154 OT THE VENTS

vent; its position was not at all altered. This single experi-ment (hews the necessity of suppressing the use of small cham-bers in guns.

121. Several observations and experiments might be ad-duced to prove that when the bottom of the vent corresponds•with the upper part of the bore, it is a long time before it•changes its form ; and that if the metal be corroded, it is•only round the lower orifice.

122. It has been observed (4) that the vents of guns runsooner as the proportion of tin is the greater; and in the secondchapter the necessity of having a hard metal to prevent cavi-ties being formed in the bore was insisted onbut this con-dition can only be obtained by increasing the quantity of tin,and thereby rendering the vent liable to run after a very littlefiring; and it is well known that when the vent is muchenlarged, there must be great irregularity and a considerablediminution of force in the discharge: it is necessary thereforeto form the vent of some more resisting metal, which whenenlarged may be easily taken out and replaced ; the best expe-dient hitherto devised, is to insert into every gun previous toits being carried on service, a screw bouch which may bereplaced at pleasure.

123. From satisfactory experiments made on the resistanceof simple and compound metals with a view to determinewhich is the best for bouching guns, it is in general foundthat their resistance is in proportion to the difficulty of fusingthem in the crucible. The following metals were provedwith the machine described in the Treatise on Powder(139) and found to decrease in resistance from the first tothe last.

1. Forged iron well worked and welded with thehammer.

2- Iron of the second smelting.

3. Pure copper.

4. German brass.

5. Gun-metal containing ^ of tin,

6. Assayed gold.

7. Aflayed silver.

8. Lead.

9. Tin.

Of these metals the three first are least corroded by theaction of powder; but forged iron is the only one that cande depended on for making screw douches (122). Gun-

metal