The curve on the surface of water in a graduated cylinder is called the meniscus. In fact, it is not only the cylinder in which water surface exhibits this feature, it is quite common in a number of other containers as well. Try using a conical flask or a burette or a pipette. The best meniscus is observed in a capillary tube, smaller the diameter, the more prominent the meniscus we observe. In scientific experiments, meniscus is used with great care. For example, when working with liquids, it is often tough to know what the upper surface is, esp. for measurements (say volumetric measurements in a burette during titration). As a thumb rule, the upper meniscus is take as top surface for colored liquids and lower meniscus is used as upper surface for clear liquids (such as water).
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