As the apron is composed of two pieces scarfed together, and used to support she scarf of the stem, it is necessary that the scarf thereof should be at some distance from that of the stem. It is formed of the same thickness with the heel of the stem; but its thickness is equal throughout. Sometimes the piece immediately under the apron forms a curve, of which the horizontal part covers the dead-wood, whilst the vertical part corresponds with the inside of the stem, to which it is fayed, making the commencement of the apron. [Illustration: _Naval ARCHITECTURE_ _PLATE. I._ ] APRON, _platine_, is also a square piece of lead fastened over the touch-hole of the cannon, to keep the charge dry at sea or in rainy weather. _Naval_ ARCHITECTURE, or the science of ship-building, comprehends the theory of delineating marine vessels upon a plane; and the art of framing them upon the stocks, according to the proportions exhibited in a regular design. All edifices, whether civil or military, are known to be erected in consequence of certain established plans, which have been previously altered or improved till they have arrived at the desired point of perfection. The construction of ships appears also to require at least as much correctness and precision as the buildings which are founded upon _terra firma_: it is therefore absolutely necessary that the mechanical skill of the shipwright should be assisted by plans and sections, which have been drawn with all possible exactness, examined by proper calculations, and submitted to the most accurate scrutiny. _Naval_ ARCHITECTURE, or ship-building, may be distinguished into three principal parts. First, To give the ship such an exterior form as may be most suitable to the service for which she is designed. Secondly, to give the various pieces of a ship their proper figures; to assemble and unite them into a firm, compact frame, so that by their combination and disposition they may form a solid fabric, sufficient to answer all the purposes for which it is intended: And, Thirdly, To provide convenient accommodations for the officers and crew, as well as suitable apartments for the cargo, furniture, provisions, artillery and ammunition. The exterior figure of a ship may be divided into the bottom and upper-works. The bottom, or quick-work, contains what is termed the _hold_, and which is under water when the ship is laden. The upper-works, called also the dead-work, comprehend all that part which is usually above the water when the ship is laden. The figure of the bottom is therefore determined by the qualities which are necessary for the vessel, and conformable to the service for which she is proposed.