For it was the unwise ruling of the courts that actual presence of slaves on a captured ship was necessary to prove that she was engaged in the unlawful trade.

Her hold might reek with the odor of the imprisoned blacks, her decks show unmistakable signs of their recent presence, leg-irons and manacles might bear dumb testimony to the purpose of her voyage, informers in the crew might even betray the captain's secret; but if the boarders from the man-of-war found no negroes on the ship, she went free.

What was the natural result?
When a slaver, chased by a cruiser, found that capture was certain, her cargo of slaves was thrown overboard.
The cruiser in the distance might detect the frightful odor that told unmistakably of a slave-ship.

Her officers might hear the screams of the unhappy blacks being flung into the sea.

They might even see the bodies floating in the slaver's wake; but if, on boarding the suspected craft, they found her without a single captive, they could do nothing.