All 3 Uses of

sentry
in
Moby Dick

And it is much to be deplored that the place to which you devote so considerable a portion of the whole term of your natural life, should be so sadly destitute of anything approaching to a cosy inhabitiveness, or adapted to breed a comfortable localness of feeling, such as pertains to a bed, a hammock, a hearse, a sentry box, a pulpit, a coach, or any other of those small and snug contrivances in which men temporarily isolate themselves.†
Chpt 34-36

Man may, in effect, be said to look out on the world from a sentry-box with two joined sashes for his window.†
Chpt 73-75 *

And this it was, this same unaccountable, cunning life-principle in him; this it was, that kept him a great part of the time soliloquizing; but only like an unreasoning wheel, which also hummingly soliloquizes; or rather, his body was a sentry-box and this soliloquizer on guard there, and talking all the time to keep himself awake.†
Chpt 106-108

Definition:

someone who stands guard