In order to illustrate the importance of iron among other metals and non-metallic products of mines, it is necessary to condense the yearly statistics of the total mining production of the world. Statistics of this kind have been given by several writers, but none of them can be said to be strictly unobjectionable. It is even difficult to obtain the ever-changing figures from those states in which statistical records on mining are kept and collected regularly, and with the utmost care; and from countries where statistics are neglected, only approximative figures can be secured. During the last thirty years, I have myself taken a lively interest in these figures. As a member of the jury of the metallurgical department of all the international exhibitions, I was favored with the best opportunities for obtaining the most accurate information upon the subject that could be secured. I now publish the following synoptical table, the figures of which are chiefly transcribed from records of the years 1861-5 as a result of my researches and observations : MINING PRODUCE OF THE WORLD IN APPROXIMATE FIGURES, EXPRESSED IN GERMAN CWTS. These figures by themselves do not prove the relative importance of iron-. In order to form a right idea of its real value, they must be converted into figures, comparable among each other by means of reduction to a mathematical standard, which can be easily understood all over the world, that is, with the money value of the produce mentioned in the table above. With respect to the precious metals, the average value is nearly equal in all countries; there is, however, a vast difference of prices for the common metals, the salt and mineral fuels in the vaiious countries. The price of the German cwt. of anthracite and lignite averages in the various countries from 5 to 50 kreuzers, silver Austrian currency (the tun from 50 cents to $5 gold). In order to determine the average price of coal, the price of the English coal may be considered as decisive as representing more than half the aggregate production In fkw of this fact, 20 kreuzers per cWt* ot two dollars gold per iuh Can be asi&umed as the average pride of coal. tf& igureg lor ilxe iron nv&tlf all rgfar %n the wMght f pig and cast iron; but the work for the smelter and metallurgist does not end here; the pig iron is transformed into wrought iron and steel, and for this reason the value of the cast and bar iron, and the various kinds of steel, must be taken into consideration. The more developed the industry of a country is, the greater will be the demand for iron in general, and more particularly for cast iron. Most frequently the demand of cast iron varies between one-fifth and one-third of the whole iron consumption, and the cost of cast iron ware can be rated at the average price of five florins (one tun at $50 gold). The price of bar iron varies between three and fifteen florins, but the real average can, at most, be rated at five florins, the price of the common English kinds being decisive in fixing the standard. The manufacture of steel has increased considerably during the last few years; formerly it was one-fifteenth, now it has probably reached to one-tenth of the bar iron production. The cwt. of steel varies from six to thirty florins per cwt. (or from $60 to $300 per tun), but the average may be fixed at ten florins ($100) per tun. In yiew of this great variation of the kinds of iron and the consequent variation in the prices of the same, and considering the loss in the weight which is consequent upon the transformation of the pig iron to cast-iron ware, and of bar iron into steel, the price of four and a half florins per cwt. ($45 per tun) appears to be a fair average for this metal. Attention may be called to the fact that the anthracite and lignite, used in the smelting of iron and steel, have to be deducted from the whole production of coal, but the deduction will be, instead of five cwts of coal for every one hundred pounds of iron (which is the actual amount of coal required for the smelting), on account of the partial uso of vegetable fuel, only three to four cwts. for every one cwt. of iron. The pound of gold (German mint pound) commands the price of 675 florins; the pound of silver (German mint pound) 45 florins. Copper, at the mines, costs 50 to 60 florins (average 57), because the better brands predominate. Lead varies between 10 to 15 florins per cwt., average 12 florins. Zinc varies between 5 to 7 thalers; average 6 thalcrs, or 9 florins per cwt. Among the other metals, which are not quoted in the above table on account of their minor significance, the mercury may be considered as the most important; then tin, platinum, antimony, nickel, etc. Their yearly production may scarcely exceed in value the sum of thirty million florins, or one hundred and fifty million dollars gold, The Value of Brains, Working as an ordinary hand in a Philadelphia shipyard, until within a few years, was a man named John L. Knowl-ton. His peculiarity was that, while others of his class were at ths ale houses, or indulging in jollification, he was incessantly engaged in studying upon mechanical combinations. One of his companions secured a poodle dog, and spent six months in teaching the quadruped to execute a jig upon his hind legs. Knowlton spent the same period in discovering some method by which he could saw out ship timber in a beveled form. The first man taught his dog to dance—Knowlton, in the same time discovered a mechanical combination that enabled him to do in two hours the work that would occupy a dozen men, by slow and laborious process, an entire day. That saw is now in use in all