The substance of part at least of this paper was given in 1876 as an evening lecture to the British Association at its Glasgow meeting.

While engaged in writing the article “Mechanics” for the Ency. Brit., I had to consider carefully what basis to adopt, and decided that the time had not yet come in which (at least in a semi-popular article) Newton's laws of motion could be modified. The article was therefore based entirely on these laws, with a mere hint towards the end that in all probability they would soon require essential modification. It is well, however, that the question of modification should now be considered; and this should be done, not in a popular essay but, before a scientific society.