406 Notices respecting New Books, 



same quantity of air must successively fill a space 3, 5, 7, 9 times 

 as great as at first, and must hence diminish its density and con- 

 sequently its pressure so as to become from | to -J of what it was 

 in the centre of the circle. The pressure of the air which passes 

 through the tube into the space between the disks is thus, al- 

 though originally greater than the pressure of the atmosphere, 

 gradually becoming less while radiating to, and escaping at the 

 edge, and becomes actually less than the atmospheric pressure. 

 It is true that the velocity of the air-particles decreases from the 

 centre to the edge, and the diminution of pressure is not quite so 

 great as would appear from our calculation, because the external 

 air opposes a considerable resistance to the escaping current, but 

 the ultimate effect is that the pressure over the greater portion of 

 the space between the disks is less than the external pressure of 

 the atmosphere : hence the remarkable result that the external air 

 presses the suspended disk against the current blown from the 

 tube, and moves it close to the fixed disk, and not until the cur- 

 rent of air diminishes in strength will the disk fall back again." 



The general principles of wave-propagation, both when the vibra- 

 tions of the particles of the medium are transversal and also when 

 they are longitudinal, are clearly explained, and a careful account 

 given of the vibrations of strings, rods, and plates, the laws obeyed 

 by them, the theory of overtones, &c. Occasionally, however, the 

 explanations are by no means adequate : for instance, on page 382 

 we find the following, relative to the action of a flue organ-pipe : — 

 "A current of air is directed through a narrow slit and strikes the 

 opposite edge of the aperture or 'mouth' of the instrument." 

 This is all : here the description comes to a sudden conclusion, 

 while the reader is left to explain for himself as best he can what 

 becomes of the air thus striking against the sharp edge, and how it 

 sets in vibration the air-column in the pipe. A little further on, 

 the pitch of an organ-pipe is spoken of as depending on its length 

 only, the lengths of two open pipes which yield notes whose 

 vibration-frequencies are as 2 to 1 being as 1 to 2. It might at 

 least have been mentioned that the pitch is a function also of the 

 cross section of the pipe. 



On page 347 pitch and tone are used synonymously. 



These, however, are small matters. On the whole the chapter on 

 Acoustics is good, the portions relating to the organ of voice and 

 the difference of vowel sounds of the same pitch as depending on 

 the presence of various overtones, differing in intensity, combined 

 with the fundamental, being especially well worked out. 



In the Optics, after some well-considered directions have been 

 given for the construction and silvering of mirrors, several simple 

 cases of reflection are described and geometrically solved. There 

 is here the same absence of general formulae which we noticed in 

 the case of the mechanics. The student is instructed to work out 

 each example separately, and to determine geometrically the rela- 

 tive sizes of an object and its image and their distances from the 

 mirror. No mention is made of spherical aberration (which the 



