8 
mechanical action of the gas and there is but little'.doubt that, could the latter’s 
escape be prevented, the result of the other evils would be of little or no im> 
portance for the utmost that high temperature and chemical action could do would 
be to render the smooth and shiny surface of the bore rough and dull. 
This mechanical action of the gas accounts for the rapidity of the deterioration 
of vents in guns with which friction copper tubes are used, whilst in those tired 
by vent-sealing tubes, the vents are preserved in good condition for an indefinite 
time. 
In fact, although platinum is proof against the products of the combustion of 
gunpowder and unalterable under the high temperature developed in guns, the 
old platinum vents became unserviceable with the same facility as those made of 
other metals. In such cases the action of the gas on the metal may be compared 
to that of the emery wheel which, revolving at a high speed, detaches the molecule? 
of the hardest substances brought in contact with it. Now that all nations are 
adopting smokeless charges, a perfect obturation between projectile and bore is of 
the utmost importance since the extreme fluidity of the gas developed by the new 
explosives facilitates its escape and leads- to more rapid erosion than has hitherto 
been caused by the combustion of gunpowder. 
Having thus shown that want of hermetic obturation, due to the faulty con¬ 
struction of the driving-bands now in use, is the main cause of the formation of 
erosion, we shall now pass to consider a new method by which this difficulty may 
be overcome. 
The obturating crown, which we here propose, consists of two parts, viz. : a 
band rigidly attached to the body of the projectile and a detachable, movable 
ring. 
The fixed band A is of the same shape as that of the ordinary driving-band 
now in use except that its anterior cone is serrated in a direction analogous to 
the twist of the rifling. 
The shape of the movable ring B, which may be of gun-metal, brass or other 
convenient metal, is as shown in the cut, and works on the tapered cone A of the 
driving-band. 
The contact surfaces of the two parts are conical, having, approximately, the 
same inclination. 
The ring B is free to slide towards the base of the'projectile in the direction of 
