240 Notiees respecting New Books. 



The Mechanical Principles of the Aeroplane. By S. Brodetsky, 

 Keader in Applied Mathematics, University of Leeds. (J. & A. 

 Churchill, 1921, 270 pages ; price 21 shillings.) 



This treatise goes very deeply and thoroughly into the mechanical 

 and mathematical calculations involved in the design of the flying 

 machine. 



An introduction discusses the possibility of flight of a machine 

 heavier than air, and might well be amplified to show the main 

 facts involved, after the resistance was taken to vary as the simple 

 sine law instead of the squared sine, as required on Newton's 

 method of treating air as a cloud of dust particles, ignoring the 

 equation of continuity. 



But as soon as the theoretical work of Helmholtz and Kirchhoff 

 on discontinuous flow of a fluid pointed out the simple sine 

 law as much more appropriate, it was seen that mechanical 

 flight was in the region of possibility, and had only to await 

 the arrival of an engine of weight per horse-power low enough. 

 This was provided happily by the advent of the motor vehicle 

 and its internal combustion engine as a present ready made, and 

 then man could really take to the air, and not merely dream of 

 it in the realm of poetical fancy. 



Such an introduction need not go beyond calculations to appeal 

 to the young scholar, and would serve to attract him further into 

 the deeper treatment given here, as in other complete treatises, 

 such as Bairstow r 's ' Applied Aerodynamics.' 



Optical Theories. By D. N. Mallik, Professor, Presidency 

 College, Calcutta. Second Edition revised. 200 pages ; 

 16 shillings. (Cambridge,- at the University Press, 1921.) 



A course of lectures delivered before the Calcutta University to 

 advanced students on Optical Theories. 



Early speculations, on the corpuscular and the undulatory 

 theory, are discussed in Chap. I. The elastic solid theory, 

 electro- magnetic theory, and electron theory follow in Chaps. II, 

 III, IV. In this new edition a discussion is introduced in 

 Chap. V of the new theory of Relativity, in its bearing on the 

 modification of former theories of the aether. 



These various theories are summarised and compared, in a 

 valuable discussion in eloquent words in Chap. VI, of the nature 

 of the electro-magnetic field. 



In this way the true inner significance is brought to light of 

 the accurate interpretation of the results concealed in the differ- 

 ential equations and triple integrals of the rigorous mathematical 

 treatment. 



