Therefore, go forth, companion: when you find
No Highway more, no track, all being blind,
The way to go shall glimmer in the mind.
Though you have conquered Earth and charted Sea
And planned the courses of all Stars that be,
Adventure on, more wonders are in Thee,
Adventure on, for from the littlest clue
Has come whatever worth man ever knew;
The next to lighten all men may be you . . .
The acknowledgments say they are from The Wanderer. I've been looking for the original poem for years, Masefield did write a poem "The Wanderer", but they are not from that. Last night I tried again and found a presentation by Ken Croasdale to a Society of Naval Architects and Marine Engineers function, identifying the source as Masefield's book The Wanderer of Liverpool. In that book they form part of a longer poem "The Ending", which tells of the end of the ship The Wanderer.
The poem ends:
"Adventure on, companions, the attempt
At high adventure brings reward undreamt.
The raging sea is grim with reefs un conn’d :
There is a way, a haven is beyond.
Way for yourself, a harbourage for you,
Where every quarry spirit can pursue
Is, in the glory of the dream come true."
So singing, she wanders the waters with white wing on wing
Star-lighted, star-guided, the sea-gleaming beautiful thing.
Archive.org has The Wanderer of Liverpool, the poem "The Ending" is here, pages 76-87