Leaving Iquitos, the _Tambo_, with the Commission on board, passed up the Amazon to the mouth of the Ucayali river, up the Ucayali past the rapids of the "Devil's Leap," and entered the Tambo river. The Tambo was found to be a narrow stream, full of rocks and rapids and not practicable for navigation by steamers. When the steamer _Tambo_ could ascend no higher, Tucker fitted out a small boat and pulled some twenty miles farther up the river, but everywhere found such obstructions as rendered it an impracticable route to the interior. It is, perhaps, to be regretted that time did not allow of an examination of the other affluents of the Usayali trending towards San Ramon and Tarmo. On his return to Iquitos, Tucker was again dispatched to the United States to procure another and smaller exploring steamer. During his absence Captain James Henry Rochelle was directed by the Government at Lima to take charge of the Hydrographical Commission as its acting president. After an absence of some months, Tucker returned to Iquitos with the new steamer, which was named the _Mayro_, and was little more than a large steam launch, intended for use where a vessel of greater draught of water could not be employed. The next expedition decided upon was for the exploration of the water route towards Huanaco, by way of the entirely unknown river Pichis. Most of the tributaries of the Ucayali had been traveled more or less by the Jesuit priests from the College of Ocopa, but none of them had attempted the route of the Pichis, the banks of which were in possession of roving tribes of Indians, who permitted no stranger to pass through their country. It was thought possible, and even probable, from the stories told by the natives, that the head of the Pichis river would be found well suited for being the eastern terminus of the trans-Andean railway. In February, 1873, the _Mayro_, with a detachment of the Commission on board, was dispatched from Iquitos, with orders to await at the mouth of the Pachitea river the coming of the _Tambo_. Tucker embarked on board the _Tambo_ on the 1st of April with the main body of the Commission, and arrived at the confluence of the Pachitea and Ucayali, seven hundred and sixty-five miles from Iquitos, on the 13th of May. The river had commenced to fall, which rendered it prudent not to ascend the Pachitea in steamers, for had one of them got aground whilst the water was falling, it would probably have remained in that situation until the next annual rise of the river.