EXPLORERS OF THE AMAZON <i> by Anthony Smith (Viking: $21.95; 344 pp.)</i>

“For many portions of the world, their exploration by Europeans was a brief affair,” observes Anthony Smith in “Explorers of the Amazon.” “The mystery of the Nile was speedily unravelled after brave individuals had ventured west from Zanzibar to encounter Lake Victoria. The first shipload of immigrants reached Australia in 1788, and within 80 years most of the major traverses had been achieved. Men first saw Antarctica in 1820 and had reached its pole within a century.” The wild interior of the Amazon basin, however, has been a challenge to adventurers for almost 500 years--and the attraction continues to this day, since the gigantic river and most of the land along its 4,000 miles have remained virtually unchanged since Francisco de Orellana first sailed its length in 1542.

In his overview of Amazon exploration, Smith writes the sort of style one could expect from a newspaperman, BBC broadcaster and founder and president of the British Balloon and Airship Club. In his chapter on the barbarous 1559 expedition of Pedro de Ursua and Lope de Aguirre (basis for the Werner Herzog film), he writes: “It is important to remember the prevailing attitude towards the Indians. They were part slave, part beast of burden, part wealth. The Pope may have decreed that they had souls to be saved, but the edict had not been welcomed by the colonials.”

Isabela Godin, the first woman to sail the length of the Amazon, did so to rejoin her husband. He had descended the river--apparently to see if it was passable--and for 16 years sought permission from the Portuguese authorities to return upriver to collect his family. When a boat finally was dispatched to collect her, she was the only member of the party to survive. The story of her sudden emergence out of the jungle made her the toast of European salons, and eclipsed news of the revolution brewing in France.

After the exploits of the early adventurers, Smith turns his attention to scientists who have worked on the river. Baron Alexander von Humboldt, Prussian botanist, geologist and linguist, set out in 1800 to find the link between the Orinoco and Amazon rivers, and collected 12,000 plant specimens on the trip. In the early 20th Century, a young American launched a crusade to expose the terrible conditions under which rubber workers toiled. Smith ends the book with an alarum about the burning of the rain forests, noting that this region that has remained unchanged for so many centuries could in a few decades be altered beyond recognition.