Outlines of Astronomy, § 856.

Philosophy of Discovery, pp. 263, 264.

See, on this point, the second chapter of the present book.

Ante, chap. vii., § 1.

It seems hardly necessary to say that the word impinge, as a general term to express collision of forces, is here used by a figure of speech, and not as expressive of any theory respecting the nature of force.

Essays on some Unsettled Questions of Political Economy, Essay V.