Page:Popular Science Monthly Volume 13.djvu/648

630 far that has brought the theory of the reduction of psychological facts to the finest point. But with respect to the substance of method and details of analysis he has, in common with Mill and Bain—in fact, with all the school—that which constitutes the organism of English psychology and gives it a physiognomy of its own in contemporary history."

little book plunges into depths where only mathematicians can follow; but its opening sentences are so characteristic of the author's clearness of perception and statement, that, as they involve no formula, and are withal instructive, we quote them:

"Just as geometry teaches us about the sizes and shapes and distances of bodies, and about the relations which hold good between them, bo dynamic teaches us about the changes which take place in those distances, sizes, and shapes (which changes are called motions), the relations which hold good between different motions, and the circumstances under which motions take place.

"Motions are generally very complicated. To fix the ideas, consider the case of a man sitting in one corner of a railway-carriage, who gets up and moves to the opposite corner. He has gone from one place to another; he has turned round; he has continually changed in shape, and many of his muscles have changed in size during the process.

"To avoid this complication we deal with the simplest motions first, and gradually go on to consider the more complex ones. In the first place, we postpone the consideration of changes in size and shape by treating only of those motions in which there are no such changes. A body which does not change its size or shape during the time considered is called a rigid body.

"The motion of rigid bodies is of two kinds; change of place, or translation, and change of direction or aspect, which is called rotation. In a motion of pure translation, every straight line in the body remains parallel to its original position; for, if it did not, it would turn round, and there would be a motion of rotation mixed up with the motion of translation. By a straight line in the body we do not mean merely a straight line indicated by the shape or marked upon the surface of the body; thus, if a box have a movement of translation, not only will its edges remain parallel to their original positions, but the same will be true of every straight line which we can conceive to be drawn, joining any two points of the box.

"When a body has a motion of translation, it is found, that every point of it moves in the same; bo that to describe the motion of the whole body it is sufficient to describe that of one point. When a body is so small that there is no need to take account of the differences in position and motion of its different parts, the body is called a particle. Thus the only motion of a particle that we take account of is the motion of translation of any point in it.

"A motion of translation mixed up with a motion of rotation is like that of a corkscrew entering into a cork, and is called a twist.

"Bodies which change their size or shape are called elastic bodies. Changes in size or shape are called strains.

"The science which teaches how to describe motion accurately and how to compound different motions together is called kinematic."

The volume is a college text-book, and the genius and position of its author are a sufficient guarantee of its originality and excellence. Prof. Clifford has broken down so sadly in health that he has been compelled to suspend work at University College, in London, and leave England for the more genial climate of Southern Europe. His work on the "Fundamental Ideas of Mathematics and Physics explained to the Non-Mathematical," with which he has been long occupied for the "International Scientific Series," is well advanced, and it is to be hoped that he will recover his health to complete it, and carry out the other important intellectual projects of which his teeming head is full.

of physical science and students who are without the aid of competent instructors and good laboratory apparatus will find this a very useful manual. The volume, in the first place, contains a great deal of valuable, practical instruction for making experiments in physics. The manipulation of apparatus, the construction of apparatus at the least possible cost, the points to be considered in the purchase of instruments, on these and many other like topics the author supplies a good deal of common-sense information, gathered in the course of his own experience as a teacher of science, and selected from technical manuals. Having aided the student in choosing his apparatus or constructing it, and given him some insight into the secrets of physical manipulation, the author, in the