descriptive rather than theoretical, but wonderfully fresh and original in treatment and full of suggestiveness, a _Primer on Political Economy_ (1878), _The State in Relation to Labour_ (1882), and two works published after his death, namely, _Methods of Social Reform_ and _Investigations in Currency and Finance_, containing papers that had appeared separately during his lifetime. The last-named volume contains Jevons's interesting speculations on the connexion between commercial crises and sun-spots. He was engaged at the time of his death upon the preparation of a large treatise on economics and had drawn up a table of contents and completed some chapters and parts of chapters. This fragment was published in 1905 under the title of _The Principles of Economics: a Fragment of a Treatise on the Industrial Mechanism of Society, and other Papers_. Jevons's work in logic went on _pari passu_ with his work in political economy. In 1864 he published a small volume, entitled _Pure Logic; or, the Logic of Quality apart from Quantity_, which was based on Boole's system of logic, but freed from what he considered the false mathematical dress of that system. In the years immediately following he devoted considerable attention to the construction of a logical machine, exhibited before the Royal Society in 1870, by means of which the conclusion derivable from any given set of premisses could be mechanically obtained. In 1866 what he regarded as the great and universal principle of all reasoning dawned upon him; and in 1869 he published a sketch of this fundamental doctrine under the title of _The Substitution of Similars_. He expressed the principle in its simplest form as follows: "Whatever is true of a thing is true of its like," and he worked out in detail its various applications. In the following year appeared the _Elementary Lessons on Logic_, which soon became the most widely read elementary textbook on logic in the English language. In the meantime he was engaged upon a much more important logical treatise, which appeared in 1874 under the title of _The Principles of Science_. In this work Jevons embodied the substance of his earlier works on pure logic and the substitution of similars; he also enunciated and developed the view that induction is simply an inverse employment of deduction; he treated in a luminous manner the general theory of probability, and the relation between probability and induction; and his knowledge of the various natural sciences enabled him throughout to relieve the abstract character of logical doctrine by concrete scientific illustrations, often worked out in great detail. Jevons's general theory of induction was a revival of the theory laid down by Whewell and criticized by Mill; but it was put in a new form, and was free from some of the non-essential adjuncts which rendered Whewell's exposition open to attack. The work as a whole was one of the most