156 OP THE VENTS
In the fourth, it requires more tabour to prepare the cavity;but a fresh screw may be inserted when the first is corroded*The old method of using screw douches was very imperfect;fince they were obliged to vary the diameter in each gun inproportion to the degree of corrosion.
125. In order to derive every possible advantage fromscrew douches, and render tueir use more simple, expeditious,and general, the following precautions should be observed.
1. To apply during the time of peace to all guns,particularly to battering cannon, a screw bouch of ironin one of the two methods expressed in figure 12 and13; they should be formed on the same model for allpieces of the fame calibre, that there may be always aquantity in store.
2. The head of each bouch should be so formed thatit may be easily screwed and unscrewed; and of such alength, that the bottom may exactly correspond withthe upper part of the cylinder.
3. blot to diminish the metal at the breech too much,
the diameter of the bouch should be less than
DQ.
; and
to prevent the necessity of frequently replacing it, itsdiameter should not be less than P-Sr
4
4. The vent should (ny) correspond with the bot-tom of the bore, which should be a plane figure. If theseoperations be executed with exactness and precision byskilful workmen, the resistance will be fully sufficient;and there will be no danger of the flames passing amongthe spirals to damage and corrode them.
126. The resistance of the screw bouch may be easily as-certained : (PI. 4, Fig, 12) since before it can yield to theaction of the powder, it must be driven forcibly out of thefemale screw and be separated from its spirals; or it mustbreak off and carry away with it the spirals of the otherscrew, which would leave a vacuum NBD Q_; or it mustbreak the spirals of both. Now as the tenacity of iron farexceeds that of gun-metal, the spirals of the female screw mustfirst yield to thole of the bouch ; and if the circumference ofa spiral answering to the diameter BD be called —(1 and itslength D Q = w> cm will be the section of the fracture; this
section