1845. J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL 



313 



and most psppci^il in'niincos for painting in lifu of marlili* ilu<t, so 

 commonly uspd by the Gropks, Romans and modern Italians, and for 

 wliicli purposes it merely requires more impalpable grinding. 



One caution perhaps may be given jiidicioiislv, en jmasanf ; the 

 carbonate of l)arytps of nalnro, tliongli not so soluble as the artitioi il 

 or prfcipn/nl(d cnrbonxte, and therefore lus injnrions, is still inimical 

 to animal life, and consequently ere ought to he taken that no waters 

 in which it is mixed or gnund (\( ground wet) slioidd be dischargeil 

 into running streams or ponds to vvliich horses or cattle may go; and 

 here it mav not be amiss to repeat, th.it for ornamental plasters and 

 fresco painting as a pigment, if Kale r free from iron lie used, it sir- 

 passes infinitely every pigrnent.d white, and tint not in beauty only 

 but utility, fir :is much as pigmental lime lai/s on Ih siir/ace .nu] h 

 too identical with the ground, whereas pigmental carbonate of b.irytes 

 is distinct in /owe and becomes part and parcel of the intonaco, and 

 therefore adds materially to the permanence of fresco. 



Now, a passing word on plaster and plaster-keyin?, which if may 

 be said also we have grossly neglected ; for, with the exeeplimi of 

 some verv cinmsv attempts by German artists to give an additional 

 t'>otli by the use of pebbles" in tlie unde-- coats, one single advance 

 Aas not been made during seven or eight centuries ; in f.ict, our ceil- 

 ings are infinitely inferior to many old Roman works, where the artists 

 in endeavouring to imitate marble took considerable pains. Nothiig 

 can be more conimcn than the separation of the coats or layers of 

 plasters, and nothing more obviously to be expected ; tliere is no sort 

 of provision for securing a connexion, much li'ss for enabling that con- 

 nexion to carry a great body and weight of fine plaster on an extended 

 •lirface. It is, indeed wonderful, that fine paintings in fresco or dis- 

 temper should have existed any length of time under sneh careless 

 and inetTicient managenienient ; and 1 consid'Tthe fri'Sfo frame men- 

 tioned in my last paper (in the August number) amply proved the 

 possihilitv of applying Smeaton's principle in forming the foundation 

 of Eddvstone light-house to plaster- keying up to the finest intonaco, 

 for eacli coat was so perfectly dovetailed into the other, at regulated 

 distances, that it would be difficult indeed to separate its strata; in 

 fact, it could only be done by the saw. Now two precautions only 

 were usi d in the work — First, to regulate the sand from the sharpest 

 specimen of sea said {mtll wnslied) to fine silver sand, as to ensure an 

 universal gradation of tuoth or frieze ; secondly, to indent by a wooden 

 tool each coat in the angles, after dividing its surface into squares or 

 diamonds, making each indenture or mortice pretty deep, so that, at 

 regular distancts, the coat of plaster added gave not onlv the covering 

 •urface but the tenon for each mortice, by which the last and finest 

 coat might have been enabled to carry a ton of plaster to every twenty 

 feet square of work, in lieu of holding only by the suction (as the 

 workmen call it> oof a very irregnlar surface carelessly and irregularly 

 wetted, and (d'ten scored by the pressure of an ordinary trowel, tlie 

 worst tool in the world, for the somewhat polished surface left by an 

 inclined, or half flattened, trowel has little or no key at all, hence the 

 frequent use of wood in scoring. 



Allow ine the use of native carbonate of barvtes from the onset of 

 the rough cast, and in very special icorks (such as for fresco in the 

 Palace at Westminster or other national buddingsj that of caustic 

 barytic w..ter,'' to wet my surfaces with, joined to this mode of keying, 

 and I will defy either time or human hand, except by the saw, to se- 

 parate one square inch in five hundred .«quare feet of surface; the 

 coats become so homogeneous a body, in fai;t, that the work is the real 

 antipodes of the commi.n layer upon layer of heterogeneous plaster 

 scarcely keyed at all, simply because it is both mechunicaliy morliced 

 and citemtcal'y united, 



W. MaRRIS DlXSDALE. 



Sept. 8, 1845. 



• Than wtii li nothing csn I 



• For fresco joinings, after c 

 Itiis ciusilc b-aylic water U 

 ■faiirpeaC edge becomes une mu 



more clumsy o.- defective or dillicult to work over, 

 long away the sui.erliiou5 .(lues of ttu- day before wiTk, 

 ivaluable; well netted with the tninuteit iuteralice or 

 J really hardwr than the general surface. 



ON DREDGE'S SUSPEXSION BRIDGES. 



Bv F. BashF'jkth. B.A., Ftllow of St. John's College., Camhridgc, 

 and of l/ie Cambridge Philosophical Society. 



Sir — Any new plan for a suspension bridge unaccompanied by its 

 torrect mathematical iheury is. iu my opinion, unwort!n of confidence. 

