worked by boys, and the air is carried along pipes to places to which it would otherwise only very slowly penetrate. Forcing-pumps are also employed to force forward the air in a similar way. Sometimes a supply of fresh air may be got by running a second level from the air into the hill, and making a communication. In that case the air may be put in action, and may enter at one level and go out by the other. Sometimes a shaft may be carried up to the open air, or let down from the open air into the level; and when that is done a current of air may be effected. Whatever is within the range of such current, of course, is well ventilated.

Such things, however, are not the general rule. In most mines there are not two levels communicating with the open air, neither can there be shafts from the open air down to the levels. Where nature does not interpose a physical impossibility, there is what is equally powerful-the dread of expense. The sum required to sink a shaft or to run a level may be so great that the mine is not worth it. The proprietor would rather discontinue working it than submit to the burthen; and the men, young persons, and boys, having no other means of existence, are eager to be allowed to work at the mine such as it is. (Dr. Mitchell, Report, § 51, 56: App. Pt. II., pp. 727,728). TEMPERATURE OF MINES.

(From the same Report.)

Coal pits are almost always comfortably warm; and in general the deeper they are, the warmer. By proper ventilation the heat can generally be so regulated as to render the temperature unoppressive, and even grateful. When cold in the main roads the heat is often oppressive in the side gates and at the workings. Oppressive heat may always be regarded as an indication of imperfect ventilation. It is stated that in the mines of the Yorkshire coal fields the thermometer stands in the main roads at from 50° to 60°, in the side roads from 60° to 65°, and at the workings from 64° to 72°. In the deep mines in the northern coal field the temperature is considerably higher. In one of the Hetton pits, in South Durham, the temperature was found to be 66° at the bottom of the shaft, and 70° in the workings; but in the Monkwearmouth colliery, the deepest in the northern coal field the average temperature ranges from 78° to 80°, and in some parts of this mine it occasionally rises to 89°.

TIN, COPPER, LEAD AND ZINK MINES.

(Cornish District.)-The natural temperature of the mines in the south western district increases so rapidly, that at the depth of 200 fathoms from the surface it varies

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from 81.2 degress to 85-6 degrees Fahrenheit. When the work is in progress, there is, of course, a rapid exchange of oxygen for carbonic acid, by means of the respiration of the miners, and the burning of the can. dles; and when the blasting takes place, the gases generated by the explosion of gunpowder are diffused, and a thick smoke fills the shaft.

IMPROVED APPLICATION OF THE SCREW TO STEAM NAVIGATION.

Sir, I make free herewith to send you some sketches of an application of the screw to steam-vessels, which I have already submitted to my friend, Mr. John Seaward, and to others, but on which I should like to have the opinion of some of your intelligent correspondents. The sketches speak for themselves,

and possibly exhibit nothing new. The shaft placed sufficiently low is meant to move two screws placed longitudinally beneath the water line in midships. Sponsings or other contrivances would protect them, when in rivers or port, from outward injury. The screw thus applied, one would think, must prove more efficient, than at the stern of the vessel, where, besides other difficulties, a considerable vacuum has been found to deprive it of much of its power.

I am, Sir, your obedient servant,
FREDERICK SCHERR.

Kew Green, May 6, 1842.

OPTICAL PHENOMENA.

Sir,-Perhaps some of your scientific readers will explain the following phenomena which I have observed.

1. When a piece of sheet zink is slightly wetted with dilute nitric acid and the finger drawn over the plate a most beautiful transparent blue colour presents itself, similar to the fine variety of Prussian blue? I have always observed the same phenomenon under the same circumstances.-2. How is it that a reflected image of a candle flame is seen on the carbonaceous film deposit on smoked glass, or any other object, when viewed at particular angles? I am, &c.

Newark.

K. DALTON.

BLASTING ROCK-SAND TAMPING-GALVANIC BATTERY.

Sir, In a description of Roberts's galvanic apparatus for blasting rock, in No. 978 of your Magazine, I find an important error promulgated with a tone of authority that may lead to a continued perseverance in a very injudicious practice; I allude to the arguments in favour of tamping with dry sand.

It is stated that Mr. Roberts "believes that many hundred weight of gunpowder would be required to blow out a column of sand 2 inches in diameter, and 18 or 20 in. in depth, placed in a solid rock."

Now I will venture to assert, that one ounce of gunpowder under such circumstances would blow out every particle of the sand. The experiment may be easily tried by any one. This mistake has long been current, and arises from assuming, as fact,

a principle that has been deduced from inference. The enormous resistance to the pressure of a volume of sand through a cylinder or tube is well known; owing to the particles by pressure becoming thoroughly wedged against the sides of the tube; but the effect of the explosion of the powder is to penetrate through all the interstices, and to separate the particles instead of wedging them together.

