Using a Sandglass

Sandglasses of different sizes and durations could measure intervals of time for ship speed and periods of duty. From Thomas Blanckley, A Naval Expositor, 1750.

Caption:
Sandglasses of different sizes and durations could measure intervals of time for ship speed and periods of duty.
Type: Illustration
Credit: Smithsonian Institution Libraries
Origin: Smithsonian Institution
Navigation Methods
 
Sandglasses of different sizes and durations could measure intervals of time for ship speed and periods of duty.