Question

1. what was the primary use of the sandglass or hourglass?

to measure the altitude of polaris
to plot the ships course on charts
to correct the magnetic compass
to measure time intervals for determining the ships speed

Explanation:

Sandglasses or hourglasses were traditional time - keeping devices. They were used to measure specific time intervals. Measuring the altitude of Polaris requires a sextant, plotting a ship's course uses charts and navigation tools other than an hourglass, and correcting a magnetic compass has nothing to do with an hourglass. Measuring time intervals with an hourglass was useful for determining a ship's speed by comparing distance traveled in a measured time.

Answer:

To measure time intervals for determining the ship's speed