not show how the discharges happen amongst the clouds? Namely, by the accumulation increasing faster than the discharge, through imperfect conductors, whereby the increase in a certain time becomes sufficient to leap the interruptions. I have constructed another frame, 5 feet square, and covered it with silk. The middle part of it for about 31 feet

square is covered with brass filings for the purpose of showing how the "sheetlightning" is produced, for I consider it to be the actual light of the clectricity generally, if not always, and not the reflection from the clouds.

I am, Sir, yours, &c. W. ETTRICK. High Barns, Sunderland, Dec. 19, 1836.

II. Explosions produced by the presence of unduly heated metal within a steam-boiler.

15. In a properly constructed steam-boiler no part of the metal is exposed to the direct action of the fire, without being immediately in contact with water: the temperature of the metal cannot be raised above that of the water, and is thus determined by the weight upon the safety valve. When, from any cause, the metal is not so circumstanced, it becomes unduly heated, and danger may arise from two sources; first, the metal is weakened and rendered less capable of resist ing even ordinary pressure; second, it serves as a reservoir of heat ready to bring into existence highly elastic steam, whenever water shall obtain access to it.

16. The first of these positions rests upon the basis of direct experiment, and is, the Committee believe, generally admitted.* Their experiments on the strength of mate rials have, however, developed a curious fact in regard to the strength of malleable iron, namely, that it slowly increases at first with an increase of temperature, and attains its maximum at a temperature above that at which any of the steam engines used in practice, are worked. Above this maximum, the decrease of strength is very rapid; so as to be, at a red heat, but about one-sixth of that at ordinary temperatures. Copper, on the contrary, is weakened by any increase of temperature above the lowest, which was tried, namely, 32° Fahr. The fact just stated in regard to iron, is interesting in its application to the proof of iron boilers, by the water-press, and as showing the great, and rapidly increasing, danger from di minished strength, as the metal is raised above the temperature of maximum strength.

17. Secondly, the heated metal serves as a reservoir of heat to furnish highly elastic steam, when water is in any way brought

In the Minutes of the Select Committee of the House of Commons there is a statement by Mr. John Steel that cast-iron is strongest at the temperature of 3009, but it is not supported by refer ence to experiment.

into contact with it. That highly heated metal can produce steam rapidly, has hitherto been a controverted position. In the experiments of Klaproth, successive drops of water thrown into an iron spoon, originally heated to redness, vaporised the more rapidly as the metal lost heat. In the experiments of Perkins and others, larger quanti ties of water in highly heated metallic vessels, vaporised very slowly. It is true that by injecting water into an iron cylinder, heated to redness, Mr. Perkins found a sudden increase of elasticity; but he attributed the effect to the hot and unsaturated steam which the cylinder contained, and through which the ejected water passed. The Committee found that the temperature of clean iron at which it vaporised drops of water most rapidly, was 334° Fahr. The develope ment of a repulsive force is so rapid above this temperature, that drops which required but one second to disappear, at the tempera ture of maximum vaporisation, required 152seconds when the metal was heated to 395°. One-eighth of an ounce of water introduced into an iron bowl, three-sixteenths of an inch thick, and supplied with heat by an oil-bath, at the temperature of 546° Fahr., was vaporised in fifteen seconds, while at the initial temperature of 507° Fahr., that of most rapid vaporisation under these circumstances, it disappeared in thirteen seconds. cooling effect of the water upon the metal is here strikingly shown, by the increased tem❤ perature to which the latter has to be raised at the beginning of the experiment, in order to give the most rapid vaporisation. A fur ther illustration of the same kind is afforded by comparing the temperature giving most rapid vaporisation, when the metal of the bowl is supplied with heat by a good and a bad conductor, or imperfect circulator, as

The

+ Franklin Journal, vol. iji. p. 418. Lond. Mechs. Mag. or Jour. Frank, Inst. vol. ix. p. 348.

Report of Com. on Explosions. Part I. "Vaporisation of drops." Reply to Query VI. Jour. Frank. Inst. vol. xvii. p. 90,

by a bath of tin and one of oil. With a rough surface, an iron bowl one-quarter of an inch thick, vaporised one-eighth of an ounce of water most rapidly by introducing it when the metal was at 555° Fahr., the bowl being in an oil-bath; while in a tin bath the corresponding temperature was 508° Fahr...

