[Footnote 13: Johnson, J.B.: The materials of construction, pp. 81-82.]
[Footnote 14: See Tiemann, Harry D.: The theory of impact and its application to testing materials. Jour. Franklin Inst., Oct., Nov., 1909, pp. 235-259, 336-364.]
[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912, XXIII2, pp. 12-13.]