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with asymptotic branches extending onwards from the diffracting edge. Prof. Stevelly conceived the dark limb of the moon to be such a diffracting edge to the slender beam of light which reached us from a fixed star; and that as the curve was at the last moment the light was allowed to pass convex towards the moon, the portion of the ray which last entered our eye before the star disappeared, being the direction in which we should then see the star, if produced backwards, would meet the moon on her dark surface.

Sir D. Brewster said that if two observers were placed near one another, one will see the phenomenon and another will not. Besides

, if it arose from the cause supposed by Prof. Stevelly, it should be observed when the edge of a distant spire or other terrestrial object appeared to pass over a star, which he was not aware had ever been noticed. In his opinion, the cause of it was the light of the star pass. ing occasionally through small spots in the atmosphere, which differed from the surrounding portions, producing an effect on the image of the star something like mirage. ---Prof. Challis observed, that if so, the edge of the moon would be rendered discontinuous at that partSir D. Brewster replied, that the new property of the retina which he had described yesterday, supplied an answer to that objection; for it appeared that when two parts of a luminous line were disconnected, the retina filled up the chasm, and rendered the line continuous.—Sir W. Hamilton said he considered it rather favorable to Sir D. Bretster's view, that in some states of the atmosphere, he had obserred the edge of the moon notched, particularly when she was near the horizon.

ON THE CHEMICAL CHANGES OCCURRING IN IRON FURNACES, by Dr. Lyon Playfair, and Prof. Busen.—This report went very extensively into the various methods adopted by the authors to insure an accurate determination of all the gaseous products of the hot-blast iron furnaces. It was found that coking was effected in the furnace 10 the depth of 24 feet. That the distillation of coa reached its maximum at the depth of 14 feet—That the formation of tar took place at between 17 and 14 feet.

Hence the coal had to travel 24 feet from the mouth of the body of the furnace to the boshes, to be entirely coked. A great diminution of oxygen is found to occur at those points where the gases become developed

, and hence they pass away without undergoing combustion and it has been estimated that 91 per cent of the heating material in the form of gaseous products are lost in the hot-blast furnaces. The combustible gases driven off from the furnaces were expelled with a forte superior to that used in driving coal gas through the mains for the purpose of lighting towns. These matters having been thorouges examined—and all the gaseous product submitted to analysis, man! of the results being of a very curious character—the authors suggest the propriety of building a canal just above the point at which the gases are given off, for the purpose of conveying these products 10 other parts where their high heating and illuminating powers may be employed advantageously. These gases in combustion, with a due

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supply of oxygen, would give a temperature higher than is necessary for smelting iron; and although the authors do not propose that it should be used for that purpose, they suggest the advantage of employing that waste material for heating steam apparatus and many manufacturing processes.

ON THE MANUFACTURE OF A COLORED Glass, by M. Splittgerber. Specimens of glass were exhibited, into the composition of which gold entered as a chloride. These specimens were white, but upon gently heating them in the flame of a spirit lamp they became a deep red, transmitting the red rays of light only. If again the same reddened glass is exposed to the heat of an oxygen blow-pipe it loses nearly all its color, a slight pinkness only remaining. M. Splittgerber considers these results to arise from the oxidation of the chloride of gold in the siliceous compound.

ON THE VENTILATION OF COAL MINES, by Prof. Ansted.-Prof. Ansted gave an outline of the methods of working coal mines, and then proceeded to narrate the particulars of certain accidents, the circumstances of which seemed capable of suggesting improvements in future; and he referred more particularly to the fact, that, in the Haswell explosion, a considerable number of the persons employed in other panels might have been saved if the communication between them had been more effectually shut off, and if, also, there had been a separate passage or air-drift to the upcast shaft. He also described the explosion in the Killingworth Colliery, where, if the Davy lamp had been in use, the accident would, in all probability, not have happened; and Prof. Ansted urged the necessity of certain conditions being observed in all collieries. 1. That there never should be less than two shafts, because an explosion can scarcely happen without destroying the partition at the bottom of the shaft, and thus checking or entirely stopping the ventilation. 2. That the panels should always be of moderate size, and the air-courses never exceed a certain length. 3. That separate air-drifts should always be made from distant workings direct to the pit bottom. 4. The exclusive use of the Safety Lamp in those mines, at least, where the escape of gas from fresh workings, faults, &c., rendered the working at all dangerous.

Prof. Faraday said, the subject of mining accidents had long occupied his attention. The more he pursued the inquiry, the more he was disheartened at the apparent hopelessness of finding out any good general remedy. The explosions were not simply the effects arising from the mixture of gases, but from the combustion of the coal-dust and coal-gas which the first explosion made. In the fatal case at Haswell, the place where the accident originated had been ascertained; and the progress of the fire could be traced on the scorched beams and props of the galleries, and the deposits of coke made from the coal-dust which the explosion raised. To this circumstance the great force of the explosion was due, and not to the first escape of gas. A similar explosion had been known to take place in a cotton-wadding manufactory, the whole atmosphere of the place being fired by means of the particles of cotton in it. Of all the workmen killed in the Haswell accident, perhaps not one was really burned to death

, but suffocated by the choke-damp. In one part of the workings the explosion had produced sharp vibrations, like the firing of gunporder, and in another the burning went on slowly, like a common fire. But although two panels were blown into one, and solid stoppings of brickwork thrown down, there was no indication of the accident in the shaft. If the stoppings had not been blown down, and the supp! of air had continued, the mine would have taken fire, and the mea been burnt instead of choked. Since the late investigation, Mr. Faraday and Mr. Lyell have had many hundred plans submitted to them

