100 tons, in consequence of which mistake of the engineers, the ship was a failure.

It does not seem to have occurred to any party, that an 800-horse engine would require more coal than a 500, as no provision seems to have been made for the difference. I will grant that there were enough coals for the 800horse engine, though I do not think so, and make a present of that portion of the argument, and proceed to show that this mistake, supposing it to have been made, was wholly insufficient to account for the defects in that ship; and I think it scarcely fair of his Lordship to lay the blame elsewhere than where it rightly attaches, namely, to the late surveyor and his assistants; for though his Lordship says that he speaks on authority, till he gives up that authority, the injustice inust attach to him.

I need not tell you, Sir, that the engines must have been put together in the factory when the mistakes as to size and weight were discovered, if not before, and communicated to the builder, who himself, or by his assistant, should have ascertained whether these were so great as to cause a failure, and if so, they ought to have prevented the engines from being put in.

The following arguments will prove to the lowest comprehension, that her deficient stability could not have been caused by the mistake alluded to, and that the whole of the errors which have produced the unhappy result called the Retribution. must be laid at the door of the constructive department of the Navy.

If we assume, what was most probable, that the centres of gravity of the intended 500 and 800-horse engines were equally high, then we have only the extra 100 tons of weight to deal with, as respects stability, which, having its centre at the centre of gravity of the engine, and much below the centre of gravity of the ship, would have increased rather than decreased the stability, so that this solution of the difficulty would tell the other way.

However, lest we should be said to exaggerate, we will grant, that the height of the centres of gravity of these engines differed in the amount usual in engines of similar construction, but differed in power as the of the ratio of their respective powers, 5 to 8, and let

it be assumed that the centre of gravity of the 500-horse engine was intended to have been 10 feet high, this would place it about 8 feet below the centre of gravity of the ship and lading (without the engine and boilers). The weight of these engines would then have been about 340 tons, and the ship's displacement about 2660 tons - from which we have, as the effect on the common centre of gravity of the system, by placing the 500-horse engines as above stated, 340 × 8=2720÷ 2660-1 feet, the quantity which the centre of gravity would have been lowered, and the stability proportionally increased.

Following our assumption, the centre of gravity of the 300-horse engine would have been 12:6, or 5'4 feet below the centre of gravity; and with 100 tons more in weight, we have 440 × 5'4=2376÷2660

=

89 feet, the quantity which the common centre of gravity would have been lowered by the 800-horse engine; that is to say, the centre of gravity was lowered less in the amount of one-tenth of a foot by the 800-horse engines than it would have been had those of 500 been put in; from which it would appear that the original design would have had a very slight amount more stability than that which obtains in the Retribution. I use the expressive appear advisedly; for, no doubt, the greater immersion in the latter case, will have given a larger area of load water-line, by which the metacentre would be raised; in other words, the stability would be increased, so that, on the whole, there would not have been less stability in the existing ship, than in that originally designed.

Yet, we may grant him the full value of this one-tenth; for she would have been a failure irrespective of this alteration which produced it, supposing it to have been injurious, for she would have been so if she could not have carried 300 tons of coals, or other weight, upon her upper deck, without its materially injuring her properties (a quantity which some of our steamers have carried). Her upper deck is about 15 feet above the centre of gravity, from which we have as the effect on the centre of gravity 300 × 15-4500+30001 foot; the

quantity which the new centre of gravity would be raised to, by placing 300 tons 15 feet above the previous centreor, in other words, the amount of altera

The

length of tube, 8 inches; diameter of ditto, half an inch; diameter of circular box, 4 inches; depth of circular box, §-inch; holes in upper surface, about fifty in number, in diameter about one-tenth or one-sixteenth of an inch; one hole in the under surface, in diameter one-sixth of an inch. cylindrical vessel may be made of copper, tin, gutta percha, glass, or earthenware, according to the properties of the chemical agent employed. The cover may be of cork, with two perforations for the two tubes, or it may be of the same material as the vessel itself. In the former case, the short tube may be of glass; in the latter, of the same material as the case itself, and soldered to it. In either case, a piece of vulcanized India-rubber tube, of convenient length, may be attached to the short tube, and terminate in a mouth-piece. For facility of breathing, it is of consequence that the tubes should be of good width. A strip of moistened bladder, wrapped round the junction of the cover with the vessel, will make it air-tight.

