"the periodic heavy dull report of a howitzer firing"
M1905 Howitzer used by Allied Forces in World War I
Public DomainM1905 Howitzer used by Allied Forces in World War I - Credit: Wikimedia Commons

A howitzer is a type of field gun, characterised by a relatively short barrel and used to fire heavy shells at high trajectories with a steep angle of descent. 

They were typically employed against fortifications.