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The conditions were approximately taken from practice, without reference to this point, and the circumstance was not noticed at first.

The value of the cylinder vacuum, and also the value of the friction air-pump power absorbed, must be estimated, to obtain the net horse power, according to each person's views; and it is as easily managed with the total steam pressure expressed as horse power, as with the pressure in lbs. per square inch. Taking the condenser at 4 lbs., and the mean difference between it and the cylinder at

=

coals per day. The work done by the boilers of the Great Western, in the evaporation of water, is just 10 to 1; but the effect of the steam on the piston is only as 5 to 1. The quantity of water consumed at each stroke of the cylinder is not quite exactly the same in both engines, as 5 has been substituted for 493 of a cubic foot per minute, in the Cornish engine.

7 lbs. of water per stroke.

1 lbs. we shall have 24 lbs. resistance on the under side of the pistons of the Great Western's engines, and consequently 4th part of 800 horses power, or 133 horses power resistance against the piston. The allowance is less in the Cornish engine, on account of the pause between the strokes. The friction allowances must be made at pleasure. I conceive the mean power in crossing the Atlantic will not reach 600 horses power. I remain your obedient servant,

S.

ON THE CAUSE OF EXPLOSION IN STEAM-BOILERS, AND THE REMEDY. BY MR. WILLIAM SAMUEL HENSON.

[We have in a former Number (493,) described an improved mode of working steam expansively, which forms the most important of " Certain Improvements in the Steam-engine," lately patented by Mr. Henson. Another of Mr. H.'s improvements consists in the application of a governor to the safety-valve of steamengine boilers, by which the safety-valve is raised when the engine is at rest, and the danger of explosion from the sudden stoppage of ebullition in the boiler thereby prevented. The present paper ex. plains Mr. H.'s views in this improvement; they are ingenious, shrewd, and original, and well deserving of attention. -ED. M. M.]

I find by the Government Report on steam-vessel accidents, published in 1839, that, out of twenty-three explosions, nineteen occurred whilst the vessels were on the instant of starting, or were stationary; three whilst steaming; and the time when the remaining one took place was not ascertained. In two instances only was it proved that steam was blowing through

the safety-valves at the time of the explosion, showing the valves to have had an insufficient area, being only from onefourth to one-fifth of a square inch to each horse power, instead of one square inch, as recommended by the most eminent engineers. In the other seventeen cases of the nineteen, the ebullition had not been continued in the boilers while the engines stopped.

The greatest number of boilers have ruptured below the water-line, caused apparently by some sudden action under water. The most violent explosions have generally taken place just at the instant of setting the engines in motion after standing quiet some time with no steam escaping, and consequently no ebullition. These explosions have generally been attributed to the lowness of the water in the boiler, and the exposed parts getting red hot, whereby, when the water is agitated by the engine being set on, or by the safety-valve being suddenly opened, or even by the oscillating of the vessel, a thin sheet of water has washed over the

ON THE CAUSE OF EXPLOSION IN STEAM-BOILERS, AND THE REMEDY.

red-hot parts, causing, as suggested, the sudden formation of such an immense volume of steam, that no safety-valves could relieve the boiler in time to save it. But if ebullition had been continued when the engine stopped, this cause of explosion could not have arisen, as the ebullition would most probably have prevented the boiler getting red hot, at least those parts near the water. The water has been known to get very low while the engines were at work, without any accident happening, yet the same boilers have exploded whilst the engines were stopping, with every reason to suppose there was plenty of water, and the safety-valves not overloaded. In several instances it has been proved that a sufficiency of water has been in the boilers at the time of the explosions, and the vessels have performed in safety regular voyages across seas which required good seaworthy vessels and strong boilers, yet these boilers have exploded whilst preparing to start, or on the instant of starting from the quays or ports where they have been stopping, and weakness and insufficiency of stays has been attributed as the cause.

Many persons have contended that the extreme violence of some explosions is caused by the over-heated parts of the boilers decomposing the steam and generating a highly explosive mixture of gases. It is true that red-hot iron will decompose steam, but in doing this the oxygen combines with the iron, and the hydrogen alone is set free, which is not explosive by itself. In no instance, I believe, have the two gases been proved to be produced under these circumstances.

