A kind of chronometer or instrument for measuring intervals of time. It is constructed of glass, and consists of two bulbs, one above the other, connected by a narrow neck. The time is measured by the running of dry sand from the one bulb to the other, the quantity being adjusted to the time which each glass has been constructed to indicate. In the case of an hour-glass, as much sand is placed in one bulb as will take an hour to pass from it to the other. Hour-glasses were very much in use in churches during the 16th and 17th centuries, and specimens of very fine workmanship are still to be seen in several churches in England. (See Log.)