the shipyards of the country. It cuts a beam to a curved shape as quickly as an ordinary saw-mi]] saw rips up a straight plank, Knowlton continued his experiments. He took no part in parades or target shootings, and in a short time afterwards he secured a patent for a machine that turns any material what' ever into a perfectly spherical form. He sold a portion of hie patent for a sum that is equivalent to a fortune. The machine was used cleaning off cannon balls for the Government. When the ball comes from the mold the surface is incrust-ed, and the ordinary process of smoothing it was slow and wearisome. This machine almost in an instant, and with mathematical accuracy, peels it to the surface of the metal, at the same time smoothing out any deviations from the perfect spheroidal form. The same unassuming man has invented a boring machine, that was tested in the presence of a number of scientific gentlemen. It bored at the rate of twenty-two inches an hour, through a block of granite, with a pressure of but three hundred pounds upon the drill. A gentleman present offered him ten thousand dollars upon the spot for a part interest in the invention, in Europe, and the offer was then accepted. The moral of all this is tliat people who keep on studying are sure to achieve something. Mr. Knowlton doesn't consider himself by any means brilliant, but if once inspired with an idea, he pursues it until he forces it into tangible shape. If everybody would follow copy, the world would be less filled with idlers, and the streets with grumblers and malcontents. TAE FRENCH ATLANTIC GABLE.—The manufacture of the French Atlantic cable is rapidly approaching completion. Up to the 14th of April the total length manufactured was 3,034 nautical miles—about 2,214 miles of the section intended to be laid between Brest and St Pierre, and 716 miles of the section between St. Pierre and the United States. Only 474 miles of the former section and 57 of the latter remain to be completed, The whole iength of the core for both sections was finisned April 15th; at the Gutta Percha Works. The Great Eadefn has taken on beard 1,750 miles of the first section, and *h teamei* BcdMeHa 450 wiles of thfc Second section. Improved Self-Holding Adjustable Plow. The object of this device, as stated by the inventor, is to provide a simple and convenient arrangement for adjusting plows to the varying width and depth of the furrows, as may be required. Two views are shown in the accompanying il-lustrations,one exhibiting one side,and the other the opposite side of the plow with the truck attachment. The plow itself is an ordinary plow, such as is generally used,the attachment being capable of application as well to plows now in use as to those which may be built to receive the device. This itself is very simple : it being only two wheels of different diameters, on independent axles, the larger one to run in the furrow already made, and the smaller one to run on the untouched surface. By'this contrivance any required depth and any required width of furrow may be assured, and the share made to take and sustain any angle. On the plow beam, in front of the share, are bolted two plate sockets, one on each side, the holes in the sockets being square and vertical. In one, the shank of the bent axle of the small wheel fits, and is secured to any position by a set screw in the sleeve or socket. The other receives a bar similarly secured, the lower end of which embraces the straight axle oi the large wheel. At the end of this axle is a slotted arm the lower end of which embraces the horizontal portion.of the small-wheel axle, while a bolt passing through the slotted arm and the end of the large-wheel axle, serves to hold both axles in position. By these arrangements either wheel is made capable of vertical adjustment, and the large wheel may be also adjusted horizontally to govern the width between the furrows. The relative positions of the two wheels may be changed to adapt them to a right hand or left-hand plow. Both the uprights are provided with marked scales for adjusting the depth of the furrow. According to the inventor, a plow with this device is self-holding, the driver needing only to attend to his team; any one who can drive a team can plow better than the best plowman with the ordinary plow, without the truck; an equal furrow in depth, width, and direction; the plowshare being self-sharpening as its point is kept always level; the draft lighter, and thus the labor less on the team—the truck bearing the load usually borne by the horses; the weeds being turned under and held by the large wheel and axle until covered, and other minor advantages evident to the practical reader without special notice. Patented through the Scientific American Patent Agency March 2d, 1869. State and manufacturing rights for sale by the inventor, Joseph Clees, or J. N. Clees, Nashville, Tenu. Solid Emery Grinding and Polishing Wheels. Solid emery wheels have lately come into very general use for grinding and polishing. When well made they wear evenly and cut rapidly, and as they require no redressing, but last until entirely worn out, they are rapidly superseding the old-fashioned wooden wheel coated with emery, and even usurping some of the functions of the ordinary grindstone. The engraving presents a perspective view of a machine for carrying one or two of these wheels, fixed on the same shaft and driven by the same belt. A stand supports two bearings with their boxes, in which runs a shaft carrying, in the space between the boxes, a pulley, and on its ends solid emery wheels. A slotted projection at the base of either bearing