﻿Notices respecting New Books. 237 



city of a given motion. If we adopt the nomenclature that Dynamics 

 is the general doctrine of Force, and Mechanics the applications of 

 that doctrine to Machines, then Kinematics stands related to the 

 former of these in the same manner as Mechanism to the latter. The 

 fundamental distinction implied in the word kinematics or pure me- 

 chanism is now so familiar to students of mechanical science, that 

 we are apt to forget how few years have passed since that distinction 

 was actually effected : — 



"The science of pure motion," says the late Dr.Whewell*, writing, 

 we believe, in a.d. 1840, " has not generally been separated from 

 the science of motion viewed with reference to its causes. Recently, 

 indeed, the necessity of such a separation has been seen by those who 

 have taken a philosophical view of the science." It was mainly by 

 the labours of Mr. Willis that this separation was actually effected, 

 and the theory of pure mechanism established as an independent 

 science. This was done in his well-known treatise ' The Principles 

 of Mechanism/ the first edition of which was published in the year 

 a.d. 1841. In noticing the appearance of the second edition of a 

 work which long ago took its place as the standard work on the 

 subject, it is quite unnecessary to speak in its praise. Nor are the 

 changes introduced into the new edition of a kind to call for a long 

 notice. Perhaps the most important is the rearrangement of the 

 first and principal part of the volume. This change is as follows : — 

 One piece of a machine may communicate motion to a second piece 

 either (A) by rolling contact, (B) by sliding contact, (C) by wrap- 

 ping connexion, (D) by link-work, (E) by reduplication. Under 

 each of these divisions there are three classes : — (1) Directional rela- 

 tions of the motions constant, and velocity-ratio of the pieces constant. 

 (2) Directional relation constant and velocity-ratio varying. (3) Di- 

 rectional relation changing periodically and the velocity-ratio con- 

 stant or varying. In the first edition the classes were treated sepa- 

 rately in the five divisions. In the new edition the divisions are given 

 separately, and each, except (E), subdivided into the three classes. 

 Mr. Willis states that, in his experience as a lecturer, he found the 

 very great convenience of thus classing the elements of machines 

 with reference to the mode in which velocity is communicated. 

 The same arrangement, it may be mentioned, is adopted by Dr. 

 Rankine in part iv. of his treatise ' On'Applied Mechanics ;' and there 

 can be no doubt that it constitutes a great improvement in the form 

 of the work. 



Most of the changes in the substance of the book are such as are 

 sure to occur to an author who re-edits a book after a lapse of nearly 

 thirty years : new articles are inserted here and there ; additional 

 notes relating to the history of the subject are of frequent occurrence ; 

 and besides these there are two parts wholly new, viz. on "Combina- 

 tions for the action of which properties of friction are employed," and 

 on " Universal Joints." The effect of all this has been to increase 

 the book by about one third part of its original size. The revision 



* Philosophy of the Inductive Sciences, p. 152 (ed. 1846). 



