Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/603

Rh describing in language of much freshness and picturesque charm the view from a hill overlooking the poet s native vale of Towy. Artless in an affected age, the natural images which crowd upon one another in this charming little poem are as admirable now as when they were written, and hold an assured place in English literature. Dyer s ambition to succeed as a painter impelled him to visit Italy, and about ten years after the publication of Grongar Hill he seenis to have attained this great desire, and to have spent some time in the south of Europe. It was in consequence of this tour that he wrote his next poem, TJie Ruins of Rome, a descriptive piece in about 600 lines of Miltonic blank verse. In this work the phraseology is pompous and conventional, but there is considerable know ledge displayed, and the ardour of a true lover of antiquity. The Ruins of Rome appeared in, and increased its author s reputation. Having fallen into bad health while painting in the Campagna, and finding that he was not destined to excel in the practice of art, he determined to enter into holy orders. In he was ordained by the bishop of Lincoln, and presented with the living of Calthorpe, in Leicestershire. He was married about this time to a lady descended from the brother of Shakespeare. In he was translated to the living of Belchford, in Lincolnshire, to which was added in  that of Coningsby. In he exchanged Belchford for the wealthier incumbency of Kirby-on-Bane. In he published his longest work, the didactic epic of The Fleece, in four books, of which the first discoursed of the tending of sheep, the second of the shearing and preparation of the wool, the third of weaving, and the fourth of trade in woollen manufactures. The subject was prosy, and the stately blank verse in which it was discussed gave the poem a ridiculous air. The town took no interest in it, and the wits facetiously prophesied that &quot;Mr Dyer would be buried in flannel.&quot; He did. in fact, very shortly afterwards follow his poem to the grave, for he died of consumption on ths 24th of July, leaving a wife and four children. After his death his genius was defended and his writings analyzed by Scott of Amwell, who published a commentary on Dyer s poems. The latter were collected by Dodsley in, but they only form one small volume. Grongar Hill has been compared with Sir John Denham s Cooper s Hill, which may in some measure have suggested it. These two pieces remain the most important topographical poems in English literature, if we exclude Ben Jonson s Penskurst.  DYNAMICS properly means that science which treats of the action of force. Defining force as that which affects the motion of matter, it appears that the study of dynamics will lead to the consideration of the motion of material systems, and the laws in accordance with which this motion is changed by the mutual actions of the bodies forming such systems. But there is a sense iu which we may con template the geometrical results of the motion of bodies without studying the forces under which, or the time dur ing which, it takes place ; and hence there are many pro blems which at first sight we might be disposed to include under the head of dynamics, but which also belong to the domain of pure mathematics, and may therefore more pro perly be considered as a branch of geometry. On the other hand, there is a branch of dynamics which treats of pure motion without taking any account of its subject or the means by which it is produced or changed. In this branch, to which the term kinematics, though first employed by Ampere in a wider sense, may with propriety be confined, it may seem that no consideration of matter or of force is involved ;^ but, unlike the class just alluded to, the pro blems which come under this head involve explicitly the element of time, and it is only after studying the laws of dynamics that we are able to furnish a theoretical measure of time satisfying the demands of the human mind. Thus any subject in which the measurement of time is involved enters on this account into the domain of dynamics.

Measurement of Time.—For ordinary purposes the rota tion of the earth furnishes a sufficiently exact means of measuring time, and the observation of the transit of a known star is the best method we possess of determining the error of a clock ; but that the fundamental conception of the measurement of intervals of time is based upon other foundation than the diurnal rotation of our planet at once appears from the fact that we see no inconsistency in asking whether the length of the day is the same now as it was 2000 years ago. If our primary conceptions of the measure ment of time were derived from the earth s rotation, the absolute constancy of the length of the day would be amatter of definition. But it is not to the motion of the earth or of any other single body that we are indebted for our highest conception of the measurement of time it is rather to the dynamical principle expressed in the first law of motion ; and hence it is that the theoretical measurement of time, and of other physical quantities which explicitly involve time, must find a place under the head of dynamics. Kinematics may therefore properly be treated as a branch of dynamics, and for its discussion, as well as for the enunication and explanation of the laws of motion, the reader is referred to the article on. Perhaps there is nothing which appears to present a subject for study simpler than that afforded by the properties of space, and hence it is that geometry attained so high a reputation and made such rapid advances among the ancients. It was easy to construct material standards of length and by their means to measure approximately the linear dimensions of limited portions of space, the human mind being only too ready to believe in the constancy of the dimensions of the standards constructed ; and thus the properties of space presented a subject which, at the very outset, afforded a facility for investigation which was want ing in the study of other physical quantities. The great simplification introduced by this belief in the permanence of the dimensions of material standards will be apparent if we consider the position in which we should be placed by the adoption of a different hypothesis. Once admit the supposition that the properties of a figure, as regards dimensions or form, depend explicitly on its position in space, or upon time, either by a process of growth in them selves or because space is changing its character, and the whole subject of geometry will require reconsideration.

Displacement.—A number of points or figures may be connected in accordance with such geometrical conditions that if one or more be displaced in a given manner the displacements of all the others may be determined. The determination of the displacement of each in terms of the given displacements is a problem in pure mathematics, and the branch of geometry which treats of such questions may be called the science of displacement. If we suppose the figures here contemplated to be material bodies, and the geometrical conditions to be determined by means of material constraints such as links, guides, teeth, and the like, the science of displacement thus applied becomes that of mechanism, and it is only necesary here to cull attention to the following statements. First, in the study of dis placements, or of pure mechanism, no account is taken of any but the geometrical properties of the bodies displaced, while the forces engaged in producing the displacement are entirely neglected : the consideration of the mechanical properties of the materials of which the parts of a machine are constructed, the forces acting between those parts, and the best means of &quot; fitting &quot; them, belongs to applied mechanics and machine construction. Secondly, the ele ment of time is altogether left out of consideration ; for, 