Joshua Shaw invented a new device for firing cannons in 1828. The whole term is "percussion lock and wafer primer." Prior to this invention, cannons were loaded with powder charges and a pricker (a big iron needle) was poked through the vent hole to open the rearmost bag. A measure of power was poured into the vent hole and then a lit slowmatch or heated iron bar was used to light the powder, setting off the cannon. The wafer primer was a slug or wafer of pressure-sensitive compound that was placed in the vent hole instead of pricking and pouring. The percussion lock (http://www.history.navy.mil/pics/rodgerscoll_dahlgrena.jpg) was a striking device that was attached to the cannon. A lanyard attached to the lock allowed the gun captain to stand further away from the cannon before firing. The gun captain pulled the lanyad, releasing the lock hammer to strike the wafer primer. The primer ignited under the pressure, shooting fire into the bagged powder charge and setting off the cannon. The lock and wafer allowed for a much greater rate of fire as it eliminated pricking and priming. "Tube and priming" is a reference I found at http://www.archive.org/stream/americanprogress00deverich/americanprogress00deverich_djvu.txt and sounds like it refers to measuring out priming loads in advance and keeping them in capped tubes. I guess the gun crew would prick the bag, get a loaded tube, uncap it, dump the charge into the vent hole, and then use the slowmatch to light it off.