distributions of these freestream nuclei, the first, to our knowledge, to 

 be measured in a flow facility. It seemed quite plain then, that these 

 nuclei were the ones responsible for the travelling bubbles photographed by 

 Knapp. That being so, it would appear that if these freestream microbubbles 

 were diminished in number and size, travelling-bubble cavitation would be 

 similarly diminished. One mechanism, whereby the larger freestream bubbles 

 could be selectively removed from the low pressure region surrounding a 

 body, was proposed by Johnson and Hsieh; in their concept bubbles migrate 

 across streamlines under the action of the pressure field around the body. 

 Indeed, they show, by calculation, that the population of larger bubbles 

 near the position of minimum pressure can be reduced, and that from this 

 "screening" effect a size-dependent effect on travelling-bubble growth 

 with a given freestream nuclei distribution is found. Thus, a scale effect 

 is produced based upon the concept of travelling cavitation bubbles 

 originating from freestream microbubbles. At about the same time, Schiebe 

 (1966) proposed a scheme of inception measurement based upon the idea of 

 measuring the rate of these travelling-bubble cavitation events, a concept 

 which has subsequently received much attention and development at the St. 

 Anthony Falls Hydraulic Laboratory and elsewhere. 



This early development of cavitation-inception observation, develop- 

 ment of basic theory and phenomenological tests, may be said to have 

 culminated in the round robin tests on a standard body sponsored by the 

 International Towing Tank Conference in 1966. But before dealing with 

 these findings, we need to look in more detail at the inception scaling 

 effects that had already been reported. Of these, certainly the work by 

 Parkin and Holl (1953) has been most influential in providing a source of 

 information used by co-workers in formulating theories. There is a 

 pronounced Reynolds-number effect on these bodies, and a "size" effect (or 

 unit Reynolds-number effect) showing that additional parameters are 

 important (Figure 3) . An even larger size effect was produced in an 

 inception study on hydrofoils, Parkin (1952), as seen in Figure 4. These 

 effects were so large that it is easy to understand the intense effort to 

 explain them by many laboratories and investigators. Despite many attempts, 



