FROM a recently published part of the “New English Dictionary” it is to be inferred that the first authority for the use of the word “radian” was the “Treatise on Natural Philosophy” of Thomson and Tait, the date given being 1879—that is to say, the date of the new edition of part i. of vol. i. As the word has at least ten years of previous history, it may be desirable to put on record a few additional facts in regard to it. My own first use of it was in class-teaching in the College Hall at St. Andrews in 1869, and I possess a note-book, belonging to one of my students of that year, in which the word is used. The introduction of it was almost simultaneous with my proposal of the word “therm” in connection with the measurement of heat.