1848.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



221 



person within sight of it is more or less aft'ected by its influence, and a 

 condition of electricty is induced in that part of their bodies towards the 

 machine of a different kind from that developed by the conductor; and on 

 the opposite side, or that farthest from the machine, electricity of the con- 

 trary kind is induced. This induction of positive and negative electricity 

 on distant bodies, leads, as Professor Faraday observed, to important 

 practical consequences. He had, he said, been often consulted by Govern- 

 ment as to the propriety of having metal roofs on the powder-mills at Walt- 

 ham Abbej, and he had always objected to them as dangerous, because a 

 thunder-cloud might induce in the extended metal surface, an amount of 

 electricity capable of discharging itself to the earth. In illustration of 

 this, a large insulated metal ball, placed at a distance of two feet from 

 the conductor of the machine, was brought near a jet of gas, which became 

 ignited by the induced electricity passing off in a spark to the metal gas- 

 pipe, though the ball was far too distant from the conductor for any spark 

 to pass between them. This experiment was repeated several times, and 

 each time with the same success. 



The chief point to which Professor Faraday directed attention in 

 the seventh and concluding lecture was, the cause of the difference 

 between the phenomena of voltaic and of frictional elecricity, his object 

 being to prove that they are really identical. The marked differ- 

 ence between frictional and voltaic electricity, which were^dwelt upon 

 so much in the preceding lecture, are caused entirely by *the different 

 degrees of intensity in which the force is developed, and Prolessor 

 Faraday showed, in the concluding lecture, that, by diminishing the 

 intensity of frictional electricity, the phenomena may be rendered 

 similar; the g-eat difficulty in showing these effects being caused by the 

 very small quantity of electricity that can be evolved in a given time even 

 by the most powerful electrical machine when compared with the amount 

 evolved by the voltaic battery. Though the machine employed was a 

 plate of glass, about four feet in diameter, which yielded a rapid succes- 

 sion of strong sparks five inches long, it would require about five million 

 turns of the plate to produce a quantity of electricity equal to that evolved 

 by a grain of water in the voltaic battery. The different appearances of the 

 sparks emitted in the highest state of intensity by the electrical machine 

 from those produced by the discharge of electricity accumulated in the 

 Leyden jar, and the alterations the light and the length of the sparks may 

 undergo by being transmitted through various media, were shown in nu- 

 merous experiments. Though the rapidity of the electric spark is evi- 

 dently very great, it far exceeds, in reality, the appearance to the eye, for the 

 duration of the impression on the retina after the light is extinct occasions 

 a prolongation of the effect. It has been ascertained by Mr. Wheatstone, 

 that the duration of the light of the spark is less than the millionth part of 

 a second, and Professor Faraday exhibited the mode by which this fact 

 had been established. A concave mirror, placed horizontally, was made 

 to revolve with great rapidity by multiplying-wheels, and when in action 

 a bright light from the combustion of lime was reflected to a focus on the 

 ceiling. The rapidity of the motion caused the light to form a circle, in 

 the same manner as the turning rapidly round of a lighted stick or of any 

 other bright object seems to form a circle, in consequence of the impression 

 on the retina remaining until the effect is renewed by the return of the 

 light to its former place. When an electric spark was substituted for the 

 permanent light, each spark was seen separately, and no circle or prolon- 

 gation of the light was produced. The velocity of the mirror and the 

 number of successive sparks being known, an approximation can 

 be obtained to the duration of the light. It is in consequence of 

 this instantaneous duration of electrical discharges that they fail to 

 produce many of the effects of voltaic electricity, and if the continuous 

 action of the latter during a second could be concentrated one million times 

 its effects would be tremendous. By diminishing the intensity of frictional 

 electricity whilst retaining its quantity. Professor Faraday ignited gun- 

 powder, which was blown away without ignition by the undiluted dis- 

 charge. The following experiment afforded a good illustration of the 

 different actions of frictional and voltaic electricities caused by the concen- 

 tration of force in the former. A gold thread twisted with silk was 

 deflagrated by a discharge from an electrical battery without injuring the 

 silk, the action having been so instantaneous that there was not time to 

 burn the silk, thougli the metal was destroyed by the heat evolved. 

 When similar gold thread was exposed lo the action of the voltaic battery, 

 the silk was instantly consumed by the wire being made red hot, whilst 

 the metal remained. The statical character of frictional electricity. Pro- 

 fessor Faraday said, may be rendered current by applying a conducting 

 substance to draw it off from the machine as quickly as it is excited, and 

 the imperceptible effects of such a current prove how small the quantity 

 of electricity excited really is, and it is only by allowing it to accumulate 

 that we become sensible of its presence. The phenomena of lightning and 

 thunder are owing to the facility with which Franklinic electricity can be 

 accumulated, and thus reserved in store for an instantaneous discharge. 

 Some specimens of the effects of lightning were exhibited on the lecture- 

 table. A number of splinters from a riven oak, a branch from a mulberry 

 tree, the rent and shivered handle of a hay-fork, and the partially-melted 

 iron cable of a ship were displayed. The latter is such an extraordinary 

 exhibition of electrical power, that Professor Faraday said nothing but 

 the strongest evidence could have induced him to believe it ; the ship was 

 stated to have been struck with lightning during an earthquake at Callao. 