18. By carrying out this idea we have the clue to the action of water thrown, in considerable quantities, upon heated metal; and find, accordingly, that when the water was increased sixteen times, or from one-eighth of an ounce to two ounces, the temperature of most rapid vaporisation was raised from 460° to 600° Fah.; the surface of the metal being smooth, and the heat supplied through tin. Now, although differences in the mode of applying heat will alter these temperatures, it is clear that they rise rapidly with the quantity of water thrown upon the metal. In the case where as much water was thrown into an iron bowl as it could contain without loss by ebullition, the temperature of greatest vaporisation, upon a clean surface, was 600° Fah., or about 2000 below a red heat, and would, according to analogy, have been higher if on a rough, or oxidated, surface:*

19. These observations explain the direct experiments made by the Committee, in which highly elastic steam was always rapidly produced by injecting water into a boiler heated to bright redness.† - - In one case, by the injection of ten ounces of water the elasticity of the steam was raised, in less than two minutes, to upwards of twelve atmospheres, and a miniature explosion pro duced. The remarks made in this experiment show, that, wherever the water slid along the bottom of the boiler, the spot of contact was for the instant blackened, by the sudden reduction of temperature, and this under the unfavourable circumstance of the introduction of a limited quantity. The bottom of the boiler in these experiments was clean, but not bright. The time required for the generation of explosive steam under these circumstances does not yet admit of calculation, but this may be affirmed with certainty, that a safety-valve which, under ordinary circumstances, may be adequate to carry off the excess of steam produced in a boiler, will prove wholly insufficient for its escape, in the supposed case.

220. These experiments are entirely supported by well-authenticated cases of explosions in steam-boat boilers. Mr. Bakewell

Report of Com, on Expl. &c. Reply to Query IV. Vaporisation of increased quantities of water." Jour. Frank. Inst. vol. xvii. p. 160.

Ibid. Reply to Query II.

Letter to Secretary of Treasury, communicated to Com. on Expl. Reply to Circular of Com. No.

gives an instance, in the case of the steam. boat Grampus, where six cylindrical boilers, each 38 inches in diameter, exploded simultaneously. The engineer had discovered that they contained very little water, and had suddenly thrown in a plentiful supply. When one of the boilers of the steam-boat Car of Commerces exploded, it was well known that the pumps had not furnished the requisite supply of water; and just after an attempt to remedy this difficulty, the head of one of the boilers was thrown off. This boiler was, it seems, differently constructed from the others, with which it was connected, and which did not give way.

The first of these cases is distinctly made out, and the second cannot be resolved into a matter of opinion, as may perhaps be done with other accidents, which, though there is a strong probability that they are to be traced to this cause, the Committee refrain from quoting.

21. It is, of course, not assumed that an explosion must necessarily follow the presence of heated metal; for other circumstances must conspire to produce such a result. Facts, indeed, may be brought to show that, in certain cases, these attendant circumstances have been accidentally wanting, or have been judiciously avoided,...,

As examples of this, may be taken instances mentioned by Col. Long, in which timber on the top of cylindrical boilers has been known to take fire, though considerably remote from any fire-flue. Those to which Mr. Bakewell¶ has been an eye-witness, when the steam has been so highly heated after leaving the boiler, as still to burn the hempen packing of the steam-cylinder, and where wood contiguous to the boiler has been fired. Similar incidents not followed by explosions have occurred in the mines of Cornwall,** and in one of the Liverpool and Dublin packets.+t Examples of the second kind will be referred to subsequently.

22. With such a powerful agent present, as the highly elastic steam which it has been proved may be rapidly generated by the heated metal, it might have been supposed

XII. Also, Letter of Thos. J. Haldermann, No. XXI. of Replies.

Letter of Thos. J. Haldermann, No. XXI. of Replies to Circular, &c.

Replies to Circular, &c. No. II.

¶ Replies, &c. No. XII.

**Mr. Perkins states, on the authority of Mr. Moyle, that a ladder accidentally resting upon the top of a boiler, was set on fire by heat communicated from thence. Franklin Jour. vol. iii. p. 417, or Lond. Jour. Arts, vol. xiii. p. 95.

tt Evidence before Com. of House of Commons, 1817. Hazard on Explosions. Frank, Jour. vol. iti. Ewbank on Explosions. Jour. Frank. Inst. Vol. X.

that no other cause for explosion would have been looked for, than the action of this steam. The case is, however, otherwise, and the Committee must turn aside from their direct course to examine briefly the theory which assigns the production, and subsequent destruction, of hydrogen gas, as the cause of the explosion. According to this view, the water thrown upon the metal is decomposed, and hydrogen gas evolved; or a similar decomposition of the steam, by the hot metal, takes place. This hydrogen, becoming mixed with oxygen, is ignited by the red-hot metal, and an explosion ensues. The difficulty of furnishing oxygen for the hydrogen to combine with, has lately been met more satisfactorily by Mr. Perkins, than it had been by any preceding theorists. He asserts that air is frequently drawn in by the operation of the forcing pump, and is thus accumulated in the boiler. The primary hypothesis, in regard to the production of hydrogen, having been fully disproved by the experiments of this Committee, there is no necessity for examining the minor

to observe, the it may be well, however,

if air were introduced into a highly heated boiler, containing hydrogen in too large a quantity either to combine explosively, or silently, with the oxygen of the air, that element would be taken up by the heated metal; and that gases cannot enter, and remain without mixing with the steam, and being carried out with it. In the experiments of the Committee, which bave been referred to, water was thrown upon the bottom of a boiler, heated to orange redness, without being decomposed. In fact the scale of oxide existing upon the bottom, prevented the decomposition of water, by enfeebling the affinity which would produce it. This boiler was carefully cleaned, and in good working condition; a condition in which no one need be told, a boiler has not a bright metallic surface.

