelled by pad- 

 dle wheels ; but according to the Great Western experiments, as stated 

 in Mech. Mag. p. 255, the screw is inferior to the wheel in the propor- 

 tion of 12 to 10, or one fifth, so that instead of 14 miles her true velo- 

 city will probably be 1 1.65 miles, in still water, as aforesaid. 



Now, Cunard's packets attained a greater velocity than 11.65 miles 

 when tried experimentally, and yet their average passages, show a sea 

 speed of about 8-i miles only, the average length of passage being 13 

 days 16 hours out, and 11 days 5 hours home. The passage out gives 

 7.86 miles per hour, while that home influenced by prevailing wind 

 and current gives 9.3 miles, in this case the mean sea rate is 8.58 miles 

 per hour. 



We take these facts from the Nautical Blagazine, subsequently 

 published in the Times, it appears to be a compilation from registered 

 results, and therefore entitled to the highest credit. It states the pas- 

 sages of Cunard's line to be outwards, from 10 days 19 hours to 16 

 days 12 hours, and one reached 20 days 17 hours; the home passage 

 from 9 days 17 hours to 12 days 18 hours, and this between Liverpool 

 and Halifax only. 



We draw two conclusions from these data: the first, there is no 

 reason to suppose the Great Britain will make quicker passages than 

 the present vessels ; the second, that provision should be made for 20 

 days consumption of fuel, for we need not point out to our nautical 

 readers the position of such a vessel in the broad Atlantic, minus her 

 steam power, rigged as she is. What then becomes of the 1800 tons 

 space for measurement goods? Supposing this vessel bulfetted about 

 for upwards of 20 days (as one of Cunard's superb vessels has been), 

 she would require stowage for 103 tons X 20 days = 2060 tons of 

 coals at the moderate computation of 8 lb. per horse per hour; if we 

 reduce it to the minimum of safety, say 16 days stowage, we require 

 1648 tons, at 8 lb. per horse only; which we may say is the general 

 average of the best and most economical boilers. We know enough of 

 marine engineering to state confidently that the boilers of the Great 

 Britain will not reach this point. 



We hope we may not be misunderstood, and our remarks considered 

 inimical ; our desire is simply to correct what we believe to be erro- 

 neous conclusions and to put the proprietors on their guard, that they 

 may be moderate in their expectations. It would conduce to our ut- 

 most pleasure if their most sanguine expectations become realized, 

 not in the narrow spirit of pecuniary gain, but as a national triumph 

 and convincing proof that Great Britain is, and ever shall be, mistress 

 of the seas. 



"THE LIGHT OF ALL NATIONS." 



The laudable attempt of Mr. Bush to construct a light-house on the 

 Goodwin Sands, is now occupying public attention, we have, there- 

 fore, obtained the following particulars of the construction of the cais- 

 son and its progress, which we hope to he able to follow up in next 

 month's Journal, with a drawing, and an account of the works as they 

 progress. We are happy to liear that up to the present period, not- 

 witbstanding some severe gales and squally weather, Mr. Bush has 

 proceeded very successfully, and fully equal to his most sanguine ex- 

 pectations. We sincerely hope that his praiseworthy efforts will be 

 \ rowned with success. 



