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 tests to which it has been subjected. All clocks are constructed on the basis of this method of measurement; that is to say, on the plan of counting the repetitions of some operation, adopted solely on the ground of its being capable of continual repetition with a certain degree of accuracy, and possibly also of automatic compensation for changing conditions. Practically clocks are regulated by reference to the diurnal rotation of the earth relatively to the stars, which affords a measurement on the repetition principle agreeing with other methods, but more accurate than that given by any existing clock. We have, however, good reasons for regarding it as not absolutely perfect, and there are some astronomical data the tendency of which is to confirm this view.

The most important extension of the principles of the subject since Newton’s time is to be found in the development of the theory of energy, the chief value of which lies in the fact that it has supplied a measurable link connecting the motions of systems, the structure of which can be directly observed, with physical and chemical phenomena having

to do with motions which cannot be similarly traced in detail. The importance of a study of the changes of the vis viva depending on squares of velocities, or what is now called the “kinetic energy” of a system, was recognized in Newton’s time, especially by Leibnitz; and it was perceived (at any rate for special cases) that an increase in this quantity in the course of any motion of the system was otherwise expressible by what we now call the “work” done by the forces. The mathematical treatment of the subject from this point of view by Lagrange (1736–1813) and others has afforded the most important forms of statement of the theory of the motion of a system that are available for practical use. But it is to the physicists of the 19th century, and especially to Joule, whose experimental results were published in 1843–1849, that we practically owe the most notable advance that has been made in the development of the subject—namely, the establishment of the principle of the conservation of energy (see and ). The energy of a system is the measure of its capacity for doing work, on the assumption of suitable connexions with other systems. When the motion of a body is checked by a spring, its kinetic energy being destroyed, the spring, if perfectly elastic, is capable of restoring the motion; but if it is checked by friction no such restoration can be immediately effected. It has, however, been shown that, just as the compressed spring has a capacity for doing work by virtue of its configuration, so in the case of the friction there is a physical effect produced—namely, the raising of the temperature of the bodies in contact, which is the mark of a capacity for doing the same amount of work. Electrical and chemical effects afford similar examples. Here we get the link with physics and chemistry alluded to above, which is obtained by the recognition of new forms of energy, interchangeable with what may be called mechanical energy, or that associated with sensible motions and changes of configuration.

Such general statements of the theory of motion as that of Lagrange, while releasing us from the rather narrow and strained view of the subject presented by detailed analysis of motion in terms of force, have also suggested a search for other forms which a statement of elementary principles might equally take as the foundation of a logical scheme. In this connexion the interesting scheme formulated by Hertz (1894) deserves notice. It is important as an addition to the logic of the subject rather than on account of any practical advantages which it affords for purposes of calculation.

MOTIVE (from Lat. movere, to move), in psychology, a general term signifying any element of consciousness which prompts an agent to a decision. The older psychology usually regarded motives as strictly analogous to mechanical forces exerting pressure or tension, and explained human action as necessarily determined by the resultant of various, possibly conflicting, motives. Contemporary psychological research tends to show with increasing clearness that we must recognize a power of decision in the self, and that the analogy of mechanical forces is inadequate to explain the facts. On this view motives will be regarded as solicitations to act in a certain direction, while the self decides by throwing its volitional weight on the side of the motive which it regards as preferable. The solicitations may come from the most diverse sources: they may be mere desires to avoid some pain or to gratify some appetite; or they may be of higher origin, such as the motive of patriotism, or the desire to advance knowledge. Purposes or ends are often termed motives. “Conflict of motives” means sometimes a conflict of purposes, when the agent has adopted two different lines of action and has difficulty in combining them; or it may mean a conflict of solicitations. It is better to call purposes or ends by those names when they have been definitely adopted by the agent: while they are still under deliberation the term “motive” may be used.

 MOTLEY, JOHN LOTHROP (1814–1877), American historian, son of Thomas Motley, was born on the 15th of April 1814 at Dorchester (now a part of Boston), Massachusetts, and graduated at Harvard in 1831. He then studied at Göttingen and Berlin, becoming a friend of Bismarck at Göttingen, and after a period of European travel returned in 1834 to America, where he continued his legal studies. In 1837 he married Mary Benjamin (d. 1874), a sister of Park Benjamin, and in 1839 he published anonymously a novel entitled Morton’s Hope, or the Memoirs of a Provincial. In 1841 he entered the diplomatic service as secretary of legation in Russia, but resigned his post within three months. Returning to America, he soon entered definitely upon a literary career. Besides contributing various historical and critical essays to the North American Review, including a remarkable essay on the Polity of the Puritans, he published in 1849, again anonymously, a second novel, entitled Merry Mount, a Romance of the Massachusetts Colony. About 1846 the project of writing a history of Holland had begun to take shape in his mind, and he had already done a large amount of work on this subject when, finding the materials at his disposal in the United States inadequate, he went to Europe in 1851. The next five years were spent at Dresden, Brussels and the Hague in investigation of the archives, which resulted in 1856 in the publication of The Rise of the Dutch Republic, which became very popular. It speedily passed through many editions, was translated into French, and also into Dutch, German and Russian. In 1860 Motley published the first two volumes of its continuation, The United Netherlands. This work was on a larger scale, and embodied the results of a still greater amount of original research. It was brought down to the truce of 1609 by two additional volumes, published in 1867. In 1861, just after the Civil War had broken out in America, Motley wrote two letters to The Times defending the Federal position, and these letters, afterwards reprinted as a pamphlet entitled Causes of the Civil War in America, made a favourable impression on President Lincoln. Partly owing to this essay, Motley was appointed United States minister to Austria in 1861, a position which he filled with great success until his resignation in 1867. Two years later he was sent to represent his country in London, but in November 1870 he was recalled by President Grant. After a short visit to Holland, he again took up his residence in England, where the Life and Death of John Barneveld appeared in two volumes in 1874. Ill health now began to interfere with his literary work, and he died at Frampton Court, near Dorchester, Dorset, on the 29th of May 1877, leaving three daughters. The merits of Motley as an historian are undeniably great. He has