23. Carburetted hydrogen does no doubt exist at times in a boiler, in greater or less quantities, from the decomposition of oil, or of vegetable substances introduced to stop leaks, or to prevent deposits, but nothing warrants the idea that it can accumulate and mix with air, so as to be dangerous.

In furnaces where coal is used as a fuel, it will be seen in the sequel that gas, if prevented from escaping by the closing of a damper, may collect, and may possibly be a

The reader should refer to these, that he may see the care which was taken in them. A negative result requires so much more caution than a positive one, that more time was devoted to those experiments in order to make them satisfactory, than the Committee deemed warranted by the importance of the subject. Report of Com. on Explosions, Part I. p. 61, &c. Jour. Frank. Inst. vol. xvii. p. 217.

source of danger. The ignition of a mixture of coal-gas and air in a furnace has been known to destroy it, as also of a mixture of gas from resinous wood and air; but these are cases altogether foreign from the subject under discussion.§

24. The explosion of the steam-boat Enterprise, on the Savannah river, is said to have occurred at the instant the boat was struck by lightning. This has been advanced as confirming the hydrogen hypothesis; but no inference can fairly be drawn from an accident, in regard to which the circumstances are so little known. If there was hydrogen present, there must have been unduly heated metal, and the direct action of electricity on the non-conductors around the boiler, may have so displaced it as to bring water upon the heated metal, and thus to effeet an explosion. This, like the other supposition, is mere hypothesis. It is certainly, however, quite as contrary to analogy, that an electric spark should pass through any part of a space, like the interior of a boiler surrounded by a conductor, and thus explode a mixture of hydrogen and oxygen within it, as that it should shatter this extensive conductor by its direct action. The Committee consider the circumstances of this case as too ill-defined to draw any inference from it, certainly not one which is contrary to sound theory, by which they mean general induction from numerous well-observed facts.

25. Another case has been urged with much more appearance of directness in the testimony. A boiler in the Union rollingmills, at Pittsburgh, burst with a tremendous explosion; a cylinder with one of the heads attached was thrown out of the works, and rising to a considerable height in the air, fell nearly 200 yards from its former bed.

passenger in a boat which was near at the time, describes a stream of fire, as issuing from behind the boiler, which, according to the hypothesis under discussion, was a stream of burning hydrogen. It is almost needless to remark, that if hydrogen had been the

+ Explosion in the Gold mines as given by John Taylor. Esq. Philos. Mag. vol. i. ̧ ̧

M. Arago states this to be the fact, on the authority of M. Gay Lussac. A furnace was thus destroyed at the Paris arsenal. Annuaire du Bureau des Long. 1830, p. 197, and Jour. Frank. Inst. vol. vi. p. 54. 29t

See the case of an explosion of a sheet-iron drum attached to an anthracite-stove, with its explanation by Prof. Hare. Jour. Frank. Inst, vol. vi. p. 337. Pine shavings were used to kindle the fire, the gas from which, mixing with the air in the pipes and drum, produced au explosion, when the flame from the kind ed shavings rose into it. Refer also to the explosion of the bellows of a smith's forge. Silliman's Jour. vol. xxiv. p. 192.

cause of the explosion, it would not have -burned in a stream behind the empty boiler as it rose; the observation is, however, perfeetly well explained by Dr. Jones,* by the stream of light which appears to attend every luminous substance moving rapidly, on account of the duration of the impression upon the eye. That the boiler was red-hot, there appears no doubt.

26. From this digression the Committee return to the pursuit of their subject. They conceive that it has been fully established, that the presence of unduly heated metal is dangerous, both from the weakness of the material, and the possibility of its producing highly elastic steam. They, therefore, proceed to examine the probable causes leading to this result, and which have been suggested either in the communications made to them or in other documents, and the proposed remedies for, or precautions against, the danger.

