Commencing at the Yavari river, which forms the boundary between Peru and Brazil on the south side of the Amazon river, and following the Upper Amazon and its principal tributaries up to the head of navigation, the first place to be noted is the mouth of the Yavari river:[2] Latitude 4 deg. 18' 45" south; longitude, 69 deg. 53' 10" west of Greenwich; magnetic variation, 5 deg. 38' 54" east; thermometer (Fahrenheit), 76 deg.; elevation above sea-level, 266 feet; distance from the Atlantic ocean, following the course of the river, 1811 miles; current, in the Amazon, 4-1/2 miles per hour; width of the Yavari river at its mouth, 500 yards; width of the Amazon, 1200 yards; depth of water in the channel of the Amazon, 36 feet. As the Yavari river marks the boundary between Peru and Brazil on the south side of the Amazon, special pains were taken to ascertain correctly the latitude and longitude of its mouth; the observations for the latitude and longitude were taken on a small islet, probably overflowed at high water, in the middle of the lower mouth of the river. It was said in Iquitos that, in 1874, Captain Guillermo Black, President of the Peruvian Boundary Commission, ascended the Yavari in a small steamer a distance of 500 miles from its mouth, and 300 miles farther in canoes to a point where there was barely two feet of water in the channel, at which point the latitude was determined to be 7 deg. 1' 22" south, and the longitude 74 deg. 8' 25" west of Greenwich; elevation above the sea-level, 800 feet. TABATINGA (BRAZIL). Distance from the Atlantic, 1825 miles; current, 4-1/2 miles per hour; depth of water, 36 feet; width of river, 800 yards. Tabatinga is the Brazilian frontier post on the north side of the Amazon. Captain Azevedo, of the Brazilian Navy, gives the latitude of this place as 4 deg. 14' 30" south; longitude, 70 deg. 2' 24" west of Greenwich; magnetic variation, 6 deg. 35' 10" east. LETITIA. Latitude, 4 deg. 10' 57" south; longitude, 69 deg. 59' 21" west of Greenwich; magnetic variation, 5 deg. 57' 40" east; elevation above sea-level, 274 feet; distance from the Atlantic, 1828 miles. Letitia is the Peruvian frontier post on the north bank of the Amazon. A fort, intended to command the passage of the river, was projected but not erected at this point. It is probable that the passage of steamers up the Amazon cannot be stopped by forts and batteries at any point on the river below Tamshiyacu. LORETO. Latitude, 3 deg. 54' 20" south; longitude, 70 deg. 7' 45" west of Greenwich; magnetic variation, 5 deg. 11' 24" east; thermometer, 78 deg.; elevation above sea-level, 286 feet; distance from the Atlantic, 1865 miles; current, 3 miles per hour; width of river, 1300 yards. Loreto is the most eastern Peruvian town of any importance on the Amazon. It is situated on the north or left bank of the river. Near it resides a tribe of Indians, partly civilized, called the Ticunas. CAMACHEROS.