The shaft is held aft by a clutch placed just forward of the elevating-wheel. The clutch consists of a sleeve, supported on trunnions by a bearing bolted to a chock on the forward deck of the boat, carrying a yoke-link, loosely bolted to two lugs on its forward lower end and a detaching-lever, loosely bolted to two lugs on its forward upper end. The worm-shaft, passing through this sleeve, bears a rigid collar so placed that, when the shaft is aft, the collar is close up against the forward end of the sleeve. The yoke-link, when swung up, embraces the shaft and bears against the forward side of the collar, holding the shaft aft. The yoke-link is held up by the detaching-lever which is thrown forward between the upper ends of the yoke-link, a transverse roller in the detaching-lever, with ends projecting on either side, locking the yoke-link in place. A pin, passed through eyes worked in the upper ends of the yoke-link, prevents accidental tripping of the detaching-lever. When this pin is withdrawn and the detaching-lever is pulled aft the yoke-link falls and the shaft is free to move forward. By the worm-shaft and its attachments the elevating-arm can be rotated around the cross-beam, held in any position in its plane of rotation, or released at any desired moment. The gear and worm are protected by hoods. BOAT SPARS. =Pattern A.—Plate VII.= The spar, made of steel, consists of two tubes, 18 and 15 feet long respectively, one 4 inches and the other 3½ inches in diameter, joined together with a telescopic joint. The tubes—with a lap of 2 feet—are held together by two screws. At the larger end of the spar is an eye-bolt for the heel-rope screwed in from the inside, and at 5 feet from the smaller end is a hole for reeving the spar leading-wires. This constitutes the main spar, which can readily be taken apart for stowage by removing the screws. To assemble the spar, a feather fits into a score on the end of the larger tube, bringing the screw-holes opposite each other. Iron spar-clamps are furnished, to facilitate the assembling of the spar. The two tubes composing each spar are marked by similar letters or numbers. =_Note._=—Spars must invariably be taken apart after use, joints lubricated, and protected by a canvas cover. DETONATORS. =Plate VIII.= Detonators are cylindrical copper cases, closed at the bottom, containing 35 grains of fulminate of mercury, primed on top with dry, pulvurulent gun-cotton. A plug, made of 1 part of ground glass and 2 parts of sulphur, melted together, is cast around the detonator-legs:—tinned copper wires, No. 20 A. W. G., 6 inches in length, insulated with a double layer of cotton thread soaked in paraffine, the outer layer colored red. The inner ends of the detonator-legs are bridged by a platinum-iridium wire, 90% platinum, 10% iridium, ³/₁₆ inch long and 2 mils in diameter, having a resistance of .65±.03 ohm.