General Discussion

Vitus Bering confirmed that Asia and North America were separate landmasses in the early 1700's. By the mid-1700's the Russians had mapped the entire northern coast of Eurasia and their maps strongly suggested that the Arctic Sea completely separated North America and Eurasia across the pole.

From there it was a matter of hope. Bering's expeditions confirmed that the northern coast of Eurasia was navigable, at least briefly, during the summer. Because the northern coast of North America appeared to be at a lower latitude than the north coast of Eurasia, this widely raised hopes that the northern coast would have an ice free seaway. They knew where the Bering Strait was, they knew where Baffin Bay was, and it seemed likely that there was water connecting the two.

So, a bit of deduction and a whole lot of hope.