120 ON THE POWER OF FLUIDS IN MOTION TO PRODUCE 



on the first letting on of the water, and on the suddenly closing a stop-cock 

 v/hen the fl.uid is in one of those pulses of attenuation which occur in the mo- 

 tion of fluids through long pipes, obstructed by enclosed air, and rendered irre- 

 gular by branches. 



How very powerful these blows are is well known to the engineer, and we 

 have now only to show under what circumstances they may be reversed in di- 

 rection, and vastly increased in intensity. 



The inverted direction will occur w^henever the pipe, pressed from without 

 by the atmosphere, and any large mass of fluid, as the water of a well or pit 

 through which it may pass, is left unsupported within by the sudden separa- 

 tion and contraction of the fluid which follows an hydraulic blow. Such in- 

 version will always occasion a strain from without, more powerful than the in- 

 ternal strain that produced it, but the severest strains of this kind will occur 

 when the original force is such as we have termed an interior vibration of im- 

 mense momentum and infinitely little extent, compared with the motions and 

 mass of the support. 



It is true that such a case is never practically attained, and that it far more 

 usually happens that the internal strain has an extent of motion approaching 

 to, or equalling that of the support; in which case the reversed pressure be- 

 comes the sura of the interior and exterior forces; but as this forms one limit 

 of the practical action, so may the explosive forces that we have described be 

 said to form the other; and I shall therefore consider it proper to give their 

 theory as connected with the subject under discussion. The very instance, 

 indeed, which Dr. Hare describes, and that which fell under my own observa- 

 tion, approach this class, as will be evident in the former case, from considering 

 that a pipe passing through a large metal reservoir, or chamber, might have 

 the lateral pulses propagated in the water it conveyed, small with regard to the 

 expansion which the chamber was capable of enduring; and this to the greater 

 extent, as we speak not of the total lateral motion of the pulse, but of that 

 lateral expansion which it would undergo during its extremely rapid transit 

 through the chamber. 



Such forces would nearly resemble those internal explosions that we assume ; 

 and as the sides of the reservoir in the case alluded to were of copper, and of 

 no great thickness, they will approach the remaining conditions of the problem, 

 partaking of the nature both of flexible and of elastic bodies, and having little 



