The army Hyder assembled was one of the largest seen in southern India, estimated to number 83,000. Carefully co-ordinating the actions of his subordinate commanders, he swept down the Eastern Ghats onto the coastal plain in July 1780, laying waste the countryside. Due to Hyder's secrecy and poor British intelligence, officials in Madras were unaware of his movements until the fires of burning villages just away were seen in Madras. Hyder himself organised the Siege of Arcot, while detaching his son "Karim Khan Sahib" to take Porto Novo. The movement in August of Sir Hector Munro with a force of over 5,000 from Madras to Kanchipuram (Conjeevaram) prompted Hyder to lift the siege of Arcot and move to confront him. Word then arrived that Munro was awaiting the arrival of reinforcements from Guntur under Colonel William Baillie, so he sent a detachment under Tipu to intercept them, and eventually followed in strength himself, when Munro sent a force from his army to meet Baillie. Tipu and Hyder surrounded Baillie's force, and compelled the surrender of about 3,000 men in the Battle of Pollilur on 10 September; it was the first effective use of rocket artillery and made a strong impression upon the British. Hyder then renewed the siege of Arcot, which fell in November.