Accurate time is essential to dead reckoning. Water-clocks (clepsydras) and portable sundials suffered obvious disadvantages aboard ship, so the sandglass or hourglass was the timepiece most often used in navigation. The most common glasses were the four-hour and half-hour sizes. Days at sea were divided into six four-hour shifts or watches. A ship's boy carefully tended the half-hour glass, turning it as soon as the sand had run through and calling out or striking a bell for all aboard to hear. At the end of four hours, he turned the four-hour glass. (Hence the system of bells and watches still used aboard many vessels.) The texture of the sand could affect its rate of flow, as could condensation within the glass, so several glasses were used together for accuracy.