From various incidents I have been led to believe that there may be another cause of explosion which has hitherto escaped observation. I will endeavour to explain it as briefly as possible. It is well known that water boils, under the ordinary pressure of the atmosphere, at 212° Fahr., and that it takes about five times as long to convert a given quantity of water into steam at 212° as it does to raise the water from the ordinary temperature to the boiling point. It follows that this steam contains about five times the quantity of caloric to its equivalent in water, or, in other words, that the steam contains five times as much heat as it contained when in the state of water at 212°; but the additional heat is not sensible to the thermometer, because it is expanded

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throughout a greater space. Therefore, as every particle of water requires a much higher temperature than 212° before it can expand into steam, it appears that if heat could be communicated equally to every particle of water, and the water kept perfectly still at the same time, the water would attain a much higher temperature than 212° Fahr, before the whole of it flashed into steam. This I am aware is not easy to accomplish, on account of water at rest not being so good a conductor of heat as when in motion; and those parts which are hottest, being lighter than the other parts, will rise to the surface and disturb the stillness. This tendency of the most heated parts to rise to the surface causes a number of currents to move in various directions, and these currents appear to assist materially in the formation of steam, by enabling certain portions of water to concentrate sufficient heat in themselves, from the surrounding portions, to form steam.

I have found by experiment that water kept very still, and heat communicated gradually, it did not boil, although considerably above the boiling point; but upon agitating the water a little, even when the fire was removed, a portion of it instantly flashed into steam, driving some of the water with considerable violence against the upper side of the vessel, and a very brisk ebullition continued for a space of about a minute afterwards, until the temperature of the water was reduced to the boiling point. This experiment was tried at a low temperature, with a close vessel, from which the atmospheric air was excluded. The upper part of the vessel was kept at a low temperature (about 60° Fahr.) and the lower part heated very gradually by interposing dry sand between the fire and the vessel. I have by this means heated the water something more than 100°Fahr. above the boiling point. As it is very probable that the effect above described is produced equally at high temperatures, I think its violence is quite sufficient to account for some of the phenomena of steam-boiler explosions. Again, if a bottle containing a little cold water, or almost any other liquid, be corked lightly, and then shaken well, there will be sufficient vapour formed by the agitation of the water, and the escape of the gases contained therein, to blow out the cork. But to produce a still greater effect, put

a little water into a deep bottle and cork it up, leaving a small aperture open to the atmosphere, and then boil the water by means of a spirit-lamp; when the steam has heated the whole of the bottle, and escapes freely from the aperture, remove the bottle from the lamp; and when the steam has ceased to blow out, and the ebullition stopped, turn the bottle on one side, or give it a good shake, when a considerable volume of steam will instantly be generated, which will blow out the cork. This experiment shows the necessity of having a large surface of water for the steam to escape from in a steam-boiler, and the danger of allowing the water to remain quiet. I will also observe, that with a sufficiency of water in a boiler, and good safety valves, not overloaded, there is less danger with a brisk fire than with a slow one, as the former would continue the ebullition while the engine was stopping, by generating sufficient steam to force open the safety valves, thereby preventing the formation of great quantities of

steam.

The sudden commencement of ebullition has also a tendency to strain parts of a boiler by the contraction of the iron arising from the cooling effect peculiar to evaporation at all temperatures. The well-known experiment of taking a vessel containing water boiling hard from the fire, and resting it upon the hand without pain, though it cannot be borne for a moment after the ebullition has quite ceased, is sufficient to prove this fact. The Americans appear to be well aware of the danger to their steam-engine boilers of stopping, without knowing whence the danger arises; but by disconnecting the paddle-wheels from the engine, they are enabled to stop the vessel without stopping the engine, though in this case a fly-wheel is necessary, or the engine would not work at all. But it is not requisite to continue the engine at work if a certain quantity of steam be allowed to escape; the effect in the boiler will be exactly the same as if the engine was at work, and water may be very readily supplied to the boiler without the assistance of the engine.

