In 1840 the islands in the mouth of the Gallinas River, situated near the present Sierra Leone-Liberia border, formed a principal departure point for slave ships. In November of that year - a couple of months after William Loney left Wanderer - Sir Richard Doherty, the Governor of Sierra Leone discovered that Prince Mauna, the son of the King of the Gallinas, Seacca, was holding two British subjects: the black woman Fry Norman and her child. Joseph Denman was ordered to rescue them. Thanks to his close blockade, the eight Spanish-owned barracoons (slave factories) were all full when, in the boats of Wanderer, Rolla and Saracen, he crossed the river bar, and initially freed 90 slaves who the owners were trying to evacuate to the mainland. Denman set a guard over the barracoons, and demanded that the King not only freed Fry Norman and her child, but also sign a treaty abolishing the slave trade throughout his dominions. After some prevarication, and helped by Denman's threats of violence, he freed the Normans and agreed to the treaty, allowing the destruction of the barracoons, the liberation of the slaves, and the expulsion of all the slave traders in his dominions.
After three days of destruction, initiated by firing incendiary rockets into the barracoons, Denman transported a total of 841 slaves, and their Spanish owners, back to Sierra Leone. The Commissioners there estimated that the action had cost the Spaniards between £100,000 and £500,000, as well as a claim on 13,000 slaves already paid for to the local natives. They optimistically predicted that relations between the slave traders and the natives had been so badly damaged that "there is now so serious a feud established as to render impossible, at least for a considerable time to come, the reestablishment there of slave-factories". Unfortunately this optimism proved to be unfounded. A year later they reported that "we have received information that during last rains no less than three slave-factories were settled in the Gallinas, wither the factors and goods had been conveyed in an American vessel".
The authorities were initially very pleased with Denman's action. Foreign Secretary Lord Palmerston said: "Taking a wasp's nest ... is more effective that catching the wasps one by one". Demman was posted (promoted to Captain) on 23 August 1841, when a full report reached London, and he and his men received a bounty of £4,000. However, one of the Spanish dealers, Buron, sued Denman for immense damages, and it was only in 1848 that the Court of Exchequer pronounced in the latter's favour. In the mean time, after a number of similar actions by other officers of the West African Squadron, doubts began to rise concerning their legality, and of the policy of close blockade. In May 1842 the Admiralty was informed that the Advocate-General "cannot take upon himself to advise that all the proceedings described as having taken place at Gallinas, New Cestos and Sea Bars, are strictly justifiable, or that the instructions to Her Majesty's naval officers are such as can with perfect legality be carried into execution. The Queen's Advocate is of the opinion that the blockading of rivers, landing and destroying buildings, and carrying off persons held in slavery in countries with which Great Britain is not at war, cannot be considered as sanctioned by the law of nations, or by provisions of any existing treaties". Unfortunately this opinion was interpreted by many as a re-legitimisation of slavery.