the way in which he could " sling " the symbols. Tait was not only an intellectual, but likewise a physical, giant. I am nearly six feet high, but standing beside Tait, I used to feel diminutive. He was well-built, and muscular. He wore a long beard, the hair on the top of his -head had disappeared at an early date, and left exposed a massive forehead. To protect his head while lecturing it was his custom to wear a skull cap. On the street he wore a sack-coat and a soft felt hat, and with cane in hand, was always walking rapidly. About the time of his moving to Edinburgh he married a lady who proved a genuine helpmate. She took full charge of all the affairs of the household, so that her distinguished husband might have perfect leisure for his scientific labors; and her influence was also such as to steady his attachment to religion.
Before the year 1860, when Tait became a professor of physics, Joule had made his determination of the mechanical equivalent of heat, thus establishing the first law of thermodynamics; Thomson, Rankine and Clausius had established the second law; and Rankine had drawn the outlines of the science of "energetics." In the first edition of Dynamics of a Particle there is no mention of the doctrines of energy; it is probable that Tait's experimental work with Andrews led him to study the papers of Thomson, Joule and Rankine. Anyhow the main object of Thomson and Tait's Treatise on Natural Philosophy was to fill up Rankine's outlines, expound all the branches of physics from the standpoint of the doctrine of energy. The plan contemplated four volumes; the printing of the first volume began in 1862 and was completed in 1867. The other three volumes never appeared. When a second edition was called for, the matter of the first volume was increased by a number of appendices and appeared as two separately bound parts. The volume which did appear, although judged rather difficult reading even by accomplished mathematicians, has achieved a great success. It has been translated in French and German; it has educated the new generation of mathematical physicists; and it has been styled the "Principia" of the nineteenth century. Such was his admiration of Newton
