A marine sandglass is a timepiece of simple design that is a relative of the common hourglass, a marine (nautical) instrument known since the 14th century (although reasonably presumed to be of very ancient use and origin). Sandglasses were used to measure the time at sea or on a given navigational course, in repeated measures of small time increments (e.g., 30 minutes). Used together with the chip log, smaller marine sandglasses were also used to measure the boat speed through the water in knots.

Although vital to maritime navigation, marine sandglasses were not accurate measuring instruments for the passage of time; many design and environmental factors could affect the duration of sand's flow, and therefore its reported time. Their use continued through the early 19th century, when they were supplanted by reliable mechanical timepieces and by other advances in marine navigation.

Marine sandglasses were very popular on ships, as they were the most dependable measurement of time while at sea. Unlike the liquid-based clepsydra, the motion of the ship did not affect the hourglass. The fact that the hourglass also used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it during temperature changes. In conjunction with a record of a ship's speed and direction, seamen used the hourglass to determine their position with reasonable accuracy.

Marine sandglasses originally consisted of two glass bottles, one inverted above the other, connected by a small tube, with the ends wrapped and so joined together. Over time, the later progress in the art of glassblowing allowed them to be made in a single piece. The marine glass was filled with sand or a suitable material such as finely ground eggshell, lead or tin chips (used to avoid humidity). This flowing material was chosen with two main objectives–to avoid the humidity, and to absorb motion– which are both required for shipboard use.

Placed in the upper half, the sand would flow slowly and steadily towards the lower half by the action of gravity, taking a certain time to empty (that was calibrated during their design and manufacture). Once the upper portion of the glass was empty, the glass could be turned to measure another time period.

The origin of the hourglass is unclear, although unlike its predecessor the clepsydra, or water clock, which may have been invented in ancient Egypt, the first referenced use:

From the Roman time it disappears completely from historical records until it is re-introduced in medieval Europe. By the 8th century it is mentioned by a monk named Luitprand, who served at the cathedral of Chartres, France. But it was not until the 14th century that the marine sandglass was seen commonly, the earliest firm evidence being a depiction in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti.

For more than 500 years (from 1300 to 1800) the instrument for measuring time at sea was "the Ampoule of hours". In Catalonia there are references of its manufacture since the middle of the 14th century made of Catalan transparent glass according to the formulas of the alchemist Guillem Sedacer, using as a fundent the soda ash obtained by burning barilla (a plant whose ashes dissolved in water and cast with a sieve have been the basis from the "laundry" to wash clothes from the Neolithic). Barilla grows on the Catalan coast from Orihuela to Montpellier and the Italian manuals for making Italian glass (La Sedacina and Arte Vetraria) say very strictly: "bisogna comprare la soda di Spagna" ("you must buy soda from Spain").