Simply by cutting berdi reeds ""in August,"" Thor Heyerdahl builds a leakless, buoyant craft that magnificently overcomes all that Mother Nature contrives--only to succumb to pollution and politics. The result is a less rousing, less exciting adventure than that of the Kon-Tiki or the two Ra, but a more focused, scientific expedition. ""The ability to navigate to given destinations had become more of a challenge,"" Heyerdahl writes, than the ""drifts"" of the Ra with the winds and the currents. Assembling his international crew at the aptly-named Garden of Eden Rest House on the Tigris River in Iraq, Heyerdahl copies the age-old methods of construction and form of a Sumerian magur--as much as can be gleaned from ancient sources and living traditions; then, with the usual multilingual mishaps, he proceeds down river to the Arabian/Persian Gulf to trace one of the earliest trade routes (ca. 3000 B.C.) from the cradle of civilization, the Tigris-Euphrates Valley, to legendary Dilmum (Bahrain), Makan (Oman?), Meluhha (the Indus Valley in Pakistan?), and Punt (Somalia?). Highly speculative about the connections between the Old and the New Worlds, Heyerdahl distinguishes himself from those with ""simpler minds"" (as he calls them) who believe in extra-terrestrial visitors by actually putting his theories to the test; and the expedition does indeed establish the ancient trade routes--if not the actual ports of call--already hinted at by archaeological finds. With persistent political impingements (including some stray Russians) and only passing shipboard perils, not as dramatic a story as its predecessors--but rewarding, still, as an eye-witness passage into the dim past.