The Livens smoke drum was designed for use with the 8-inch Livens projector, so as to produce a smoke screen of large volume and long duration at long ranges. The drum, as adapted for combustion smokes, weighs 17.5 pounds empty and 49 pounds loaded. The smoke-gas mixture was specially adapted for use in the Livens drum. [Illustration: FIG. 96.—Livens Smoke Bomb.] Smoke mixtures in Livens were never used to any considerable extent in the war and it is questionable if they ever will be. A Livens can usually only be fired once before resetting, hence Stokes mortars are used whenever possible. SMOKE FUNNEL The smoke funnel was developed for the production of a white smoke cloud from the stern of a vessel. The smoke producing materials are liquid ammonia and silicon tetrachloride, with carbon dioxide as a compressing medium. This is the most satisfactory compressing medium, because: (1) The silicon tetrachloride is forced out at nearly constant pressure. (2) The carbon dioxide is easily compressed to a liquid and can be handled in this form. Further, it has a vapor pressure of 800 pounds at 60° F., and a cylinder can be nearly emptied without loss in efficiency. (3) Carbon dioxide is sufficiently soluble in silicon tetrachloride to cause the latter to effervesce and thus materially aid in its evaporation on spraying. (4) Liquid carbon dioxide, behaving in a manner similar to liquid ammonia, affords a means for the silicon tetrachloride to “keep pace” with the ammonia, under changes in temperature, and thus ensures a more nearly neutral, and therefore the most effective, smoke. [Illustration: FIG. 97.—Navy Smoke Funnel.] The smoke funnel proper consists of an open end cylinder, about 2 feet in diameter and 7 feet long, mounted in a horizontal position on an angle iron frame. At one end is an 18-inch fan securely fastened to the cross supports. This fan is operated by hand, through gears giving a ratio of about 30 to 1. The silicon tetrachloride enters the cylinder through a pipe, which terminates in four spray nozzles, while the ammonia enters through a single nozzle. The air forced into the funnel serves to hydrolyze the silicon tetrachloride and mixes the vapors. The resulting reaction evolves a dense white cloud of very large volume and high obscuring power. One set of cylinders is capable of maintaining this cloud for over 30 minutes. Under normal conditions the discharge is at the rate of 2 pounds of silicon tetrachloride to 1 pound of ammonia. To stop the smoke, the silicon tetrachloride is closed first, the ammonia allowed to run about half a minute, and the fan is shut off last. [Illustration: FIG. 98.—Navy Smoke Funnel in Operation.] SMOKE KNAPSACK