With respect to firing charges by a galvanic apparatus, it may be applicable to verg great explosions, but the delicacy of the arrangements, and their want of simplicity for use by common workmen, as well as the expense of its application, including that of the portion that must be destroyed at each blast, would preclude its employment under any ordinary circumstances; its adoption is the less necessary, since so much facility is given to firing charges either under water or otherwise, and such perfect security from accidents obtained by the invention of Bickford's Safety Fuse, an article that cannot be too strongly recommended.

I am, Sir, your obedient servant,

J. F. B.

THE ROLLER SUBSTITUTE FOR WHEELS.

Sir, It may be worthy of notice as a matter of historical curiosity that there was a patent granted as far back as the 3d of William and Mary (1691), to a person of the name of Kendrick Edisbury, for an invention, having precisely the same object in view as that of your correspondent, Mr. George Robinson (p. 386 present vol.). As the practice of requiring enrolment of specifications had not at the date of that patent come into use, it is impossible to say whether Edisbury's invention resembled Mr. Robinson's in its details, but the following extract from the patent itself will show that the result was the same.

"Whereas, Kendrick Edisbury hath by his humble petition represented unto us, that with great danger, and much time spent, he hath invented and found out a new art or invention of certain rollers to be used under the bodies of carriages, carts and waggons instead of wheels, which will be far more useful than wheels, by amending and preserving, as well the highways as private grounds, which said invention was never used in England before, and prayed us to grant him our Letters Patent for the sole use thereof for the term of fourteen years; know ye therefore, that we, being willing to cherish and encourage all laudable endeavours and designs of such our subjects, as have by their industry found out useful and profit

able arts, mysteries and inventions, and that the said Kendrick Edisbury may accordingly reap some fruit and benefit of his labour and charges in and concerning the premises of our especial grace, certain knowledge and mere motion have given and granted by these presents," &c.

It appears further, from Mr. Webster's Patent Cases,* that Edisbury's patent was afterwards the subject of an action for an infringement, when a verdict was given in his favour.

I am, Sir, your obedient servant,

WARDER.

MR. WALKER'S WATER ELEVATOR-CHALLENGE TO THE CENTRIFUGALISTS.

Sir,-In your Number 981, "W. P." states, that with a proportionate power his centrifugal pump will do five times the work of Mr. Walker's machine for raising water. I wish to know whether I rightly understand him to mean that, with the same power applied to each, in the same time, the centrifugal pump will raise five times as much water? I find that Mr. Walker's machine raises one-third more water than my old one with the same power in the same space of time. In fairness, therefore, to all parties, I should like to see this centrifugal pump in action. I do not know how it acts, but if "W. P." will show me that he can at the same expense, or first outlay, affix an apparatus to raise fluids, say 30 feet, in the same space of time (say one hour,) with the same power (say one man), and raise as much as one-tenth more than I do with Mr. Walker's apparatus, then "W. P." may hear of something to his advantage. On his acquainting me (by a note addressed to your care,) when he will be prepared to exhibit his centrifugal pump in action, I shall immediately adopt means to test the comparative efficiency of the two instruments-not by a private, but a public trial in the presence of scientific persons-the results of which, whatever they may be, shall also be made known to all the world. I am, yours, &c.

June 3, 1842.

T. Y.

P.S. I have sent a copy of this to Mr. Walker, who will give my address-and who is quite agreeable to my proposal.

A new work in course of publication. by the learned author of the "Law and Practice of Patents," the best book on the subject which has yet appeared.-ED. M. M.

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WALKER'S WATER ELEVATOR. Sir, I should have answered the letter of "W. P." sooner, but I was waiting the arrival of one of my large machines from Messrs. Ransome's manufactory, which I have now received, and which any of your readers may see if they feel inclined. It is not so outrageous in weight as "W. P." would have us to believe. It is calculated for one or four men, and weighs only 1 cwt. 2 qrs., and will deliver 50 or 200 gallons per minute, according to the size of the tube. "W. P." has so miscalculated and misrepresented an invention that he knows nothing of, that were I to write for one whole year in your Magazine, to prove that it is superior to his friend "Centrifugal," we should, without a fair practical trial, be just where we began, and your readers no better informed. I therefore call upon 16 W. P." to come forward in an honest straightforward manner, and substantiate the challenge which he has made in the Mechanics' Magazine, May 28.