The size of the cylindrical vessel will vary according to the height of the fluid above the perforated box, and according to its viscidity, with a view to provide plenty of bubble room. When the fluid is viscid, the bubbles are persistent, and, after a few inspirations, occupy a large space; though this may be advantageous in causing a longer contact between the air and the solution, it incurs the necessity of a larger vessel. The whole apparatus may be conveniently suspended from the shoulders, and both hands left at liberty. It is not necessary that the vessel should be cylindrical; it may be of a concavo-convex shape, so as to apply itself to the side without being cumbersome.

The author then explains, under different sections, the use of the apparatus-to save persons from suffocation on occasion of fires

- to remove poisonous gases from the airto purify a pestilential atmosphere-to remove carbonic acid from the air-and to neutralize the influence of acid and poisonous fumes in chemical manufactures; illustrating each branch of the subject as he proceeds, by notices of such remedial means as have been proposed by others, and by familiar expositions of the scientific principles bearing on the matter in hand. We quote, as a sample of the instructive and useful information which the pamphlet contains, the section of which carbonic acid is the theme :

:

Use of the apparatus in removing carbonic acid gas from the air.-This apparatus may be applied to the purpose of removing carbonic acid gas from air, when it is necessary to breathe such a mixture. I have made many experiments to ascertain to what extent this poisonous gas may be absorbed by this means. The vessel was filled with a solution containing 25 per cent. of caustic soda, till it reached an inch above the perforated box. After the mixed airs had been exposed to this solution by forcing them through the perforations in the box, I found that more than half of the carbonic acid was removed. I varied the proportion of the gas from one-half to 1·20th. The same experiment did not always afford the same result, because the air could not always be forced through the solution with the same rapidity, and the size of the bubbles would also vary. I may mention that I endeavoured to effect the passage of the air through the apparatus with the same rapidity as in breathing.

If it be true that we can thus deprive air of one-half of its carbonic acid, we become able to breathe an atmosphere which would be otherwise poisonous. Suppose it to be a matter of importance to enter a locality, the atmosphere of which contains 10 per cent. of carbonic acid, a proportion which would soon produce insensibility, by breathing through this apparatus the quantity of carbonic acid would be reduced to 5 per cent. a proportion which might, I think, be respired for a considerable time, without much inconvenience.

It is a common opinion that the burning of a candle in a suspected mixture of carbonic acid and air is a satisfactory proof that it may be respired with safety; but Dr. Taylor in his manual of "Medical Jurisprudence," has shown this to be erroneous. He found, by experiment," That a candle will burn in air which is combined with ten, or twelve and a half per cent. of its volume of carbonic acid; and although such mixtures might not prove immediately fatal to man, yet they would soon give rise to giddiness, insensibility, and ultimately death." But if the burning of a candle, in a mixture of carbonic acid and air, is not of itself a sufficient indication that air is respirable, yet its combination with the purifying apparatus would be useful in this way, that wherever the candle continued to burn we might infer that the quantity of carbonic acid was not so great, but that the apparatus would remove so much as to make the air respirable for a considerable length of time. Under ordinary circumstances, the lighted candle would prove not only that the carbonic acid was limited at most to 10 or 12

per cent., but that oxygen was not materially deficient. I do not forget that Dr. Taylor has shown that a candle will burn in a mixture of 25 parts of oxygen and 75 parts of carbonic acid; but such a mixture is artificial and not likely to occur in nature.