 This rending power of frictional electricity cannot be imitated by the 

 voltaic battery, but all other phenomena of the one kind can be produced 

 by the other. The decomposition of chemical compounds by the discharge 



of the Leyden jar was shown by the decomposition of iodide of potassium, 

 small indeed in effect, but corresponding with the quantity of electricity 

 which the electrical machine evolves. The alliance of the phenomena of 

 the two electricities had been shown in the course of these lectures, 

 Professor Faraday observed, by their physical effects in communicating 

 shocks, by tlie equal rapidity of their transmission, by their decomposing 

 and heating powers, and by the communication of magnetism. The dif- 

 ference between the two consists solely in the degree of intensity, the elec 

 tricity of the machine exciting a small quantity in a high state of intensity, 

 whilst the voltaic battery evolves a much larger quantity in a low state of 

 tension. Professor Faraday, taking a small flock of gun-cotton and ex- 

 ploding it in the flame of a candle, observed that the chemical force thus 

 instantaneously called into action was equal to the production of an 

 amount of electricity greater than would be contained in .500,000 charges 

 of the powerful battery of Leyden jars which he had employed to defla- 

 grate metal wires and gold leaf; and the important problem now remaining 

 to be solved was, the conversion of such rapid chemical actions into current 

 forces. Chemical decomposition, he said, when taking place less ener- 

 getically, had been shown to evolve electricity, which became manifest 

 and available as a current force by the voltaic battery, and it was quite 

 within the reach of scientific discovery to render the most energetic phe- 

 nomena of chemical force sources of continuous power. 



PROCEEDINGS OF SCIENTIFIC SOCIETIES. 



ROYAL INSTITUTE OF BRITISH ARCHITECTS. 



Mai/ 29. — Ambrose Poynter, Esq., V.P., in the Chair. 

 A paper was read " On the AppUcalion of Sculpture and Sculptured Or- 

 nament to Architecture." By H. B. Garling, Associate. Being the Essay 

 to which the Medal of the Institute was awarded on 21st February, 1848; 

 and which is given in full in our present Number (p. 201). 



June 12. — Sidney Smyrke, Esq., V.P., in the Chair. 



A paper was read " On the Theatres and Porticoes of Ancient Rome." By 

 the Rev. Richard Burgess, B.D. 



June 26. — A paper was read •" On the Triforium of the MeditBval 

 Churches." By the Rev. R. Willis, M.A. 



This evening's meeting closed the session. 



Next month we will endeavour to give an abstract of both the above 

 papers. 



INSTITUTION OF CIVIL ENGINEERS. 



May 30.— Sir. Field, the President's Conversazione. 



The accustomed annual conversazione of the Piesident, which was hereto- 

 fore held at the residence of the President, was this year held at the House 

 of the Institution in Great George- street, a change we do not approve of; as 

 also the one limiting the conversazione to one evening, which led to the 

 rooms being most inconveniently crowded — so much so, that many of the 

 numerous models and works of art could not be seen. 



On the walls of the theatre we observed three faithful portraits, by Lucas, 

 of Mr. George Stephenson, Mr. Robert Stephenson, M.P.. and Mr. Bidder; 

 they were contributed by Messrs. Graves, for whom they are about being 

 engraved. The Electric Telegraph Company— Mr. Bain, Mr. Brett, and Mr. 

 Reid — contributed instruments exhibiting their various systems of tele- 

 graphic communication, electric clocks, and electric printing. Mr. Rand's 

 fly press, for raising at one blow the collapsible tubes or capsules. Mr. 

 Whitworth's beautiful machine for knitting stockings was worked by a young 

 girl sent up from the factory of Messrs. Ransorae and May, of Ipswich, and 

 formed an attractive object.' The models of greater interest were those of 

 Mr. Stephenson's wrought-iron tubular bridge erected at Conway ; with that 

 of his wrought-iron tubular girders for large spans. Mr. Fowler's steamboat 

 floating landing stage, with its long approach over the mud banks, all to be 

 supported upon Mitchell's screw piles, for the Humber Ferry. Mr. Brunei's 

 excellent truss, of 110 feet span, used by him in the Somerset Bridge, on the 

 Bristol and Exeter Railway, ilr. Fowler's plan for opening or shutting 

 simultaneously four gates for a level crossing on the line of the Manchester, 

 Sheffield, and" Lincolnshire Railway. Messrs. Taylor, Williams, and Jordan's 

 model of their machine for carving by machinery ornamental objects, figures 

 or groups, such as we have previously noticed. This machine, by means of 

 a tracer which guides the cutting tools, is enabled to perform the most deli- 

 cate and elaborate work with great speed and at a cheap rate. Messrs. Sea- 

 ward and Capel contributed a large collection of models of paddle-wheels 

 and screw-propellers. Messrs. JIaudslay and Field also contributed an in- 

 teresting series of models of steam-engines, screw-propellers, and paddle- 

 wheels I as did also Mr. Penn, of his horizontal trunk steam-engine. Mr. 

 Clarke exhibited a beautiful model of the Great Britain steamer, full-rigged, 

 and containing/ffc-simiYes of the engines, with the screw-propeller complete, 

 and working by means of condensed air, the whole only weighing 1 o* 



