Convicts forming a mortar battery in the Woolwich marshes

The authors describe visiting the convicts at work on a mortar battery: "Right before us is a vast earth-work, all, as we are told, raised by convict labour! It is a 5-mortar battery. We approached it ... and found the prisoners, with their brown jackets thrown off, and some with their legs buried in water-boots, reaching to their thighs, digging the heavy, black, clayey soil, and carrying it away in barrows, under the eyes of two guards, with their cutlasses at their sides and two non-commissioned officers of the sappers and miners, who were directing the works."