"W. P." may have the choice of height from 10 feet to 50, and the trial to come off within a month, before practical men, for no less a sum than 1007. Perhaps, Mr. Editor, you will favour us with your presence, as the trial must be on the banks of the Thames for the convenience of scientific men residing in London.

When all this has been done, then Mr. "W. P." may reason with certainty, and your readers be in a position to judge for themselves.

I remain, Sir, your most obedient,

ABSTRACTS OF SPECIFICATIONS OF ENGLISH

PATENTS RECENTLY ENROLLED. OGLETHORPE WAKELIN BArratt, of BIRMINGHAM, METAL GILDER, for certain improvements in the precipitation or deposition of metals. Enrolment Office, March 8, 1842.

Mr. Barratt claims

First, a "mode of cleaning copper and its alloys by means of a galvanic or other battery, and also recovering by precipitation the metal dissolved during the process." The article to be cleaned is suspended from a wire connected with the negative metal of the battery, and a plate of copper of a larger superficial area than the article is suspended from the other wire, which plate receives the metal deposited from the solution. "In from five to ten minutes the scale or oxide of copper will be removed."

Secondly, a mode of precipitating zink upon other metals, by passing an electric current through a cold solution of zink in sulphuric acid. "Other acid and saline solutions of zink may be employed, as those in muriatic and acetic acids, and the muriate of ammonia, or the sulphate of zink of commerce, in connexion with the battery; but I have found the former (the sulphuric acid solution) succeed perfectly."

Third, the following improvements in the precipitating of copper upon iron and other metals. 1. He connects the article to be coated by a copper wire to a plate of zink, and immerses them in a saturated solution of sulphate of copper-taking care first to wrap the zink in cloth or strong paper, to prevent any deposition of copper on the zink plate. "I have been able by this arrangement to produce a more perfect contact of the metals, particularly where iron is desired to be coated, than by any of the modes heretofore recommended or described when using acid solutions of copper." 2. He precipitates the copper from solutions of copper in the cyanides and carbonates of potash and soda. These solutions may be used at different temperatures, but the action is most rapid when boiling. And, 3. When he wishes to convert the copper coating into brass, he deposits zink upon the copper surface first obtained, (by the process described under the second head of his improvements,) and then subjects the article to a heat of about 300° Fahr. in a muffle or other convenient apparatus.

Fourth, the precipitation of platinum and palladium from their solutions, and depositing them as coatings on other metals, either by immersing a plate of zink or some other positive metal in the solutions - "taking the precaution to have free acid present,'

or by dissolving the platinum or palladium in muriate of soda, (twelve parts by weight,) alum, (two parts,) and cream of tartar, (one part.) "Copper, iron, and other metallic articles introduced into the (latter) solution when boiling, are speedily coated; if & stronger coating be required, I attach the battery, and an anode of metallic platinum, and continue the action till the desired thickness be obtained."

Fifth, the precipitation of gold, silver, or platinum, upon other metals, by employing boiling solutions of these metals in hydrate of potash, with or without the aid of the galvanic battery.

And Sixth, the precipitation of metals in a state of alloy, "by obtaining them in solution, and using an anode of an alloyed metal in the same proportion as in solution."

Mr. Barratt also describes, but does not claim, the following mode of "precipitating copper from waters of copper mines, and other waters containing copper." "I form pits (similar to those commonly used, but deeper) to hold the solution of white copper, or other metal. I place in the solution a porous vessel of earthenware, or divide the pit by any convenient porous material; into the porous vessel, or the (parts of the) pit partitioned off by the porous material, I put a solution of muriate of soda, and into the solution I place the iron, and connect it with wires with sheets of copper, lead, or other metal in the solution of copper, and which are to receive the deposit of pure metal. The solution, after all the copper is precipitated, will serve to dissolve the iron in the porous cell, instead of the muriate of soda."

EDWARD FRANCOIS JOSEPH DUCLOS, OF CLYNE WOOD METALLURGICAL WORKS, SWANSEA, for improvements in the manufacture of copper.-Enrolment Office, May 11, 1842.

The first improvement specified consists in calcining all sulphurous ores of copper, technically called sulphurets, and other artificial products of the same nature in large kilns, whereby a large portion of the coal and labour required in the ordinary treatment are economized; and the sulphurous acid gas and sulphuric acid formed in the operation, irretrievably lost in the ordinary method, are made applicable to the manufacture of sulphur or sulphuric acid.

Secondly. When the ores have been sufficiently calcined to treat them for the reduction of any metallic oxides they may contain, in a blast furnace similar to those used in the manufacture of iron, with the addition of chambers of condensation and feeding apparatus,—such additions being