"When the Arab wishes to trade in dangerous circumstances—for example, across the Sahara with the Negro—he drives his caravan south to some oasis, and goes out some distance into the open waste, and piles up on the sand those goods he offers for trade. Then he withdraws to some remove beyond spear-range, yet still nigh enough that he is able to keep watch over his goods. The Negro now feels safe in venturing to the same place, where he piles up nearby those goods he offers in exchange. He withdraws and the Arab ventures out again, inspects the Negro’s offerings, and adds to or takes from his pile. And so it goes, back and forth, until one is satisfied, which he betokens by packing up and hauling away the counterpart’s offerings. The other waits until he has withdrawn, then goes out a last time to take possession of what remains."