t

Sir,-Having occasion to fit a footwheel to a mandril with three groves, I find considerable difficulty in ascertaining what the relative circumferences of the three groves on the foot-wheel must be, to enable the same band to fit the corre sponding circles on both. Not being much of a mathematician, I have, after a few unsuccessful attempts to find a rule applicable to the case, given up the calculation in despair, and unless some one of your talented correspondents will assist me, I shall be obliged to have recourse to diagrams, and constructing circle after circle on the foot-wheel, by actual measurement ascertain when they approach the proper size. I have no doubt that by this means a result may be found near enough for most practical purposes; but it will be a most tedious and unsatisfactory way of arriving at it, and when found will be only applicable to this one particular case. I have stated my difficulty to several practical men, and they have acknowledged they are unacquainted with any rule which can assist me, and that they have themselves been similarly inconvenienced from the want of one. If, therefore, any of your readers can favour me with a solution of the following question, and any rule which is applicable to such cases generally, you will enlighten several of your readers and very much oblige,

I am, Sir,

Your constant subscriber,

H. F. H.

Question. Suppose a mandril-wheel has three groves, the diameter of the largest being 8 inches, of the second 5 inches, and of the smallest 3 inches. A band passes round the smallest circle on the mandril, and round the largest circle of a foot-wheel, which circle is 26 inches in diameter, of what diameter must the other two circles on the foot-wheel be, to correspond with the other two

circles on the mandril, so as to enable the same band to fit them equally well; the centre of the mandril being fixed 30 inches above the centre of the foot-wheel?

YODURABILITY OF WOOD.

(From the Genesee Farmer.)

The subject of the durability of different kinds of wood when exposed to the action of air and moisture, though one of great importance to the farmer, and the public at large, does not seem to have received that degree of attention and elucidation it deserves. The Teak tree (Tectona Grandis) of the East Indies, and the Live Oak (Quercus virens) of our southern forests, appear to be nearly indestructable when employed as timber, and in naval architecture. But these from their natures cannot be cultivated in the northern States, and we must seek in our forests trees which shall approach or rival the above in excellence and durability. Fortunately these qualities are found nearly in perfection in the Yellow Locust (Robinia pseudacacia), and the Red Cedar (Juniperus virgininaa), and in a lesser degree in many others; a knowledge of the compa rative durability of which, properly ascertained, would be of essential service to the public.

Thirty-two years since, in enclosing some newly cleared field, we had occasion to set some bar posts, and, at a point where four fields cornered, we placed one so that it served for four pair of bars, one to each field. The post was the common white cedar (Cupressus thuyoides), cut from a thrifty tree fourteen inches in diameter, the holes on the four sides cut in the usual manner with a narrow axe, the bark stripped from the whole, and the large end set two feet in the earth, which at that place was rather moist. After standing more than twenty years, or until the basswood fences with which it was connected had rotted down, it was removed to another position, where it has since served for two pair of bars and one gate. At the time of removal it appeared quite sound, and present appearances indicate a duration of another twenty years at least. From our experiments we are convinced that large posts are far more durable than small ones, and that those which occupy the whole diameter of the tree, are better than sawed or quartered trees of equal size.

In the Railroad Journal, in an article on the advantages of lime, as a preservation of timber, the following instance of its effect on the durability of the white pine (Pinus ables), is given: The planks were a parcel

of pine planks used as platform on the ground, on which to make live mortar.This platform was laid by the informant's grandfather in a corner of the yard, and used every year more or less for the purpose of a mortar bed. His father continued it in the same use; himself, the grandson, continued it for a time, as long as he had occasion, after which it lay some years unused, and overgrown with grass and weeds. At length, wanting the ground for another purpose, he had it torn up and removed, expecting to find the planks entirely rotted, but, to his surprise, found them sound, and, to use his own forcible expression, "as hard as a bull's born." This was after they had lain exposed to all the vicissitudes of the atmosphere, and in contact with the surface of the earth about sixty years.

In the year 1800, a Mr. Atkinson, in the employ of the Hudson's Bay Fur Company, discovered, on Old Factory Island, in James' Bay, a branch of the Hudson's Bay, a cedar post, about a foot square and five feet high, on which the following inscription had been cut, and all the letters of which were distinctly visible:"In the year 1692 wintered three ships at this Island, with one hundred and twenty-seven men, under the government of Captain James Knight. Then we erected this monument in remembrance of it."

This furnishes the greatest instance of duration of timber set in the earth, and constantly exposed to atmospheric influences, we have any where noticed, and we believe there are few kinds of timber which would endure so long. This notice does not state the kind of cedar employed, but, from its frequency on those islands and coasts, there can be no doubt of its being the red cedar, a kind which, as stated above, is almost imperishable.

Next to the kinds above stated, rank in durability the various kinds of pine and spruce; the white oak, chestnut, red elm, black walnut, and red beech; all furnishing timber of good quality, but not such as will, like the above, resist for a long period the attacks of time. In purchasing timber, price should not be so much regarded as quality and durability, as a rail or a post that will last fifty or sixty years, is worth far more than the usual difference charged between sucb, and those which will endure only twenty-five or thirty.

FORCE OF THE WATERS.

(From Audubon's Ornithological Biography, Vol. ii.)

It was the month of September. At the upper extremity of Dennisville, which is itself