percolation rate . The rate, expressed as either velocity or volume, at which water percolates through a porous medium.

alveolus (al-ve-o'-lus). (a) In invertebrates, a small cavity or pit; e.g. a minute blind cavity in the keriotheca of some fusulinids, the conical cavity in the anterior end of the guard of a belemnite, or a pit in the skeleton of a bryozoan colony. (b) In vertebrates, the socket of a tooth. Pl: alveoli. Adj: alveolar. Syn: alveole.

magnetic north . The uncorrected direction indicated by the north-seeking end of the needle of a magnetic compass; the direction from any point on the Earth's surface of the horizontal component of the Earth's magnetic lines of force connecting the observer with the north magnetic pole; the northerly direction of the magnetic meridian at any given point. It is the common zero-degree (or 360-degree) reference in much of navigational practice. Cf: true north. Syn: magnetic meridian.

diluvium (di-lu'-vi-um). (a) An archaic term applied during the early 1800s to certain widespread surficial deposits that could not be explained by the normal action of rivers and seas but were believed to be produced by extraordinary floods of vast extent, esp. the Noachian Flood; these deposits are now known to be mostly glacial drift. (b) A general term used in continental Europe for the older Quaternary, or Pleistocene, glacial deposits, as distinguished from the younger alluvium. Syn: drift [glac geol]. Var. spellings: deluvium; diluvion.