mortar battery, the pits of which are large and open to the rear, the booths were placed on the opposite side of the railroad track. The fundamental idea of the design was to transmit ballistic data visually. This is done by means of sliding blackboard panels supported in an iron frame, one-half of which projects inside, the other half outside the booth. As shown on the drawing there are five such panels, upon which are written in proper order, beginning at the topmost panel, the several distinct portions of the data, as received over the telephone or telautograph. By using panels instead of a solid board, each bit of information is made available as soon as received. To make the whole of the panel board visible from every part of a pit, each booth was located off toward one side of the pit, or its walls placed oblique to the rear wall of the battery. For night service each panel board is illuminated by two electric 16-candlepower lamps with protecting hood and reflector. As shown, the attempt has been made to give light by day through skylights. These are of doubtful utility and are difficult to make water-tight. I should omit them in future constructions. The illustration shows two peepholes in one corner of the booth, closed by heavy porthole fixtures on the inside. These openings overlook every portion of the corresponding pit. During drill and target practice a megaphone was placed in one of these openings and the data shouted to the pit as soon as it was received, and thus made available before it could be written on the panel boards. The latter were always used to check the data received through the megaphone. The walls, floors, and ceilings were strengthened by bars of steel and have developed no cracks during the target practice. As far as I have learned the booths gave general satisfaction. POROUS HOLLOW-TILE WALLS AND CEILINGS IN 6-INCH BATTERY. The walls are 6 inches in thickness and separated by a space of 4 inches from the adjacent mass of concrete. The bricks in the walls are the ordinary 4-duct hollow brick made smooth on the exposed faces and scored only on the mortar faces. The roof is of hollow brick known under the trade name of "Herculean Arch." To guard against possible leakage through the mass of concrete overhead, the roof is covered by a layer of Paroid roofing paper, laid shinglewise, the arch being given a slight slope for drainage. The order of construction was as follows: Concrete walls first; next, the hollow-tile walls; then the herculean arch. The concrete overhead is supported on concrete beams strengthened by twisted steel. Wherever practicable the 4-inch space around the hollow brick walls and ceilings was made to connect through openings with the outside air. These openings are permanently closed by iron gratings. In addition to the 4-inch air space above referred to there is a gallery 2 feet wide extending from one flank to the other around and in front of all the rooms of the battery. There has been no condensation whatever on the walls, but there has been evidence of moisture on the ceiling where the brick were hard-burned and not very porous. The arch brick has to be burned harder than is desirable in order to give it the desired strength, which renders it not so suitable as a lining for magazines as the ordinary hollow porous partition brick. On the whole, however, the ceilings |