It is generally believed that explosions have taken place when there has been a sufficiency of water in the boilers at the moment of opening the safety valves suddenly, or of setting the engines in motion. I have already shown what may be the

effects of the slow communication of caloric to the particles of water. I will now point out how the conditions necessary for that purpose are fulfilled in the generality of steam-boat boilers. The Hues which contain the fire-grate, and conduct the heated air through the body of water, pass longitudinally through the boiler. The greater part of the heat is absorbed by the water on the top and sides of these flues, but still a considerable portion is absorbed by the lower side. When the engine is at rest, and no ebullition going on, that portion of water situated just under the flue, in consequence of being heated on the upper surface, absorbs the heat very gradually, without causing motion amongst its particles, because those portions on the upper side nearest the bottom being lighter on account of being hotter than the portions immediately underneath, have no tendency to cause those currents in the water which appear to assist so much in causing ebullition. Thus that portion of water directly under the flue becomes heated very considerably above the boiling point, and when any thing occurs, as the starting of the engine, &c., to cause agitation or vibration, a great body of steam is instantly formed, which impinges against the under side of the flue, and the bottom of the boiler. The water is by this means driven for a moment against the top of the boiler, choking up the safety-valves, and by the great agitation into which it is suddenly thrown, causing every part to give out an additional quantity of steam, whereby the under side of the flue, if not very strong, will probably be collapsed; and it is a fact that in most cases of collapse the flues have ruptured on the under side. The violent force with which the greater part of the water may be thrown against the upper surface of the boiler by this means, may account in some measure for the singular but well-known phenomenon of an entire boiler being lifted from its seat, and the great additional volume of steam which is given out by boiling water when violently agitated, may explain the fact of its bursting in the air.

From these experiments and investigations I have been led to form the opinion that if the ebullition in a boiler can be constantly kept up, explosion is not likely to happen; and to continue the ebullition, therefore, while an engine is

PROGRESS OF FOREIGN SCIENCE.

stationary, I have introduced the improvement above described, into the boilers of steam-engines.

PROGRESS OF FOREIGN SCIENCE.

[In continuation from page 203.] The Gases evolved in Blast Furnaces. On the 17th of January last, M. Ebelmen read a memoir to the Academy on his method of employing usefully the gases given off by iron furnaces when in blast, and on the constitution of these gases.

The chemical reader is probably aware that a good while ago M. Bunsen, of Marbourg, whose laborious, dangerous, and beautiful researches on alkarsin and its compounds, have made his name celebrated, conducted also a long course of experiments upon the gases evolved in blast furnaces.

The results both of his, and of Ebelmen's researches, though of considerable chemical interest, do not seem to have thrown much additional light upon the metallurgy of iron. The value of the economic applications of the combustible gases given off, as proposed by the latter, has yet to be proved.

Electro-chemical properties of Gold. M. Becquerel has commenced reading to the Academy of Sciences of Paris, the first of a series of memoirs on "The Electro-chemical properties of the simple bodies, and on their applications in the arts." The first memoir is On Gold-it is of great length, and treats minutely of several of the most important operations in the metallurgy of this metal, methods of gilding, &c. When this series of memoirs shall be complete, a translation published in a cheap form would be a most useful addition to our scientific literature.

The new French Telegraph

of M. Vilallougue.

This is the age of telegraphs and telegraphing. We have electric telegraphs for regaining our top-coats when left behind on the railways; and semaphores, to tell us the cream of the news as it comes across the Atlantic by steam; and we get the first of our news from India, whether good or bad, across France by telegraph.

The existing telegraphs in France consist of three arms, moveable in the same vertical plane; the principal arm, called

229

"the regulator," carries at each end a smaller arm, called "an indicator." The regulator, moving on an axis in the middle of its length, is either horizontal, vertical, or at 45° of inclination; each indicator turning on its extremity, is perpendicular, or at 45° to the regulator, and never takes six positions with reference to the latter. Recently in some of the government telegraphs, the regulator has been fixed horizontally, and the place of its four positions supplied by a separate bar, placed above, and moving like the beam of a balance; this upper bar the French call "mobile," for which it is not easy to find an English word.

M. Vilallougue's telegraph adopts the same principles of notation as this latter, but his mechanical arrangement is such as gives greater facility in working the machine, greater clearness in hazy weather, &c.; and enables the same instrument to answer as a day and night telegraph, with only the loss of two minutes time to change it from one to the other.

His telegraph tower is square, and painted black externally. On one of its faces it carries three large dials, like clock dials without figures, made of wood, or sheet iron, and moveable in a vertical plane round their respective centres. Each of these is about 9 feet in diameter. The two lower ones are placed side by side on the same level; the third is placed centrally and above them.