receives an ordinary rest, Such as is used on a lathe for hand turning, that is held in position by a nut and bolt. The machine is bolted to a bench at any convenient point. The machine can be used for grinding tools of every description, is a great saver of files in reducing and polishing surfaces, and does the work in either case much more rapidly than can be done on the grindstone. Parties having them in use commend them in the highest terms. The wheels used are those manufactured by the Tanite Company, Stroudsburgh, Pa. For further information, address American Twist Drill Company, WooEsocket, E. I* Cleaning the Exterior of Buildings. This question, says the Mechanics' Magazine, has been recently taken into reconsideration by our Gallic neighbors, and toward the end of last year, an order was issued by the Prefect that the fagades of all dwellings in the 3rd, 4th, 9th, and 10th divisions (arrondissements) of Paris should be periodically cleaned, the law to take effect on and after May 1, of the present year. So far back as 1852 there was a law promulgated to the same intent, but its injunctions have been so frequently neglected that the authorities have thought it requisite to call Drominent attention to it bv issuinerwhatmiffhtbe termed anew edition. The old act ran as follows: "The fagades of houses are to be kept in good repair. They are to be rubbed, plastered, painted, or the surface either renewed by cleansing in some manner or another, at least once in every ten years, at the expense of the proprietor. A noncom-pliance with this regulation will subject the offending party to a fine not exceeding 5. Although the legislation thus insisted on the general principle, the particular modus operandi, or means of putting the principle into execution, was left altogether to the discretion of the owner. The favorite method which has been successfully practiced for the last two years, is that of cleaning the walls by the employment of a jet of water projected under steam pressure. There are many advantages attached to this plan of proceeding. It not only restores the fagades to their original appearance, but it does not injure the more delicate, decorative, and ornamental portions of the building, neither does it destroy the then protecting coat which the stone has received from the influence of the atmosphere. By this method we insure the fulfillment of several valuable conditions. First, cheapness; second, the preservation* of the more fragile and sculptural work upon the edifice; and, third, universality of application. If, in addition to the enforcement of some regulation of this description, with respect to the buildings in our principal streets and thoroughfares, those in our narrow courts and alleys were brought under the same jurisdiction, the result, in a sanitary point of view, might not be inconsiderable. It has been calculated that were the exterior of the buildings in London kept in a clean fresh condition, instead of being nearly black from top to bottom, there would be a gain of nearly half an hour's daylight in every twenty-four hours. Collecting and Utilizing Sewage. The two main points in the sewage question are, the effectual removal of refuse and faecal matter from our dwellings, and its efficient utilization upon our lands. Upon these points there exists a great variety of opinions, some ad- vocating one method of removal and utilization, and some another. Into the various methods proposed, suggested, oi in use, we need not here enter; they are sufficiently well known to all who know anything at all about the matter, We point to our Metropolitan main drainage as a sufficient answer to the first point in question, and to the Croydon irrigation works as an equally sufficient answer to the second. But it may be said that our cities and towns are not Londons or Croydons, and that many clusters of habitations are neither cities nor towns. Then naturally follows the question of how to deal with a limited amount of sewage ? Of course, everybody will say, there are many ways of doing that. We admit there are, and we will now point out one of them, the most recent that has come under our notice. This is the system of M. t)elbriel, which was explained to a meeting of gentlemen practically interested in the sewage question on the evening of 7th April.' The meeting was held at the Inns of Court Hotel, Holborn, the Duke of Castellucc io in the chair. M. Delbriel's system of collecting and utilizing sewage is better-known in France, where it is practically applied in several places as vidance a vapeur. It consists in using a traction en-gine,to which are attached tanks, into which the sewage is pumped by the engine. During the extraction of the sewage, the mouth of the cesspool is covered with sailcloth steeped in sulphate of zinc. The mephitic vapors are drawn off from the tanks by means of pipes which communicate with the engine furnace in which they are burned. By these means, it is affirmed that no unpleasant smell or noxious vapor ever finds its way to the air, while the sewage in the highest condition for fertilizing purposes does find its way on to the lands of the farmers. Depots are established where the sewage is deposited, and from which it is distributed to the farmers. Or otherwise it is supplied on to their lands direct from the tanks. In all this there sounds to our ears— who have been accustomed to a widely different dealing with the same question—a return to the old cesspool system, the engine doing duty for the horse and men of the old night cart. But it must be borne in mind that with all our sanitary progress there are yet many spots in