, urging ill-considered and contradictory measures. They had enamined every part of the Haswell Colliery, accompanied by the mileviewer, and received recommendations from the best-informed men upon the spot; and they were convinced that the conditions under which the accident happened were so variable, that no general pracical rule could be obtained. Far more information, however, was required. The plan of splitting the air-courses was good, as far as the power of the upcast shaft admitted ; but, if carried too far, it woail produce stagnant points, which could not be prevented by any as rangement consistently with the ever-moving condition of the works The abolition of the use of gunpowder and lighted candles would

, o some cases, double the price of coals. But the great source of danger was the mental condition of the miners. With regard to the preza race this was so hopeless, that nothing could be done for them; athough smoking was strictly forbidden, they had been known 10 contrive to light their pipes in dangerous workings even from the Davy lamp; and Mr. Faraday had himself on one occasion sat dora with an open candle to watch the preparations for blasting, and when he inquired for the gunpowder was told he was sitting on it. N: Faraday took an opportunity, also, of expressing his firm convicta of the safety of the Davy lamp when properly used, and of its being a complete and practical contrivance, to which he would wiling! trust his own life, as he had already done on many occasions.

ON THE INFLUENCE OF GALVANIC ELECTRICITY ON THE GERNISATION OF SEEDs, by Prof. E. Solly.--In a series of experiments in which the seeds of barley, wheat, rye, turnips and radish were er posed to the influence of a feeble current of electricity, the plazas came up sooner and were healthier than others that had not been electrified. On the other hand, a number of experiments on other seeds had given opposite results,-proving, either that the germina tion of some seeds was retarded whilst that of others was facitatea by electricity, or that the effects observed in both cases were actor dental. Out of a series of 55 experiments on different seeds, a appeared in favor of electricity, 10 against it, and 25 showed no etico whatever; and in carefully counting the whole number of seeds up in the entire series, there were found 1,250 of the electrified, and 1,38 of the non-electrified. In conclusion, Prof. Solly stated that he fel doubtful whether the effects observed were really due to the initence of electricity.

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Description of the Great Britain Iron Steam Ship, with Screw Propeller; with an Account of the Trial Voyages. By Thomas RichARD GUPPY, Esq., C. E.

(From the Proceedings of the Institution of Civil Engineers.] The Great Western Steam Ship Company originated with a few directors and proprietors in the Great Western Railway Company, who entertained the idea, that, on the completion of the railway from London to Bristol, a direct line of communication, by means of steamboats, to New York, as the focal point of the New World, might be established with advantage.

Hitherto, attention had been directed to the south-western harbors of Ireland, and the nearest ports in America, as the extreme distance between which steamboats of the greatest power then supposed to be practicable, would be enabled to carry a sufficient quantity of coal for the voyage; but this company, placing confidence in the opinion of Mr. I. K. Brunel (their engineer,) ventured to build the Great Western, a steamer exceeding in size any that had previously been constructed, and with engines of so much greater power, that the predictions of many experienced and scientific men were unfavorable to the project.

The Great Western did, however, fulfil the expectations entertained of her by her projectors, in all respects, except in that, like many other moderate-sized steam vessels, so large a part was occupied by the inachinery, relatively to that which could be appropriated to pasVol. X. 3RD SERIES.No. 5.- NOVEMBER, 1845.


sengers and goods, the deficiency of space was soon found to operate disadvantageously in a pecuniary point of view.

At first it was intended that their second ship should be of timber, but the superior advantage which the introduction of iron appeared to hold out, induced a very careful comparison, and an investigation into the state of some small steam vessels already constructed of this material, and the result was the abandonment of the previous resolution.

As no example of an iron steam-ship of sufficient size existed, on which to base any calculation of the thickness of the iron to be employed in its construction, or of the disposition of the material, in order to obtain the greatest relative degree of strength, much consideration was requisite, and it became necessary to organize an establishment for building iron instead of wooden ships, before the keel of the ner vessel was laid. The principal dimensions of the hull of the Great Britain, are

Ft. l. Length of keel,

289 0 Length aloft,

329 0 Main breadth,

50 6 Depth of hold,

32 6 The tonnage, according to the usual mode of builders' measurement, is therefore, 3,444 tons.

The weight of iron used in the hull is about 1040 tons; which is equal to an average thickness of 24 inches.

The weight of the wood-work in the decks, fittings, &c., is about 370 tons.

And the weight of the engines and boilers (exclusive of the water) is 520 tons.

The total weight, therefore, is 1,930 tons; which, at a draft of water of 10 feet 6 inches forward, and 13 feet 7 inches aft, corresponds exactly with the calculation of the displacement of the hull

, which is as follows:

Drall Fore Body. (After Body. Total.



Tons. Tons. Tong.
1053 851

14 1315 1099 2414
16 1594 1386 2980

18 1904 1714 3618 She will therefore be able to take 1000 tons of coal, and 1000 tons of measurement goods, weighing perhaps 400 tons, at a draft of 11 feet forward, and 17 feet 6 inches aft.

The keel plate consists of plates žths of an inch in thickness, by 20 inches wide, which were welded into lengths of 50 feet to 60 feet, and these lengths were joined together, by very accurately made scarpus 1 foot 6 inches in length, and riveted all over, at distances of 41 inches apart.

The end pieces of the keel, which are more liable to touch the ground, are full i inch in thickness.

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