﻿194 Canadian Record of Science. 



no account of this has been given, beyond a few remarks in the 

 beginning of tlie cliapter (70 pages before), in which it is stated that 

 " the ether vibrations pass ofif in all directions by a species of 

 wave-motion." The properties of wave-motion are not described at all 

 till in the following chapter on Sound, where, after some examples of 

 vfbration, we find the statement that "To represent a sound-wave a 

 curve is used, called the sinusoidal curve," and the curve is figured. 

 But no proof is given that this does represent a sound-wave, nor 

 is simple harmonic vibration anywhere investigated. 



On p. 456 the explanation of the condenser is given thus : " The 

 reason for the greater capacity of one of the plates when near the other 

 is due to the attraction between the two charges." But Capacity 

 is not explained till p. 458, where the definition is reached that " The 

 capacity of a body for electricity is measured by the amount of 

 electricity required to raise its potential by unity." This itself will 

 puzzle a student who has only been told (twenty pages before) that 

 "when neighboring bodies differ in such a way that electrical 

 phenomena are observed in the region between them, the bodies are 

 said to be at dijfereiit potentials.'" He may wonder not only what kind 

 of a difference this is, but also what is a unity of it. The excellent 

 hydraulic illustrations given later (p. 478) will help to relieve his 

 perplexity. 



After the parallelogram of forces has been established, it seems 

 a fault of method to recur to experiment for the proof of the principle 

 of moments (p. Ill) and of central forces (p. 113.) The fewer the 

 experimental principles from which a science can be deduced, the 

 better. Other experimental results then come in as verifications 

 of theory. 



Of actual mistakes we have observed few. But on p. 29 occurs the 

 following : " It is found also to be true that the amount of work done 

 to produce a given acceleration in a given object is the same at what 

 ever velocity the particle is already moving ; for instance, to accelerate 

 its motion by 10 feet a second would require no more work if the 

 object is moving a mile a second than if its velocity is only a foot 

 a second, or if at the outset it was zero." The student who relies on 

 formulae will wonder how 5290^ — 5280- can be the same as 11- — 1- ; or 



2 2 



if he is in the habit of thinking out his dynamics, he will see that to 

 produce a given acceleration requires a given force to act for a given 

 time ; and that if the body is travelling at a greater rate during that 

 time it will cover a greater space, and the force will consequently have 

 to do more work. The source of the confusion is made clear by the 

 question on p. 33 ; " If a ball is at rest upon the floor, and you set it 

 in motion so that its velocity is one foot a second, is the work done by 

 you any greater or any less than if the ball had been moving with 

 a velocity of 5 feet a second and you had increased it to 6 feet ? 



