Be slowly lifted up, thou long black arm,
Great gun towering towards Heaven, about to curse;
Sway steep against them, and for years rehearse
Huge imprecations like a blasting charm!
The ‘long black arm  personifies  the gun. Owen attributes its human traits to emphasise its monstrosity. 
 
 The pace of the line is slow with an appropriately heavy tread, achieved by the elongated vowels in ‘slowly’ and ‘long, black arm’. The gun is dignified by the archaic ‘thou’; language that is almost Biblical.