N. B. I wish to express my acknowledgment of the help given to nie by 

 Mr. C. B. Smith, Ma.B., in carrying out many of the experiments and in 

 checking the calculations. I have also been ably assisted by Mr. Withy- 

 combe, the foreman of the Laboratories, who has devised many mechanical 

 dvices wliiith have greatly facilitated the work. 



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