ciently apparent from the above comparison, yet a question might arise relative to the quality of the substances composing the olefiant gas in each of the specimens, and an answer to this question was therefore imperative before a definite conclusion respecting the merits of the two gases could be arrived at.

The illuminating power of the hydrocarbons grouped together in the above analysis under the term "olefiant gas" depends directly upon the weight of carbon contained in a given volume, and hence by ascertaining the quantities of carbonic acid which this portion of the two gases yields on explosion with excess of oxygen, it is easy to calculate their relative value. My experiments have led to the following proportion:

Illuminating Power of Equal Volumes of the Olefiant Gases.

Manchester coal olefiant gas hydrocarbon olefiant gas=3.62: 2.8.

Hence the olefiant principles in the Manchester coal gas have a higher value than the same volume of the olefiant gas contained in the hydro-carbon gas, and this must therefore be allowed for in calculating the relative value of the two gases.

According to the above proportion the true value of the 5.5 per cent. of olefiant gas will be 7-11, and hence the illuminating power of the average hydro-carbon gas will be to the Manchester coal gas as follows:

I. Relative illuminating power of hydrocarbon gas unpurified and Manchester coal gas:

chester coal gas, and that 1,000 cubic feet hydro-carbon gas after purification are equivalent to 1,125 cubic feet of the Manchester gas, and further, that at the present market price of the articles consumed and produced, 1,000 cubic feet of average hydro-carbon gas before purification can be produced, exclusive of rent, taxes, wages, wear and tear, at the cost of 9 d. to 18. 14d., according to the mode of working, whilst 1,000 cubic feet of the same gas purified will cost from 104d. to 1s. 24d.

A distinction must be made between unpurified coal gas and unpurified hydro-carbon gas: the former contains many deleterious ingredients, which entirely prevent its use; the latter does not contain any noxious principle, but simply has its illuminating power diminished by the presence of carbonic acid.

It is also evident that a moderate heat is better adapted for producing good gas economically than a stronger one, which, although it produces much more gas, yet does so at the expense of the oil, which becomes much diminished in quantity, and thus the cost of the gas is greatly increased, whilst its quality also appears slightly to suffer. This is seen in the produce of the first, second, and sixth days, when a more moderate heat was employed compared with that of the third, fourth, and fifth days, when the heat was much higher; and although the yield in gas was considerably less than on the latter days, yet its quality was somewhat better, and the yield in oil being much greater, its price was reduced in a proportionate degree.

It appears to me a mistake to suppose that the hydro-carbon gas requires a different form of burner from those used for coal gas, as I could find no difference in the powers of the gas when coal gas burners were substituted for those generally used with the hydro-carbon; possibly, however, a larger burner might be required if the gas contained much carbonic acid. A careful determination of the specific gravities of the hydro-carbon and Manchester coal gases, which I here append, shows that they do not materially differ in this respect, and thus confirms the opinion that no difference of burner is necessary if the gas be properly manufactured:

In conclusion, its purity of composition and freedom from all substances which can, during combustion, produce compounds injurious to furniture, drapery goods, &c., gives the hydro-carbon gas great advantages over coal gas, which always contains more or

Manchester coal gas 52364.

less bisulphuret of carbon, a volatile substance that has hitherto defied all attempts to remove it or diminish its quantity by any process of purification, and which, during combustion, generates sulphurous acid, the compound to which all the mischief produced by coal gas is probably owing. The odour of the hydro-carbon gas while it is sufficiently strong to give warning of any escape, is far less nauseous than that of the coal gas, and might even by some persons be deemed pleasant, whilst the process of manufacture is so simple that any person of moderate intellect can at once conduct it.

E. FRANKLAND,

Ph. D., F.C.S. Owen's College, Manchester, June 23, 1851.

ON THE CONSTRUCTION OF STEAM BOILERS AND THE CAUSES OF THEIR EXPLOSIONS. BY WM. FAIRBAIRN, ESQ., C.E., F.R.S.

(Continued from page 15.)

The preventives against accidents of this kind are well-constructed boilers of the strongest form, and duly proportioned safety valves-one under the immediate control of the engineer, and the other, as a reserve, under the keeping of some competent authority.

