EAST AFRICAN SLAVERY.,[REUTER'S TELEGRAM.! ADEN. FED. 20. Advices brought by the mail from Zanzibar report that on the If'th January H.M.S. Nassau and Rifleman, with part of the crew of the London, under Captain Sullivan, bombaidcd and took possession of Fort Mombazique after hve hours' engagement. The enemy had 1 < killed and 51 wounded. No Joss was suflercd by the British. Mombazi has been handed over to the Sultan of Zanzibar. H.M.S. Thetis lias captured two slavers, one containing 192 and the other 110 slaves. The Portuguese attacked a slave barracoon south ot Mozambique, containing 1,000 slave-, but we**c repulsed with loss. The Thetis subsequentlyproceeded to attack the barracoon.