The Manila expedition having been abandoned, and the Spanish fleet which had been employed on the Pacific coast having returned home, Tucker requested permission to visit Lima, in order that he might lay before General Prado, President of the Republic, a plan for making an exploration and survey of the Peruvian or Upper Amazon River and its tributaries. The President heartily approved of the enterprise, for the Government was at that very time considering the practicability of opening better communications between the west coast and the eastern part of the country, and of finding an outlet by the waters of the Amazon for the rich productions of the interior. Tucker resigned his commission as rear-admiral in the Navy of the Republic, and was immediately appointed President of the Peruvian Hydrographical Commission of the Amazon. He left Lima with a full corps of assistants, and made his way across the mountains to the head of navigation on the Palcazu river, where the party was received on board a Government steamer that had been dispatched from Iquitos to meet them. The headquarters of the Commission was established at Iquitos, the principal settlement on the Upper Amazon river, and the place where the Government factories and magazines were located. In the small steamer _Naps_, belonging to the Government, Tucker made an exploring expedition of two hundred and fifty miles up Yavari, the river which forms the boundary between Peru and Brazil. None of the Peruvian steamers on the Amazon being suitable for exploring and surveying purposes, the Government at Lima ordered Tucker to proceed to the United States and procure such a vessel as was required for the duty pertaining to his Commission. In obedience to this order Tucker spent some months in the United States, and had a steamer built by Messrs. Pusey, Jones & Co., of Wilmington, Delaware, expressly adapted to the navigation of the shoals and rapids of the Upper Amazon. This vessel, named the _Tambo_, was delivered to Tucker at Para, the Brazilian city at the mouth of the Lower Amazon. Embarking on board the _Tambo_, Tucker took the steamer up the river to Iquitos, where supplies were taken on board sufficient to last for several months. He then proceeded to make an important expedition up the Upper Amazon, the Ucayali and the Tambo rivers. The Tambo river had never been explored, and it was thought that it presented a feasible route for navigation to San Ramon, a military station in the heart of the interior, only about thirty miles distant from the large and important city of Tarmo, which is connected by railway with Lima.