2nd. Explosions from deficiency of water. This division of the subject requires the utmost care and attention, as the circumstance of boilers being short of water is no unusual occurrence. Imminent danger frequently arises from this cause, and it cannot be too forcibly impressed upon the minds of engineers, that there is no part of the apparatus which constitute the mountings of a a boiler which require greater attentionprobably the safety valves not exceptedthan that which supplies it with water. A well - constructed pump, and self-acting feeders-when boilers are worked at a low pressure-are indispensable, and where the latter cannot be applied, the glass tubular gauge steam and water-cocks must have more than ordinary attention.

In a properly constructed boiler, every part of the metal exposed to the direct action of the fire should be in immediate contact with the water, and when proper provision is made to maintain the water at a uniform height and depth above the plates, accidents can never occur from this cause.

Should the water, however, get low from defects in the pump, or any stoppage of the regulating feed valves, and the plates over the furnace become red hot, we then risk the bursting of the boiler, even at the ordinary working pressure. We have no occasion, under such circumstances, to search for another cause, from the fact that the material when raised to a red heat has lost about five-sixths of its strength, and a force of less than one-sixth will be found amply sufficient to bear down the plates direct upon the fire, or to burst the boiler.

When a boiler becomes short of water, the first, and perhaps the most natural action

is to run to the feed valve, and pull it wide open. This certainly remedies the deficiency, but increases the danger, by suddenly pouring upon the incandescent plates a large body of water, which, coming in contact with a reservoir of intense heat, is calculated to produce highly elastic steam. This has been hitherto controverted by several eminent chemists and philosophers; but I make no doubt such is the case, unless the pressure has forced the plates into a concave shape, which for a time would retard the evaporization of the water when suddenly thrown upon them. Some curious experimental facts have been elicited on this subject, and those of M. Boutigny and Professor Bowman, of King's College, London, show that a small quantity of water projected upon a hot plate does not touch it; that it forms itself into a globule surrounded with a thin film, and rolls about upon the plate without the least appearance of evaporation. A repulsive action takes place, and these phenomena are explained upon the supposition that the spheroid has a perfectly reflecting surface, and consequently the heat of the incandescent plate is reflected back upon it. What is, however, the most extraordinary in these experiments is, the fact that the globule, whilst rolling upon a red hot plate, never exceeds a temperature of about 204° Fahr.; and in order to produce ebullition, it is necessary to cool the plate until the water begins to boil, when it is rapidly dissipated in steam.

The experiments by the committee of the Franklin Institute, on this subject, give some interesting and useful results. That committee found that the temperature of clear iron, at which it vaporized drops of water, was 334° Fahr. The development of a repulsive force which I have endeavoured to describe was, however, so rapid above that temperature, that drops which required but one second of time to disappear at the temperature of maximum vaporization, required 152 seconds when the metal was heated to 395° of Fahr. The committee goes on to state that-" One 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°, was vaporized in fifteen seconds, while at the initial temperature of 507°, that of the most rapid evaporization was thirteen seconds."

The cooling effect of the metal is here strikingly exemplified, by the increased rapidity of the evaporization, which, at a reduced temperature of 38°, is effected in thirteen instead of fifteen seconds.

This does not, however, hold good in every case, as an increased quantity of water, say from one-eighth of an ounce to two ounces, thrown upon heated plates, raised the temperature of its evaporization from 460° to 600° Fahr.: thus clearly showing that the time required for the generation of explosive steam under these circumstances is attended with danger, and it may be doubted whether the ordinary safety valves may not be wholly inadequate for its escape.

Numerous examples may be quoted to show that explosions from deficiency of water, although less frequent than those arising from undue pressure, are by no means uncommon-they are nevertheless comparatively fewer in number, and the preventatives are good pumps, self-acting feeders (when they can be applied), and all those conveniences, such as water-cocks, water-gauges, floats, alarms, and other indicators of the loss and reduction of water in the boiler.

