What is "post-captains"

Usage examples of "post-captains".

Real captains, full captains, we call post-captains - we say a man is made post when he is appointed to a sixth-rate or better, an eight-and-twenty, say, or a thirty-two-gun frigate.

I have seen so many squadrons formed, delayed in port, delayed still longer, the date put off, put off again, and then, when their officers had all their stores aboard for say a six months' voyage, dispersed, the whole scheme given up, the commodore sent back among the mere post-captains and reduced to begging in the Street, having spent his last guineas on a rear-admiral's gold lace.

He had scarcely relied upon it: on the remoter stations there were few post-captains to spare.

So there I was, hauled up in front of half a dozen post-captains and two admirals: Lord Keith was one of the admirals.

After a longish pause Jack said, 'You do not rate post-captains and admirals very high among intelligent beings, I believe?