They meant to keep watches during the night to prevent a surprise.

You boys keep watches day and night.

The men keep watches, as aboard ship, so that somebody is always awake in case anything should go wrong.

Purvis is a very good man, but he is no navigator; and we shall have to keep watches regularly.

Then he reminded himself bitterly that street clocks were abundant and might be looked at by simpletons who couldn't keep watches.

"Do you, then, keep watches to compare with your chronometers?" asked Rose, with interest.