A unit of time measured by an hourglass or sandglass. At sea, time was traditionally measured with half-hourglasses, making the glass a nautical unit of time equal to 1/2 hour. In this use the glass is another name for the bell
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}