We are thus furnished with a means of exploring coal mines where choke damp prevails. As carbonic acid gas is very generally the result of combustion, and formed at the expense of the oxygen of the air, it is of the greatest consequence that the apparatus should be accompanied by a lighted candle, the degree of brilliancy of which may indicate that there is a sufficient quantity of oxygen. What amount of oxygen is necessary to maintain life in a human being I do not know; but I understand that Dr. Boswell Reid concludes, from some experiments which he has made, that if the carbonic acid be removed as fast as it is formed, an animal may remain in a limited quantity of air without much inconvenience, until nearly the whole of its oxygen is exhausted. The experiments of Dr. Snow, however, do not warrant the above conclusion. He found that birds and mammalia introduced into an atmosphere containing only 10 to 104 per cent. of oxygen, instead of 21 per cent., soon died, though means were adopted for removing the carbonic acid formed by the respiration. He is of opinion that any notable diminution of oxygen, even when no carbonic acid is present, cannot take place without danger to the warm-blooded animals.

Dr. Reid, in his work on ventilation, states that in mines it is common for the men to work in an atmosphere too impure to allow a common candle to burn, though an oil lamp, in consequence of its greater tenacity of combustion, may be maintained in action without difficulty. Mr. Buddle, in his evidence before a Committee of the House of Commons, on accidents in mines, states, "That in his own experience, he knows that carbonic acid gas may be breathed when no flame will exist in it; and has had occasion, frequently, to travel in a mixture of this kind when a candle would. not burn, and a steel mill give hardly any sparks. He feels oppression of breathing, not by any means dangerous, while the steel mill will elicit any sparks." Mr. Nicholas Wood, coal miner, of Northumberland and Durham, states in his evidence before the same Committee, that he has met with carbonic acid gas in such quantities that the candles would not burn in it; but the miners are not injured by it till a considerable time after the candles are extinguished. He has not known carbonic acid emitted in such quantities as to produce an immediate sense

of sinking, and thinks that the lights being put out a considerable time before it injures the workmen, guards against any danger of that kind. From these facts we may infer, that as long as a candle exhibits the feeblest glimmer in a coal mine, there is little danger of life for a long time, provided we remove a large portion of the carbonic acid, and no other poisonous gas is present, as carbonic oxide. The danger is not immediate even after the lights are extinguished; and how much less imminent must it be if we are in possession of means of depriving the air of a large proportion of its poison. I do not anticipate that workmen will use this method, for the obvious reason that it would interfere with their work; besides they become acclimated to a carbonic acid atmosphere, in which a new comer could not live; but I think that an occasional suck through the apparatus for a quarter of an hour would revive them considerably. It would prove more acceptable to owners, surveyors, or coal-viewers, who enter an impure atmosphere occasionally only; or who have to explore localities which are not worked, whose atmospheric conditions are not known, and where the presence of choke damp may be suspected. Carbonic acid

abounds chiefly in old workings and abandoned mines; when it is necessary to enter them the use of the apparatus would, I think, contribute much to the safety and convenience of the explorers.

It is for this purpose of rescue, that I conceive that the air purifier would be of the greatest value. An alarm is given by some one who has had the good fortune to escape, that his companions are overcome with the choke damp; to proceed unprotected to their help, is only to add to the number of the sufferers; but the apparatus will probably make the difference between respirable and irrespirable air, and enable us to approach and save those who would otherwise inevitably perish.

The explosion of fire damp is less fatal from the accompanying heat and violence, than from the vitiated air of the after damp. There is reason to believe that many die gradually of suffocation, after an explosion, in consequence of the abundance of carbonic acid, and deficiency of oxygen, caused by the combustion of the gases. Timely help might snatch these from destruction. It is extremely probable, that the air-purifying apparatus would, in some instances, render this after-damp comparatively innoxious, and enable us to rescue them. Some ready method of analyzing the foul air, or cautious trials to breathe it through the apparatus, would give some information how far we might venture.

