Introdiiction xv 



In conclusion the author desires to tender his best 

 thanks to the Rev. Cecil Grant of St George's School, 

 and to Mr W. J. Ashby of the Wye School, for having 

 allowed him the use of their schools and appliances 

 during the progress of these lessons. Especially are 

 his thanks due to Mr Lionel Armstrong for much help 

 ungrudgingly rendered in collecting material, taking 

 photographs, and supervising the experiments. 



E. J. R. 



Habpendbn, 



February, 1911. 



