The annexed engravings illustrate a method of constructing subaqueous foundations by the use of iron pile planks.  These latter, by reason of their peculiar form, present a great resistance, not only to the vertical blow of the pile driver (as it is indispensable that they should), but also to horizontal pressure when excavating is being done or masonry being constructed within the space which they circumscribe.  Polygonal or curved perimeters may be circumscribed with equal facility by joining the piles, the sides of one serving as a guide to that of its neighbor, and special pieces being adapted to the angles.  Preliminary studies will give the dimensions, form, and strength of the iron to be employed.  The latter, in fact, will be rolled to various thicknesses according to the application to be made of it.  We may remark that the strength of the iron, aside from that which is necessary to allow the pile to withstand a blow in a vertical direction, will not have to be calculated for all entire resistance to the horizontal pressure due to a vacuum caused by the excavation, for the stiffness of the piles may be easily maintained and increased by establishing string-pieces and braces in the interior in measure as the excavation goes on.

[Illustration:  Fig. 1.—­Construction of A dock wall behind PAPONOTS iron pile planks.]

The system is applicable to at least three different kinds of work:  (1) The making of excavations with a dredge and afterward concreting without pumping out the water. (2) The removal of earth or the construction of masonry under protection from water (Fig. 1). (3) The making of excavations by dredging and afterward concreting without pumping, mid then, after the beton has set, pumping out the water in order to continue the masonry in the open air.  This construction of masonry in the open air has the great advantage of allowing the water to evaporate from the mortar, and consequently of causing it to dry and effect a quick and perfect cohesion of the materials employed.

[Illustration:  Fig. 2.—­Traverse section of two piles connected by mortar joints.]

This system may likewise be employed with advantage for the forming of stockades in rivers, or for building sea walls.  A single row of pile planks will in many cases suffice for the construction of dock walls in the river or ocean when the opposite side is to be filled in, or in any other analogous case (Fig. 1).

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.