The piles are driven by means of the ordinary apparatus in use. Their heads are covered with a special apparatus to prevent them from being flattened out under the blows of the pile driver. They may be made in a single piece or be composed of several sections connected together with rivets. They are designed according to circumstances, to be left in the excavation in order to protect the masonry, or to be removed in their entirety or in parts, as is done with caissons. In case they are to remain wholly or in part in the excavation, they are previously galvanized or painted with an inoxidizable coating in order to protect them and increase their durability. The points of the piles, whatever be their form and arrangement, are strengthened by means of steel pieces, which assure of their penetrating hard and compact earth. [Illustration: FIG. 3.--DREDGING WITHIN A SPACE CIRCUMSCRIBED BY IRON PILE PLANKS.] Fig. 2 represents a dredge at work within a space entirely circumscribed by pile planks. Here, after the excavation is finished, beton will be put down by means of boxes with hinged bottoms, and the water will afterward be pumped out in order to allow the masonry to be constructed in the open air. Fig. 3 shows a transverse section of two of these pile planks united by mortar joints. This system is the invention of Mr. Papenot.--_Revue Industrielle._ * * * * * AN ATMOSPHERIC BATTERY. Great ingenuity is being shown in the arrangement of new forms of primary batteries. The latest is that devised by M. Jablochkoff, which acts by the effect of atmospheric moisture upon the metal sodium. A small rod of this metal is flattened into a plate, connected at one end to a copper wire. There is another plate of carbon, not precisely the same as that used for arc lights or ordinary batteries, but somewhat lighter in texture. This plate is perforated, and provided with small wooden pegs. The sodium plate is wrapped in silk paper, and pressed upon the carbon in such a manner that the wooden pegs penetrate the soft sodium. For greater security the whole is tied together with a few turns of fine iron wire; care being taken that the wire does not form an electric contact between the sodium and the carbon. The element is then complete, the carbon and the small copper wire being the electrodes. The sodium, on exposure to the air, becomes oxidized, forming caustic soda, which with the moisture of the air dissolves, and drains gradually away in the form of a concentrated solution; thus constantly exposing the fresh surface of the metal, which renders the reaction continuous. The price of the element is lower than would be expected at first sight from the employment of so expensive a metal. The present cost of sodium is 10 frs. per kilogramme; but M. Jablochkoff thinks that on the large scale the metal might be obtained at a very low figure. The elements are grouped in sets of ten, hung upon rods