i873-j Xoticcs of Books. 409 



eight individuals would, with a slight addition, amount to the 

 present population of the world. Considered that the limit to 

 the population per square mile of land should be that of France 

 (about 168), and that the thirtieth reduplication would bring up 

 this average density throughout the globe, Mr. Ponton supposes 

 that the limit reached, the number of the earth's inhabitants 

 would thenceforward remain almost stationary. May, then, this 

 reduplication have been governed by some law ? The reasoning 

 by which Mr. Ponton proceeds to trace out this law possesses 

 the deepest interest. The results are carefully tabulated, and 

 our readers will find the work well worthy of study. 



Elements of Natural Philosophy . By Professors Sir W. Thomson 

 and G. P. Tait. Part I. London: Macmillan and Co. 1873. 



Natural Philosophy, as the good old English term runs, is too 

 often so taught as to place the power of correlation as distant 

 as possible. Indeed, the ordinary method works somewhat in 

 this manner. Professors Sir William Thomson and G. P. Tait 

 have chosen what appears certainly a more philosophic course ; 

 for, setting out with Newton's definition that " mechanics is the 

 science of machines and the art of making them," and that the 

 science which investigates the action of force is properly termed 

 dynamics, we are led to the consideration of force acting in two 

 ways (that is, so as to compel rest or prevent change of 

 motion, and so as to produce or to change motion), as in 

 Statics and Kinetics. It has been usual, in our text-books, 

 to deal first with the laws of statics or the balancing of forces ; 

 but evidently the laws of kinetics (or rather of kinematics) 

 present more obvious points to the student than do the laws of 

 statics, which are necessarily subject to the limitation of 

 equilibrium. 



Yet this we conceive not the most important phase of progress 

 exhibited in the treatment of the subject. Let the student have 

 acquired his knowledge, let him have commenced his course of 

 original research, there is still one higher step to be made, which, 

 if not gained, will render his results of small worth. We refer 

 to the means of becoming acquainted, by experiment, with the 

 material universe and the laws which regulate it. "In general," 

 to quote our authors, "the actions which we see ever taking 

 place around us are complex, or due to the simultaneous action 

 of many causes. When, as in astronomy, we endeavour to 

 ascertain these causes by simply watching their effects, we 

 observe; when, as in our laboratories, we interfere arbitrarily with 

 the causes or circumstances of a phenomenon, we are said to 

 experiment." To observation, for instances, we owe the data of 

 astronomical, meteorological, and geological science ; to ex- 

 periment the deductions of spectrum analysis, electricity, and 