I have stated that in my experiments more than one-half of the carbonic acid was removed by passing it through the caustic solution of soda: by enlarging the apparatus it would be easy to remove a much greater proportion. This might be done, either by having a greater depth of the solution above the perforated box, (say two inches, instead of one), or by passing the air to be purified through two vessels containing the solution. In the latter case it would be necessary to assist respiration, by forcing the air through the apparatus by means of a small pair of bellows under the arm into a bladder, which would become a reservoir of purified air, and to which a mouth-piece shonld be attached. This addition to the apparatus would not necessarily make it cumbrous and unmanageable.

It must not be forgotten, that the alkaline solution will only take up a certain quantity of carbonic acid, and that its power of absorbing is constantly diminishing. We will suppose that the solution contains 546 grains of caustic soda; and that 29 cubic inches of air, containing five per cent. of carbonic acid, are inspired at each inspiration, and that 20 inspirations are made in a minute: it results, by calculation, that the whole of the caustic soda would be converted into a carbonate in 80 minutes, supposing the air to be deprived of half its carbonic acid before each inspiration; but this is not the case, for as the quantity of caustic alkali diminishes its power of absorbing, carbonic acid will also diminish; but not rapidly, as I had the satisfaction to find by experiment. With a view of imitating the effect produced by a long continued passage of carbonic acid through the solution of caustic soda, I reduced the strength of the solution employed in the experiment by one-half, by diluting it with an equal quantity of water, and adding carbonate of soda, till the solution was saturated: this took place before the solution contained as much soda per cent., whether caustic or in the form of carbonate, as it did before the dilution. In passing a mixture of air and carbonic acid gas through the solution, I found that its power of absorbing the latter gas very little diminished; it still absorbed about 50 per cent. We may infer, therefore, that with a charge of 546 grains of caustic soda, and under the conditions above stated, the efficacy of the apparatus would be but little impaired after the lapse of 40 minutes; that is to say, after half its caustic soda had been converted into carbonate of soda. It is obvious that the charge of soda should vary, according to the impurity of the air and the length of time it has to be breathed. An experienced

person would cease breathing through the apparatus when he had reason to believe the air sufficiently pure, and thereby husband his store of alkali.

DESIGN FOR A MAGNETIC BORING MA-
CHINE. BY MR. JOHN THOMSON.

The enormous labour, delay, and other vexations attendant on rod boring, are well known, and do not require examples to enforce conviction; yet, strange to say, that same old awkward system even yet holds good its footing. The Chinese method, which is much more simple and effective, is being adopted in Europe. It consists of an armed weight (say one, two, or three hundred pounds, as the case may be) of a particular form, attached to a peculiar rope, to withstand the friction on the sides of the bore hole whilst at work, and being suspended from above by a spring, on which two men are seated. The weight is made to bob up and down by their exertions, striking the bottom at each depression of the spring, the boring being advanced with considerable rapidity. The clearing of the detached matter by this method is also very much facilitated, as compared with the rod-boring system.

These are the only two methods at present known for boring the earth to any great depth, and they do not admit of much improvement.

But I contend that other methods may be found to supersede these, and would suggest the powers of the electro-magnet, an agent whose limit of energy is yet unknown. The small space through which it can exert its immense power, renders it peculiarly adapted to boring purposes, either perpendicularly, horizontally, or at any other angle; being a particular advantage gained over any of the other methods, and quite applicable to direct mining.

Suppose, for instance, a short boring instrument to have a common magnet inserted into it, with the poles uppermost. Again; suppose a mate to this magnet, of the electro description, firmly inserted into a heavy weight. Next, let us imagine these two magnets to be set upon each other, face to face, the under one with its boring tool, and the upper with the weight. It will be evident that, by connecting an electro-magnetic battery to the upper magnet, and reversing the polarity as usual with that machine, a power will be exerted between these two magnets, tending, at each alternate reversal of polarity, to separate them and produce a shock. Such is the principle upon which I propose my mining machine. If the weight