PLEASE allow me to protest against Mr. A. R. Low's attack in NATURE of January 5, p. 12, on the “slug,” which was not introduced by Prof. Bairstow, but probably by Prof. Fleeming Jenkin about thirty years ago. The slug does not lead to any evasion of Newton's laws any more than the poundal which was introduced by Prof. James Thomson. All such terms are useful so long as they are precisely defined and correctly understood; in recent years a distinguished German mathematician has been striving to introduce Prof. Thomson's “radian” in place of “Einheitskreisbogenlange.”