The grooves are of two types. Those for the Parrott and converted guns are of the plain rectangular type, the rule for their number and size being that they shall be uneven in number to bring a land opposite a groove; lands and grooves shall be equal in width, and grooves for all calibres shall be the same depth—¹/₁₀₀ of an inch. Increasing twist. The Dahlgren type of groove is similar to the modified French groove (see plate, page 187), there being three only of a regular twist. These grooves are found only in the Dahlgren muzzle-loading bronze boat-guns. [Illustration: Parrott Groove.] [Illustration: Dahlgren Groove.] CARRIAGES. Naval gun-carriages are built of plate-iron, although there are still a few wooden carriages and slides for pivot-guns. Breechings are used with broadside-carriages, except in the directing-bar type. _Boat-Carriages._ As yet the designs for an iron boat-carriage for the breech-loading gun are not finished. The wooden carriage in use consists of a double wooden slide, the bed, or lower slide, having pivot-centres in front and rear and being slotted through the centre for the compressor. The upper slide traverses freely on the lower one, having a piece projecting from its lower face into the slot in the lower slide to act as a guide. A second heavy piece, used as a compressor, travels along the lower face of the slot, and is held in position by screw-handles working on threaded shafts which are tightened or loosened readily by hand. The upper slide holds the trunnion-rests, which are made in a single casting with a bottom plate to bolt on the slide. [Illustration: Wooden Boat-Carriage.] _Field-Carriages._ [Illustration] [Illustration] The field-carriages are of two types, both of iron. That for the muzzle-loader consists of a single bar to which the axle is bolted and stayed. To its lower end is bolted an iron box for a trail-wheel and a socket for a directing handspike. A small seat for the elevating-screw is bolted at its middle on top, and at the forward end is a light iron frame for holding two ammunition-boxes. The field-carriage for the breech-loader consists of two plate-iron brackets connected by the axle and three transoms. The lower part expands into a box for the trail-wheel, and the end is finished in a loop for a drag-rope. At the forward end is a lug for attaching a caisson, and a frame on the axle outside of the brackets on each side for ammunition-boxes. The elevating-screw works in a pivoting screw-box. It will be noticed that the breech-loader has trunnions, while the muzzle-loader has only a lug, making the forward ends of the two carriages different in construction. [Illustration: Iron Field-Carriage for Muzzle-Loaders.] _Broadside Marsilly Carriage._