Ascendant, a. [222].

Astrolabe, a. [164].

Atmology, b. [137], [163].

Atom, a. [78].

Atomic theory, b. [285].

Axes of symmetry (of crystals), b. [327].

Axis (of a mountain chain), b. [537].

Azimuth, a. [181].

Azot, b. [276].

Ballistics, a. [365].