 My obj'Ct was only to call attenlion to the singnlarlydetective stale 

 •f that to whicii Mr. Dredg<- referred all for answer's to their objec- 

 tions. Now seeing that wiieu suspension bridges du fail, lives by 

 hundreds are sacrificed or placed in imminent peril, it did appe.ir to 

 Mie to be worthy of animadversion, ilad it been a mere inalter of 

 curiosity, its' f.iults would have been hurmless and coi.sequeiilly un- 



worthy of notice — but when we find the patentee of a bridge, engaged 

 in carrying out his plans in various parts of the world, referring to 

 this as an authority, the state of the case becomes entirely changed. 

 I do not dispute the fact of Mr. Dredge's having constrncied bridges 

 with a smaller quiutity of material than would have been required on 

 the old phm— but I do call in question their safely, if built on the 

 theory authorizetl bv Mr. Dredge, which he appears still determined 

 to support in face of its palpable absurdities. So long as wi' have no 

 satisfactory theory w have no means of comparing the intrinsic merits 

 of the old iind the new plan; and again 1 repeat that the complica- 

 tion introduced by the oblique suspending rods is such as in inv opi- 

 nion places it beyond the reach of the most accomplished inatiiema- 

 tician. If this be the case, and siiic(^ practical experi-'nce would have 

 to be purchaseil in every separate case at too great an expi'nse, I 

 conclude thiit the plan is not of any groat value, for it is a poor econo- 

 my to sacrifice safely to avuiij the necessary expense to ensure it. 



The theorist after som- ;jj or lU p iges of assumptions and demon- 

 strations gives an example of the application of the general theory to 

 a particular case. I pointed out distinctly the step which could he 

 allowed in the general theory, and to shew that this did not even give 

 an approximation, I pointed out that the results were impossible. 



The only mathematical theory of Mr. Dredg-'s Suspension Bri'lges 

 |>ublished by Mr. Weale, that I have seen, forms the Supplement to 

 the Papers on Bridges, and I cannot find anv other in the University 

 Library. It is anonymous anil nithout preface, but by the quotation 

 from page IV., I first learu that the treatise' is Mr. Turnbull's. 



Mr. Dreilge has had the boldness to charge me with unfairly quot- 

 ing his remarks, and unfortunately for himself has ventured to' give a 

 specimen of my ilelinqiiencies. I w.is desirous of compressing my 

 remarks as much as possible, and for that purpose deemed it sullicient 

 to give the auihor's meaning fairly and fully. 



1 found the announcement, " Mr. Weale is about to publish, &c.," 

 on one of Mr. Dredge's reprinted leiiers, and I could not sup|>ose it 

 placed there by any other than his own authoritv. In consequence of 

 this omission of the publisher's name, .Mr. Dredge wishes to make it 

 appear that 1 have put at least the respoiisibililv on him. On referring 

 to page 456, vol. II. of your valuable Journal the foUowins: letter will 

 be found. ° 



"Sir— An entire description of my patent suspension bridge would 

 occupy too much space in your Journal, and employ more time than I 

 can at present spare, and as an abridgement would perhaps produce 

 more cavilling tlian would he interesting to vour readers, or necessary 

 to the investigation of truth, I wii refer Mr. Fordham to a full ma- 

 thematical description, v,b\ch w\\\ be published in a few days, liy Mr. 

 Weale, to four foot-bridges in the vicinity of Regent's Park, and 

 similar works in various parts of the kingiliim, so that as a mathema- 

 tician, and man of science, he may he able to read, see, and judge for 

 himself, and from these evidences firm what opinion he thinks ft, and I 

 shall be most liappy to see that opinion publicly expressed through tht 

 medium oj your pages. 



" 1 remain, Sir, your obedient servant, 



"J. Drlpgi:." 



This is my justification, and taken as the standard length of Mr. 

 Dredge's sentences will I hope be deemed a sufficient excuse for hav- 

 ing quoted those parts of sentences which gave his true m-aning. As 

 he informs us that he is a malhematici in, he ought to have examined 

 the theory of whicli he took upon himselj the reponsibiiity. 



Although Mr. Dredge has come forward to say something in favour 

 of the theory, he has not thought proper to notice my objections, 

 which are most distinctly pointed out. The quotation from page IV. 

 ii.is little or nothing to do with the mitter, and shews in a remarkable 

 manner the clear notions and strict reasoning, employed by the author 

 and approved of i.y Mr. Dredge, by which they will no doubt be able 

 to prove any thing. 



Mr. Dredge must have been quite aware that I did not say tliat 

 practical experience was worthless— but this I do say — the authors of 

 the letters could not have had time and opportunities for gaining prac- 

 tical experience, and then what else than theory would remain to be 

 depended on ? 



Since I wrote my remarks, intelligence has arrived from India res- 

 pecting a bridge of 2oO feet spi;n, which was in progress at Calcutta. 

 Ill the Mechanic's Magazine for Octeber 1S44, will be found a copy of 

 a letter from a Captain of Engineers to Mr. Dredge. "With the' as- 

 sist ince of a very able and fiist rate mathematician liere I have studieil 

 the theory ol these bridges most thoroughly; and the model 1 liavg 

 made 22 feet long and t leel widih of platform, is on so large a scale, 

 that I have been able to test it. in every possible way, and it has with- 

 stood the utmost ellurts to derange its parts. The Governor-General 

 and all the S'lieniific peo|ile here, have perfectly satisfied themselves 

 of llie ethciency of the system, and all the proofs with my models as- 



