1843. J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



211 



with good materials, has been, it is contended, fully proved at Lord Palmer- 

 ston's house, which was covered with the composition in 1807; Lord Ber- 

 wick's, in 1810; Sir James Langham's, in 1812; the Pavilion at Brighton, 

 in 1816 and 1823; and nearly the whole of Buckingham Palace, in 1826 

 and 1829; the latter roofs are stated to be in perfect order at the present 

 time, and have scarcely demanded any repairs since their completion. 



The paper is illustrated by a drawing, showing the mode of constructing 

 the roofs, and the improved method proposed by the author, with specimens 

 of the composition, with slates imbedded, taken from the roof of the palace 

 during some recent alterations. 



Remarks. — Mr. Poynter presented a drawing of the mode of setting the 

 pots for moiling and preparing the composition, the proportions of which 

 he stated somewhat differently from those given in the paper. Three mea- 

 sures of ground chalk, dried and sifted very fine, were mixed and kneaded 

 up with one measure of tar ; these ingredients were melted in an iron pot, 

 set in such a manner that the flame should not impinge too violently upon it. 

 The first, or " skimming" coat of the covering being laid on of a thickness 

 of -^ inch, the finishing coat was composed by adding to the former mixture 

 three measures of hot sifted sand, well mixing the whole together; the com- 

 position was laid on with a tool similar to a plasterer's trowel, but much 

 stronger. Mr. Nash, when he first tried the composition, found that the 

 surface became disintegrated by exposure to the weather ; he therefore added 

 the slates imbedded in the second coat, and subsequently never used the 

 mixture without them. 



Mr. Nixon, in reply to questions from the President and other members, 

 stated, that he was employed under Mr. Nash when the palace roofs were 

 executed, and he could bear testimony to their durability and soundness. 

 The roofs at East Cowes castle, which were covered with the composition in 

 the year 1808, and those of the Pavilion at Brighton, in 1816, were now in 

 as good a state as when they were finished. The failure at Mr. Nash's house 

 in Regent Street, arose from the roof having been originally composed of 

 mastic, which soon cracked. One coat of the Stanhope composition was 

 spread over it, to stop the leaks, but it was insufficiently done, and ultimately 

 Mr. Rainy had a new roof, properly constructed, with two coats of compo- 

 sition, which had remained sound to the present time. The price of these 

 roofs, when well constructed by the person who did those of the palace, 2 

 was about five guineas per square. 



Mr. Hogg observed, that the chalk was only exposed to such a heat as 

 would evaporate any moisture it contained. The weight of the two coats of 

 Stanhope composition, including the slate imbedded in it, was about 12 lb. 

 per superficial foot. 



Mr. Sibley considered the Seyssel Asphalte, when carefully laid, preferable 

 to any composition of a similar nature ; he had used it extensively, and was 

 well satisfied with it, both for roofing and paving. 



Mr. Hogg objected to the use of asphalte for roofing, as it was liable to 

 injury, being of a brittle nature ; it was not elastic, and it shrunk from the 

 walls, thereby causing leaks. Lord Stanhope's composition did not possess 

 these faults, and he did not consider that it was superseded by asphalte. 



Mr. Moreland had covered the roof of the tread-mill at Giltspur Street 

 Compter with asphalte, and had found it answer perfectly. It was laid on 

 in a thickness of |-inch upon roofing boards, f-inch thick, with canvas nailed 

 on them, with an entire fall of only 9 inches ; there was not any appearance 

 of leakage. 



Mr. Davidson had caused a school-room to be floored with asphalte, four 

 years ago, and up to the present time there was no symptom of wearing 

 down, although the stones which were let into the floor, for supporting the 

 desks, &c, were considerably abraded. He believed that the only failures of 

 the asphalte had occurred from the use of inferior ingredients. Gas tar had 

 been used instead of vegetable tar, and in those cases the result had not been 

 successful. 



ON BRIDGES. 



At the ordinary general meeting of the Royal Institute of British Archi- 

 tects, held on Monday evening, the 15th May last, Professor Hosking illus- 

 trated and explained his proposal to improve the design of arched bridges, 

 by the introduction of a transverse arch, groined into the longitudinal arch, 

 or series of arches; and showed the effect of this and of other suggestions 

 he has made for the improvement of bridges, in a design for re-modelling 

 Westminster Bridge. 



Mr. Hosking began by stating that the closely attentive consideration of 

 the subject of bridge designing and building, rendered necessary by his en- 

 gagement with Mr. Weale to supply a practical treatise for the extensive 

 work on the Theory and Practice of Bridges, now lately published, gave rise 

 to some suggestions of improvements in design and construction, which he 

 believes to be novel, and knows to be (as far as he is concerned) original. 



His object, on that occasion, was to explain and illustrate the more im- 

 portant suggestions he had made, that they might not be misunderstood, 

 and might be more extensively known than they were likely to become 

 whilst they rested within the covers of a professional library book. 



