THIRD

, in Music, a concord resulting from a mixture of two sounds containing an interval of 2 degrees: being called a third, because containing 3 terms, or sounds, between the extremes.

There is a greater and a less Third. The former takes its form from the sesquiquarta ratio, 4 to 5. The logarithm or measure of the octave 2/1 being 1.00000, the measure of the greater Third 5/4 will be 0.32193.— The greater Third is by practitioners often taken for the third part of an octave; which is an error, since three greater Thirds fall short of the octave by a diesis; for .

The lesser Third takes its form from the sesquiquinta ratio 5 to 6; the measure or logarithm of this lesser Third 6/5, being 0.26303, that of the octave 2/1 being 1.00000.

Both these Thirds are of great use in melody; making as it were the foundation and life of harmony.

Third-Point, or Tierce-point, in Architecture, the point of section in the vertex of an equilateral triangle. —Arches or vaults of the Third Point, are those con- | sisting of two arches of a circle, meeting in an angle at top.

THREE-legged-staff, an instrument consisting of three wooden legs, made with joints, so as to shut all together, and to take off in the middle for the better carriage. It has usually a ball and socket on the top; and its use is to support and adjust instruments for astronomy, surveying, &c.