July ist, 1887.] 



SCIENTIFIC NE^VS. 



117 



THE ROYAL INSTITUTION. 



BRIDGING THE I'lRTH OF FORTH. 



ON May 20th, Mr. Benjainiii Baker, M.Inst.C.E., gave an in- 

 teresting account of tlie great bridge now in course of 

 construction. To impress upon his liearers the e.xceptional size 

 of tlie Forth Bridge, lie said that if one of the tubes of the great 

 Britannia Bridge could be transported from the Menai Straits to 

 the Forth, they would find that it would cover little more than 

 one-fourth of the space to be spanned by each of the great Forth 

 Bridge girders. To get an idea of the magnitude of the latter, 

 let them stand in Piccadilly and look towards Buckingham 

 Palace, and then consider that they had to span the entire dis- 

 tance across the Green Park with a complicated steel structure 

 weighing 15,000 tons, and to erect the same without the possi- 

 bility of any intermediate pier or support. Let them consider, 

 also, that their rail-level would be as high above the sea as the 

 top of the dome of the Albert Hall is above street-level, and that 

 the structure of the bridge would soar 200 feet yet above that 

 level, or as high as the top of St. Paul's. It was not on account 

 of size only that the Forth Bridge had excited so much general 

 interest, but also because it was of a previously little-known 

 type. It was a cantilever bridge, and one of the first questions 

 asked by the generality of visitors at the Forth was. Why do you 

 call it a cantilever bridge ? A cantilever was simply another name 

 for a bracket, but the 1,700 feet openings of the Forth were 

 spanned by a compound structure consisting of two brackets or 

 cantilevers and one central girder. Owing to the arched iorm of 

 the under side of the bridge, many persons held the mistaken 

 notion that the principle of construction was analogous to that 

 of an arch. In preparing for liis lecture the other day, he had 

 to consider how best to make a general audience appreciate the 

 true nature and direction of the stresses on the Forth Bridge, 

 and after consultation with some of the engineers on the spot, a 

 living model of the structure was arranged as follows : — Two 

 men sitting on chairs e.xtended their arms and supported them 

 by grasping sticks butting against the chairs. This represented 

 the two double cantilevers. The central girder was represented 

 by a short stick slung from one arm of each man, and the anchor- 

 ages by ropes extending from the other arms to a couple of piles 

 of brick. When stresses were brought on this system by a load 

 on the central girder, the men's arms and the anchorage ropes 

 came into tension, and the sticks and chair-legs into compression. 

 In the Forth Bridge they had to imagine the chairs placed a third 

 of a mile apart and the men's heads to be 360 feet above the 

 ground. Their arms were represented by huge steel lattice 

 members, and the sticks or props by steel tubes 12 feet in 

 diameter and i j inches thick. There were three main piers at 

 the Forth, known respectively as the Fife pier, the Inch Garvie 

 pier, and the Oueensferry pier, and upon each of these there 

 were built huge cantilevers stretching both ways. The Fife pier 

 stood between high and low-water mark, and was separated by a 

 span of 1,700 feet from the Inch Garvie pier, which was partly 

 founded upon a rocky island in raid-stream. Another span of 

 1,700 feet carried the bridge to the Oueensferry pier, which was 

 at the edge of the deep channel. The total length of the viaduct 

 was about I5 miles, and this included two spans of 1,700 feet, 

 two of 675 feet, being the shoreward ends of the cantilevers, and 

 fifteen of 168 feet. Including piers, there was thus almost 

 exactly one mile covered by the great cantilever spans and 

 another half mile of viaduct approach. The clear headway 

 under the centre of the bridge was 152 feet at high water, and 

 the highest point of the bridge was 360 feet above the same 

 datum. 



The lecturer concluded by describing in detail the design, 

 manufacture, and erection of the superstructure, and he exhibited 

 a fine collection of photographs taken from the work in progress. 



THE SOCIETY OF ARTS. 



THE CHEMISTRY OF PUTREFACTION. 



