A barril of this sort is an assembly of seven trompes, in which each sheath is mounted on a disk with a screw, as has been described for the fire pots, by which it is attached to a base of wood having a diameter equal to that for an assembly of seven trompes. (PL VIII, figs. 8 and 9.)
The barrils are formed like those described for use on land, and they are garnished in their several sections alternately with fires to be discharged on the water and with fires to be discharged in the air. A port fire, which carries a match to the throats of the jets terminating the trompes, gives fire to all at the same time. If it is desired that only one trompe should be discharged at a time, it becomes necessary to pierce the first trompe at the bottom opposite the pot of the last set, which it must also penetrate, and to introduce into this hole the end of a match inclosed in a port fire, which conducts the fire to the throat of the jet in the second trompe; and so on as to the others.
The communications of the fire having been arranged, the assembly of seven trompes is inclosed within sheets of paper pasted over it, which give the form of a barrel. When the whole is dry, it is greased with melted tallow.
If the barril should not be sufficiently heavy to sink in water to the proper depth, a counterpoise is fastened to its bottom. This is usually a sack filled with sand, in order to add to, or take from, the amount of weight as may be necessary for the counterpoise. The adjustment should be such that almost the entire length of the barril is submerged.