Below the two lower dials a bar of wood, painted white, is placed, behind which is an aperture of the same size into the interior of the tower. This bar is horizontal, and represents the fixed regulator of the present system. Each of the two lower dials has got a radius wide. The upper dial has got a diameter, painted white, upon it, of two decimetres painted white, upon it. Means are provided inside the tower for turning these dials on their centres in any way required, and by the respective positions of the diameter, with the two radii and the regulating bar, the signals are conveyed.

The opposite side of the tower carries a precisely similar set of dials, &c., whose axes are the same (i. c. on the same shafts) as the former, so that the signals are made on two faces at once; thus the watchman at the former, or last station, always sees what signals are making by the next telegraph to him to that beyond, by which he knows if his own signals

have been correctly seen and observed. So much for the day telegraph, which, in experiments made at Perpignan, was distinctly seen with telescopes magnifying from thirty to forty times, at 8000 metres distance, which is about the mean telegraphic distance. To convert this into a night telegraph, the white bars on the several dials, and the regulator bar, are movable, and in their place, when removed, is formed a band of a built lens, that is to say, a strip cut out of one of Fresnel's lenses, (the polyzonal lenses of Brewster,) by two planes, parallel and equidistant, from a diameter. The breadth of this slice of lens being equal to that of the white strip or band before spoken of, the interior of the tower is strongly illuminated by lamps like a lighthouse, or single attached lamps are placed in the focus of each band of lens, and the whole is now in a condition to work as a night telegraph.

The acknowledged difficulties of night telegraphs are thus much reduced, if not got rid of; and the whole instrument is worked free from the inconvenience of weather, &c.

It has also been found advantageous to substitute for the band of lens two simple glazed apertures at each end of the diameter in the upper dial, and at the extremities of the radii of the lower ones, and of the regulator. There appear to be several not inconsiderable advantages secured by this arrangement, which has been approved of by the Academy of Sciences, after having been reported on by a commission of its members.

New Method of Purifying Gas. M. Mallet has had in operation for some time, at the gas-works at St. Quentin, a new method of purifying gas, which was described to the Academy of Sciences in August last. The results are said to give a gas of the highest purity, free from naphthaline, which is what makes the chief part of the smoke that blackens our ceilings, in our own gas from coal; and equally free from various ammoniacal compounds, which give much of the detestable smell to coal gas when it escapes. The gas at St. Quentin, though candidly admitted by the inventor of this process of purification not to be absolutely without smell, has yet very little, and that scarcely, if at all, offensive. It would be very desirable if our own Gas Companies would adopt something of this

sort.

Photography.

M. Nothomb has addressed a note to the Academy of Sciences, stating that he has found it advantageous to substitute proto-chloride of mercury in place of running mercury, (quicksilver,) as proposed by Daguerre. The proto-chloride is the calomel of the Pharmacopoeia,

Dilatation of Elastic Fluids.

Most persons are aware, who follow the course of science, that the coefficient of dilatation of elastic fluids, which until a comparatively recent period had been assumed the same for every gas, and such that the dilatation was part of the volume for each degree of Fahrenheit's thermometer, has more recently been submitted to new researches by Regnault, Despretz, and others. M. Magnus is the latest experimenter in this field, and has not yet concluded his researches, which are of great value: he has, however, already ascertained that the coefficient of dilatation is not precisely the same for all gases, and that the difference does not arise from the easy condensibility of some, such as sulphurous acid, into liquids.

Nicotin.

The vegetable alkali of tobacco has been carefully prepared and analyzed, with experiments of controul by M. Barral: it is a colourless anhydrous fluid, which does not freeze at 10° cent.; it has a burning taste, and is volatile at 250° cent., and is a violent poison; a single drop placed on the tongue of a middlesized dog poisoned him in three minutes. It reacts alkaline. Its composition is CH Az.

20 16 2

Coal in France.

The coal formation of the basin of the Soane and Loire has lately been described in a memoir by M. Burat, which is of considerable interest. The coal in this formation is different from any yet known, as to its mode of deposition: it is not regularly in beds, but rather in vast masses, which surpass in thickness or depth any thing previously known, but are of no great horizontal extent. In some places the coal is confounded with the other matters of the formation.

Artesian Well of Grenelle.

The public excitement in Paris, as to the formidable consequences which will result to the city from the well at Grenelle, unless speedily filled up, and which

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