IN the third of the Cantor lectures on the above subject, by 

 Mr. J. M. Thomson, the special subject discussed was a 

 consideration of the more important circumstances, and sub- 

 stances which retard or prevent putrefaction. The exclusion 

 of air, dryness, freezing temperature, and boiling heat, are those 

 physical conditions which retard or prevent the development 

 of putrefaction. In the last case, it is necessary with certain 

 germs that the heat be continued for a long period or repeatedly 



applied before they are completely stopped from causing decom- 

 position. Substances which act as antiseptic agents, may do 

 so in three ways, as germicides, as disinfectants, and as 

 deodorisers. These antiseptics act by abstracting water from 

 the fermentable substance, by forming with it a compound less 

 liable to putrefaction, by decomposing the ferment, by depriving 

 the surrounding air and the ferment of the necessary oxygen, 

 and, lastly, by killing the fungi and their germs. Of such 

 substances, that which is of most frequent use is cliarcoal, either 

 wood or animal. It acts by absorbing the gases evolved in the 

 decay, and for this purpose should have been freshly ignited in 

 order that any gases which it has previously absorbed may be 

 first expelled. Ammonia is the gas which is absorbed by the 

 pores of the charcoal better than others, though it will also take 

 up gases which are easily converted into the liquid condition, 

 and such are the most injurious to health, e.g., sulphuretted 

 hydrogen and sulphurous acid. Animal charcoal contains a 

 large amount of mineral matter, and is preferable to vegetable 

 charcoal when a liquid has to be deodorised. The absorbent 

 action of charcoal ceases after a certain time, so that the 

 material requires to be renewed. With sulphuretted hydrogen, 

 besides absorption, the charcoal also effects decomposition of the 

 gas into sulphur and water. Filters containing charcoal and 

 spongy iron are liable, after prolonged action, to become clogged, 

 and should, therefore, be purified or revivified by exposure to 

 heat, or by renewal of the filtering substance. Chlorine is the 

 most important chemical disinfectant and germicide. It acts by 

 supplying oxygen indirectly to the substance, in an analogous 

 way to that in which it bleaches. It, therefore, acts best in the 

 presence of moisture. 



IN the fourth and last of the Cantor lectures, Mr. Thomson said 

 that in addition to the disinfectants alluded to in the last 

 lecture, nitrous fumes have been used for this purpose. They 

 are the general result of the decomposition of nitrites and 

 nitrates, and act as oxidising agents. The heated vapour of the 

 strongest nitric acid is capable of destroying organic matter, and 

 sets fire to such substances as horsehair. Charcoal, iodine, and 

 phosphorus also undergo oxidation in nitric acid very energeti- 

 cally. Other strong mineral acids, e.g., sulphuric acid, act as 

 dehydrating agents, and thus bring about the decomposition of 

 organic substances, rich in the elements of water, by combin- 

 ing with the water they contain. Sulphuric acid has an anti- 

 septic action on substances, by dissolving them. Metallic salts 

 are important as antiseptics and disinfectants. Bleaching 

 powder solution also acts as a deodoriser and decolouriser. 

 Condy's fluid or potassium permanganate is another body which 

 is rich in oxygen, and the green manganate can be readily 

 obtained by fusing manganese dioxide with solid potash in a 

 silver dish. The green manganate can be converted into the 

 purple permanganate by the addition of any oxidising body, and 

 it is then in its most active condition. That permanganate does 

 yield up its oxygen in this way can be shown by its violent action 

 upon glycerine, which takes fire when brought into contact with 

 the solid potassium salt. Chloride of iron and chloride of zinc 

 (Burnett's) are also used as disinfectants. Arsenious acid and 

 boracic acid (boro-glyceride) are used as preservatives. Boracic 

 acid is an extremely good preservative, and is worthy of more 

 general application. Amongst organic substances, carbolic acid 

 is now used to a considerable extent, and is recognised by the 

 compound it produces with bromine, and by the purple colour it 

 gives with a solution of ferric chloride. Lastly, the lecturer 

 recommended a solution of two pounds of ferrous sulphate in a 

 gallon of water, or one of four fluid ounces of carbolic acid to 

 one gallon of water, as the safest materials to be used for house- 

 hold purposes. All materials of no value after disease or sick- 

 ness, should be burnt, and rooms well ventilated and disinfected 

 by sulphurous acid, after the room is vacated ; the sulphurous 

 acid being obtained by heating roll sulphur, or by the combus- 

 tion of carbon bisulphide. Cleanliness and ventilation are the 

 factors which should be attended to for the prevention and 

 removal of putrefaction and decay. 



THE INSTITUTION OF CIVIL ENGINEERS. 



ACCIDENTS IN MINES. 



T the concluding ordinary meeting of thesession, the paperread 

 . ... was by Sir Frederick Abel, C.B., F.R.S. He said that since 

 835 a succession of Royal Commissions and of Parliamentary 



A 



