Fore^vord 



There has long been a need in science and engineering for systematic 

 publication of research studies larger in scope than a journal article but 

 less ambitious than a finished book. Much valuable work of this kind 

 is now published only in a semiprivate way, perhaps as a laboratory 

 report, and so may not find its proper place in the literature of the field. 

 The present contribution is the second of the Technology Press Research 

 Monographs, which we hope will make selected timely and important 

 research studies readily accessible to libraries and to the independent 

 worker. 



J. A. Stratton 