Great Britain where the system would be a great boon, and to these M. Delbriel proposes its application. That it has proved a great success in France is due to the very different sanitary and agricultural conditions of that empire as compared with the United Kingdom. On the whole, M. Delbriel's system is well worthy of consideration, and, therefore, we subscribe to the following resolution, which was passed at the meeting in question : " Considering the present great waste of the sewage of towns, etc., and the necessity of diverting it from rivers and streams, and the value of applying it to the purposes of agriculture, this meeting is of opinion that M. Delbriel's system is worthy the attention of the public, and more especially all persons interested in this important question, and that it is desirable that M. Delbriel should issue a translation of his pamphlet."—Medianics1 Magazine. The Auroral Currents. We are asked if the currents produced on the wires during these displays are atmospheric, acting direct from these auroral phenomena, thus irradiating the heavens, and which weave their triumphal coronas up apparently among the planets ? Although there are, unquestionably, large masses of electric clouds sailing in the upper regions of the air during the presence of these auroral displays, yet the fact that all, or nearly all, interference from the currents then exhibited can be prevented by simply using two wires instead of the earth and wire, proves that these currents are caused by a dis-turbanca of of the earth's normal electric state. The earth's ordinary electric tension is disturbed, and its currents are, so to speak, scattered by this induced current from the vast masses of electricity in the sky, but are ever seeking, by the violent action peculiar to them, to restore themselves to their normal condition, thus causing temporary electric currents of great power and rapid changes of tension. Thus they enter a wire from one earth connection in this effort at restoration, and are chased back by another from the opposite extreme, exhibiting the violent and changeful currents which mark these magnetic storms. The earth, itself, is a great reservoir of electricity, offering no sensible resistance to the entrance of electrical currents, yet varying in its electric tension or condition at different points. This causes an almost ceaseless action of the earth's currents, and at almost all times they can be felt upon the wires which they use to effect the equalization of their tension. During the auroral displays this action is excessive. At the same time it can scarcely be regarded as incorrect to say that it is the induction of vast volumes of electricity from the upper air which causes these extraordinary currents which, as we have seen, can be utilized and harnessed for human service; and as a line can be worked by any polarity, provided the whole wire is worked with a like polarity, the changing currents do not prevent the line from being operated during the violent contest for the supremacy of the one current or the other.—Journal of the Telegraph. MATTER and motion constitute the visible universe. Improved Low-Water Steam Port. Ever since the invention of the steam engine, the attention of scientific men has been directed to the discovery of means to guard against the danger of low water in boilers. In consequence of the liability of supply pumps to become foul or defective, this danger is always imminent. A great number of devices have been tried, but nothing heretofore discovered was so eminently practical as to become a necessary appendage of the steam engine and an essential of every first-class boiler. The void so 1 ong existing is now claimed to be filled by Cochrane's low-water steam port, constructed in accordance with principles of natural philosophy, well understood, and therefore always uniform in action. A valve is made, composed of a spindle and piston united (10,12). The latter is hollow, so as to make the specific gravity about the same as that of water. A chamber is constructed (8) in which the piston moves freely. The valve seat (9) in the head of this chamber is closed, as the valve rises, by a bulb (11) on the spindle. A tube (7) extends from the bottom of the chamber to low-water mark in the boiler. When there is a sufficiency of water, the steam forces it up the tube and fills the chamber. This sustains the piston and the pressure of steam upon the spindle and closes the valve. On the other hand, when the water is below the opening of the tube, the chamber is filled with steam instead of water, and the weight of the piston causes the valve to descend and open, allowing the steam to escape. The action of this simple steam port is just as certain as the laws of nature on which it rests. It alwaysgives timely notice of low water, and continues the warning till the boiler is supplied. The engineer will be greatly relieved, as it performs perfectly and constantly one of his most important and onerous duties. It does not merely act at the point of danger, but gives information in time for pumping to begin. Hence the boiler may always be worked with safety at the minimum of water, and with corresponding economy of fuel. Should this invention be the means of guarding against all danger from low water, its general use will mark an era in the history of the steam engine. The inventor is J. C. Cochrane of Rochester, N. Y., who has secured patents in the United States and Europe. It is probable that the United States patent will be placed in a stock manufacturing company, either in New York or Boston, unless superior advantages are presented elsewhere.