This text comes from Poems on Slavery, a historical songbook transcribed on Wikisource and reproduced from the pinned revision linked below.
In Ocean's wide domains,
Half buried in the sands,
Lie skeletons in chains,
With shackled feet and hands.
Beyond the fall of dews,
Deeper than plummet lies,
Float ships, with all their crews,
No more to sink or rise.
There the black Slave-ship swims,
Freighted with human forms,
Whose fettered, fleshless limbs
Are not the sport of storms.
These are the bones of Slaves;
They gleam from the abyss;
They cry, from yawning waves,
"We are the Witnesses!"
Within Earth's wide domains
Are markets for men's lives;
Their necks are galled with chains,
Their wrists are cramped with gyves.
Dead bodies, that the kite
In deserts makes its prey;
Murders, that with affright
Scare schoolboys from their play!
All evil thoughts and deeds;
Anger, and lust, and pride;
The foulest, rankest weeds,
That choke Life's groaning tide!
These are the woes of Slaves;
They glare from the abyss;
They cry, from unknown graves,
"We are the Witnesses!"