Mission of Gravity — Hal Clement

For a profit -- and adventure -- Barlennan would sail thousands of miles across uncharted waters, into regions where gravity itself played strange tricks. He would dare the perils of strange tribes and stranger creatures -- even dicker with those strange aliens from beyond the skies, though the concept of another world was unknown to the inhabitants of the disk-shaped planet of Mesklin.
But in spite of the incredible technology of the strangers and without regard for their enormous size, Barlennan had the notion of turning the deal to an unsuspected advantage for himself . . . all in all a considerable enterprise for a being very much resembling a fifteen-inch caterpillar!
This book also contains Hal Clement's classic article "Whiligig World."