Plumb or Pendulum LEVEL, that which shows the horizontal lines by means of another line perpendicular to that described by a plummet or pendulum. This instrument, (fig. 10.) consists of two legs or branches, joined together at right angles. The branch which carries the thread and plummet is about a foot and a half long; and the thread is hung towards the top of the branch, at the point 2. The middle of the branch where the thread passes is hollow, so that it may hang free everywhere: but towards the bottom, where there is a little blade of silver, on which is drawn a line perpendicular to the telescope, the said cavity is covered by two pieces of brass, making as it were a kind of case, lest the wind should agitate the thread. For this reason the silver blade is covered with a glass G, in order that it may be seen when the thread and plummet play upon the perpendicular. The telescope is fastened to the other branch of the instrument, and is about two feet long; having an hair placed horizontally across the focus of the object-glass, which determines the point of the level. The telescope must be fitted
at right angles to the perpendicular. It has a ball and socket, by which it is fixed to the foot, and was invented by M. Picard.