On a former occasion in that room he had made some remarks upon the 

 subject of bridge building generally, and had urged that the piers of bridges 

 were built of much greater substance in thickness than was necessary for 



2 Mr, Millson, No. 6, Frances Street, Tothill Fields, Westminster. 



either safety or agreeable effect ; that they might therefore be greatly re- 

 duced in bulk both for economy and for their effect upon the water way, and 

 without diminishing their efficiency. It had been objected to him, however, 

 at that time by some of the members — with the too common fault of ar- 

 chitects, who would sacrifice use to effect, instead of compelling the useful 

 to be effective — that his proposal tended to destroy the due proportion in 

 appearance of the pier to the opening. The eye that had been accustomed 

 to the bridges upon the Tiber at Rome, of which the piers are rarely less 

 than one-third the span of the larger of the two arches resting upon them 

 respectively, would be offended by the absence of that proportion of solid to 

 void in London and Waterloo Bridges, in which the same relation is but one 

 sixth ; whilst the eye accustomed to the bridges upon the Thames at London, 

 would condemn the bridges at Staines, and the bridges of Jena and Neuilly 

 on the Seine, of which the piers are but one-eighth, one-ninth, and one- 

 tenth of the span of the arches resting upon them. Nor have we yet 

 reached the limit to which the diminution of proportion may be reduced 

 with safety anil good effect. Further to justify such further reduction, was 

 one of the ends to be answered by the arrangement he was then to explain, 

 which has the effect of reducing also the weight to be sustained by the 

 piers of an arched bridge. The idea had occurred to him, and he had ma- 

 tured it so far, as to be able to speak of it with confidence on the former 

 occasion alluded to above, but as he was then unprepared with illustrative 

 diagrams, he had thought it better to withhold it for the time. 



The proposed improvement consists in groining a bridge arch, or in car- 

 rying a groined transverse arch through the length of a series of arches ; 

 and the advantages derivable from this plan consist in lessening the weight 

 of the bridging constructions ; in reducing the thrust upon the abutments, 

 and consequently confirming the stability of both arches and abutments ; 

 in diminishing the liability of the bridge constructions to vibrate under the 

 action of pulsating or of rolling bodies ; and generally in greatly reducing 

 the cost of construction. 



The weight is obviously lessened by the difference between the massive 

 haunches of the main vaults, and of the requisite backing to them through 

 the extent of the transverse arch, and the comparatively light inner trans- 

 verse arch, which being of slight span, may be of stones of much less 

 depth than the main vaults require ; the thrust of the main vaults is clearly 

 dissipated throughout so much of the width of the bridge as the inner 

 transverse arch occupies, and so that if the latter occupy the proportion of 

 the width that might be given to it, the abutments of the bridge may be re- 

 duced to mere wing walls ; the vibrations arising from the traffic upon the 

 bridge are checked at the groin points as at nodal points in a vibrating cord 

 — and the groins lie directly under the carriage road where alone any action 

 that could be felt in a heavy mass of masonry can arise ; — and the cost of 

 construction is reduced by the reduction in quantity of the materials in the 

 piers and in the vaults — by the reduction of labour required for the softer 

 stone available for the inner transverse arch, and by the lighter centering 

 sufficient for the same. 



He had endeavoured to illustrate his suggestions by applying what he 

 proposed upon a compartment of London Bridge, as a familiar instance, but 

 without anv idea of reflecting upon the existing condition of that magnificent 

 work. [Here Mr. Hosking explained the diagrams, which were merely en- 

 largements of the plate which illustrate the same subject in the Treatise on 

 Bridges.] 



The only indication of such an arrangement as that he suggested, in any 

 existing work with which he was acquainted, is in Perronet's Bridge of St. 

 Maxence, where low arches are introduced over the divided parts of the 

 piers transversely of the bridge, to take the springings of the great longitu- 

 dinal arches, but these have neither the intention nor the effect of what is 

 proposed, and are a source of weakness and expense rather than of economy 

 and endurance. [The diagrams which illustrated this, showed that the 

 transverse arch was low and flat, instead of rising to the lull height of the 

 great longitudinal arches, and must therefore exert a great thrust upon the 

 divided portions of the piers which abut it ; and as the vaults spring upon 

 the backs of these transverse arches, there is no relief either in thrust or 

 weight, by groining.] 



He was well aware that the suggestion he had made was exposed to con- 

 troversy upon the presumption that the transverse arch may not have suffi- 

 cient abutment within the length of a pier, transversely of the bridge, and as 

 the theory of the groined arch has not been satisfactorily determined, if in- 

 deed it have been really investigated, he must claim to refer to experience 

 and assert upon example that the inner arch, as he had drawn it in the dia- 

 gram, was superflously abutted. Under any circumstances, indeed, it can be 

 only a question of greater or less span of the inner transverse arch with 

 reference to the abutments afforded to it by the springings of the outer and 

 greater longitudinal arch to which it is groined, since there can be no ques- 

 tion but that if the abutments are sufficient to restrain the arch, the opera- 

 tion may be safely carried out. In the example the transverse inner arch 

 occupies but half the length of the pier, leaving the minimum abutment equal 

 to half the span of the arch, with the means of increasing it to almost any 

 extent by raising buttresses upon the heads of the cutwaters. 



Numberless instances exist of arches of far less rise in proportion to their 

 span than the present example shows, abutted only by the piers on which 

 they rest, or rather by a substance upon their haunches extending only to 

 the thickness of their piers ; the piers being far less in proportion to the 

 span than in the example, whilst the proportion of abutment to span should 

 increase a.s that of rise to span diminishes. Trajan's Bridge over the Tagus 



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