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One way of defining pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. Although a manometer may use any fluid in principle, common fluids like water give heights that can't be contained in a normal room. A water column needs to be of the order of 10 metres to give atmospheric pressure. Therefore, a very dense fluid is required - mercury. Normal atmospheric pressure can support around 760mm of mercury; hence 1/760th of an atmosphere, or 1 mm of mercury, has been a convenient measure of pressure for a long time, and is sometimes called a torr.
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