Reflecting LEVEL, that made by means of a pretty long surface of water representing the same object inverted which we see erected by the eye; so that the point where these two objects appear to meet is a level with the place where the surface of the water is found. This is the invention of M. Mariotte.
There is another reflecting level consisting of a mirror of steel, or the like, well polished, and placed a little before the object-glass of a telescope, suspended perpendicularly. This mirror must make an angle of 45 with the telescope; in which case the perpendicular line of the telescope is converted into a horizontal line, which is the same with the line of level. This is the invention of M. Cassini.