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A gerb is a type of firework which produces a jet of sparks, usually lasting between 15 and 60 seconds. It is a thick-walled tube filled with pyrotechnic composition and possessing a choke, which is a narrowing in the tube. Gerbs are often referred to as 'fountains'.
Sometimes a small charge of black powder is added to the bottom of a gerb to make it finish with a bang: this charge is known as the 'bounce'. These are commonly used at sporting events.
Gerbs intended for use indoors near a proximate audience, such as at a rock concert, typically have shorter durations (from 1/8 to 30 seconds) and heights (4 to 50 feet).
Gerbs are usually measured in terms of time and height. For example, a 4x4 burns for 4 seconds at a height of 4 feet.
Eric Newby's book The Last Grain Race (chapter two Mountstewart) contains a 1938 reference to what appears to be the original gerb - in effect a mortar bomb fired from a mortar.