3rd. Explosions produced from collapse. Accidents from this cause can scarcely be called explosions, as they arise, not from internal force which bursts the boiler, but from the sudden action of a vacuum within it. In high-pressure boilers, from their superior strength and circular form, these accidents seldom occur, and the low pressure boileris effectually guarded against it by a valve which opens inwards by the pressure of the atmosphere whenever a vacuum occurs. In some cases a collapse of the internal flues of boilers has been known to take place, from a partial vacuum within, which, united to the pressure of the steam, has forced down the top and sides of the flue, and with fatal effect discharged the contents of the boiler into the ash-pit, and destroyed and scalded everything before it. A circumstance of this kind occurred on the Thames on board the steamer Victoria, some years since, when a number of persons lost their lives, and serious injury was sustained in all parts of the vessel within its reach. This accident could not however be called an explosion, but a collapse of the internal flues, which were of large dimensions, and the consequent discharge of large quantities of steam and water into the space occupied by the engines.

One or two cases which bear more directly on this point are however on record, and one of them, which took place in the Mold mines, in Flintshire, was attended with explosion. The particulars, as given by Mr. John Taylor, will be found circumstantially recorded in the first volume of the Philosophical Magazine. This occurrence seems to prove that rarefication produced in the flues of a high pressure boiler may determine an explosion. The boiler which exploded belonged to a set of three feeding the same engine; the fuel used was bituminous coal. The furnace doors of all three of the boilers had been opened, and the dampers of two

had been closed, when a gust of flame was seen to issue from the mouth of the furnace of these latter, and was immediately followed by an explosion. The interior flue of this boiler was flattened from the sides, the flue and shell of the boiler remaining in their places, and the safety-valve upon the latter not being injured.

Other similar cases of collapse might be stated, but as most of them have been attended by a defective supply of water in the boiler, the plates over the fire having become heated, they can scarcely be included in the category of this class of accidents, and more properly belong to those of which we have just treated-explosions from a deficiency of water in the boiler.

It is nevertheless necessary to observe that cases of collapse should be carefully guarded against, as the great source of danger is in the escape of hot water, which, with the steam generated by it, produces death in one of its worst and most painful forms.

The remedies for these accidents will be found in the vacuum valve, and careful construction in the form and strength of the flues.

4th. Explosions from Defective Construction.

This is, perhaps, one of the most important divisions that can possibly engage our attention, and on which it shall be my duty to enlarge. In a previous inquiry, I have already shown the nature of the strain and the ultimate resistance which the material used in the construction of boilers is able to bear. We have not, however, in all cases, shown the distribution and position in which that material should be placed in order to attain the maximum of strength, and afford to the public greater security in the resisting powers of vessels subject to so severe and sometimes a ruinous pressure. This is a subject of such importance that I shall be under the necessity of trespassing upon your time, in endeavouring to point out the advantages peculiar to form, and the use of a sound and perfect system of construction.

For a number of years the haycock, hemispherical, and wagon-shaped boilers were those generally in use, and it was not until high-pressure steam was first introduced into Cornwall, that the cylindrical form with hemispherical ends, and the furnace under the boiler, came into use; subsequently, this gave way to the introduction of a large internal flue extending the whole length of the boiler, and in this the furnace was placed. For many years this was the best and most economical boiler in Cornwall, and its introduction into this country has effected great improvements in the economy of fuel, as well as the strength of the boiler. Several attempts have been made to improve this boiler by cutting away one-half of the end, in order to admit a larger furnace. This was first done by the Butterley Company, and it has since gone by the name of the Butterley boiler. This construction has the same defects as the haycock or hemispherical and wagon-shaped boilers; it is weak over the fire-place, and cannot well be strengthened without injury to the other parts of the boiler, from the vast number of stays necessary to suspend the part which forms the canopy of the furnace. Of late years a much greater improvement has, however, been effected by the double flue and double furnace boiler, which is now in general use, and has nearly superseded all the other constructions. It consists of the cylindrical form, varying from five to seven feet in diameter, with two flues which extend the whole length of the boiler; they are perfectly cylindrical, and of sufficient magnitude to admit a furnace in each. The boiler is the simplest, and probably the most effective that has yet been constructed. It presents a large flue surface as the recipient of heat, and the double flues, when riveted to the flat ends, add greatly to the security and strength of those parts. It moreover admits of the new process of alternate firing, so highly conducive to perfect combustion, and the prevention of the nuisance of smoke.

