Mercury plumb bobs (Fig. 36) tare usually made of hollow steel rods filled with mercury or quicksilver. Consequently they are unusually heavy in proportion to their size, and their centers of gravity are low. Their comparatively small diameters also allow them to be used close to corners and walls; they are not easily affected by draughts of air; and they can be packed in a small space. As a result, they may be used to advantage almost anywhere.