Air Chamber?

[Soot]

This is a pic of a Blakely rifle with the caption:
Unfamiliar with the characteristics and purpose of an air chamber, the Confederates burst one tube at the first fire.
I think the pic is damage from blowing up the gun to prevent capture.
I've never heard of an air chamber in a cannon before. Can someone explain this cause I'm as confused as the Confederates.
A drawing might be helpful.
It came from here: http://robinsonsbattery.org/79215.html

[seacoastartillery]

     From our reading of CSN Commander John M. Brooke's journal and letters, we learned that he was informed of the accident involving the 12.75 Inch Blakely Rifle at Charleston, SC.  After a few calculations, he determined that the "Air Chamber" was NOT to be loaded with powder, but rather be left empty and the powder charge placed immediately ahead of it at the bottom of the bore.  He deduced the Air Chamber's purpose which was to act as a spring to reduce peak pressure when the gun was fired.

     As we all know, water cannot be compressed, but air can.  Hence, the inventor's design incorporated the chamber within the breech of the gun to hold a volume of air in the 5" Dia. X 16" long void.  Brooke issued orders to load the gun correctly and Confederate repairs of the bronze chamber and surrounding cast iron, (replaced with a bolted-on wrought iron enclosure) held until both guns were blown up at the end of the war.  The gun's designer wrote Brooke to say that he had indeed designed the chamber with air compression in mind to reduce peak pressure.  Also, he thought Brooke's method of placing the powder completely in front of the chamber was better than his idea of a narrow tendril-like powder bag extending into the chamber to some extent.

Regards,

Mike and Tracy

P.S.  Not known for it's accuracy, the 60,000 pound gun threw it's 600 pound bolts out with a "respectable velocity".  The energy must have been terrific!  As we maintain in our website, even if one of these tumbled, if it struck your Federal Monitor, it would most likely knock your "cheezebox off of your raft".

[Soot]

That is extremely interesting, thanks for the info and I'll make sure I batten down my cheesebox.

[A.Roads]

This is certainly a subject that one rarely encounters, the principle of “air-spacing” received much attention in England as it was especially valuable where violent powders were used in the larger natures. Long charges effected a reduction in muzzle velocity, as when the bore space was shortened and the volume for expansion was decreased, any powder beyond a certain capacity was wasted. The effect of having an air space within the cartridge was that it allowed a heavier charge to be used thereby obtaining a higher muzzle velocity without over stressing the piece. Long cartridges also tended to generate a pressure wave, but with a paper cylinder placed in the centre of the charge this was largely overcome. Breech loading Armstrong guns were for some time plagued with vent breakages and I believe that with "air spacing", and the attendant reduction of the pressure wave, the breakage of vents on was reduced dramatically.
Adrian