5th. Explosions arising from mismanagement or ignorance.

To mismanagement, ignorance, and the misapplication of a few leading principles in connection with the use and application of steam, may be traced the great majority of accidents which from time to time occur. Many of these accidents, so fruitful of the destruction of property and human life, might be prevented, if we had well constructed vessels judiciously united to skill and competency in the management. To convey a few practical instructions to engineers, stokers, and engine-men, would be an undertaking of no great difficulty. A young man of ordinary capacity would learn all that is necessary in a few months; and if placed under competent instructors, he placed might be made acquainted with the properties of steam, its elastic force at different degrees of pressure, the advantages peculiar to sensitive and easy working safety valves, the necessity for cleanliness and keeping them in good working condition; the use of water gauges, fusion plugs, indicators, signals, &c., &c., connected with the supply and height of water in the boiler. The dangers to be apprehended from a scarcity of water, the danger of explosion when the engine is standing, or when the usual channels for relieving the boiler of its surplus steam are stopped, all these are parts of elementary instruction which the stoker, as

well as the engineer, should be acquainted with, and no proprietor of a mill, captain of a steam-ship, or superintendent of locomotive, should give employment to any person unless they can produce certificates of good behaviour, and a knowledge of the elementary principles of their profession.

If these precautions were adopted, greater care observed in a selection of men of skill and responsibility in the construction of boilers, and a more strict and rigid code of laws in the management, we might look forward with greater certainty to a considerable diminution, if not a prevention, of those calamitous events which so frequently plunge whole families into mourning by unexpected and instantaneous death.

As an individual I would cheerfully lend my best assistance to the development of a principle of instruction, calculated to relieve the country of the ignorance which pervades that part of the community on which the lives of so many depend. A resolution on the part of those who employ persons of this description, and whose interests are so much at stake, to take none whose knowledge and character does not come up to the requisite standard, and pay for it, would soon find from the economy of the management and the increased security of their property, a very important change in all the requirements of the economy, as well as the application of steam. How often do we find implements of danger, and vessels containing the elements of destruction, in the hands of the most ignorant and reckless practitioners, whose insensibility to danger, and total incompetency to judge of its presence, renders them above all others the most unfit to be employed. And why? because they are the very persons, from their defective knowledge, to increase the danger and aggravate the evils they were selected to prevent. It is not the first time that engineers, to secure (if I may use the expression) an insane pressure, have fastened the safety valves, and screwed down the steam valve, closing every outlet, without ever thinking of the fire that was blazing under the boiler. (To be continued.)

PANNELL'S REGISTERED RETORT CALORIFERE, FOR CONSERVATORIES, GREENHOUSES, ETC.

(John Pannell, of 56, Fetter-lane, London, Agricultural and Horticultural Engineer, Proprietor.)

The engraving represents a vertical section of this apparatus. A A is the boiler, which is built into the furnace like a retort, being completely surrounded by the fire, which is supplied with fuel through the door N. Bisa partition, which divides the boiler into two com

Sir,-I have not had an opportunity of seeing the Mechanics' Magazine for several weeks, and consequently am not even yet in possession of the information forwarded to you by the Rev. Mr. Reynolds, and other correspondents on the rotation of the earth. I have, however, just seen the article on this subject by Mr. B. Rozzell, printed in the Magazine of last week, from which I regret to find that there is one reader of your journal to whom my discussion of that interesting topic is unintelligible. I am sorry that it is out of my power to make the matter plainer. Your correspondent expresses his regret that I should have

contented myself "with merely illustrating the theories of others." If such be the fact, I shall be thankful for an opportunity of restoring to the rightful claimant whatever there may be, in the trifle referred to, which belongs to another. I can only say that, up to this moment, I have never seen a single line of the investigations of Binet and the other Parisian analysts: indeed, I have never seen any mathematical details on the subject at all. It would be quite futile for me to attempt anything like a reply to Mr. Rozzell: each of us, I see, is quite unintelligible to the other. Mr. R. reads in my paper what I never