4 Stromeyer, Explosions of Steam Pipes. 



suggests that these two velocities may be the same, but 

 gives no very precise proof. Dr. Hopkinson seems to have 

 overlooked the point on which Dr. Ritter insists, viz., 

 that the pressure of a pressure-wave in a prismatic bar is 

 independent of the length of the bar. Dr. Hopkinson's 

 formula is 



Tension = (i-e Ti • 



He made some very valuable experiments which 

 prove one of the points on which both investigators insist, 

 vis., that the greatest stress is first experienced not at the 

 moving end, i.e., near the falling weight or moving piston, 

 but at the fixed end. These experiments will be dealt 

 with later. 



The problem of water-hammer will be considered on the 

 lines followed by Dr. Ritter. It is a very common experience 

 at country stations to see a goods train either being 

 brought to rest or being set in motion by the engine. 

 In the latter case the trucks are in contact and the 

 couplings slack, the engine starts, moving with a velocity V, 

 which, for a few seconds at least, may be considered 

 constant. The engine at first pulls only the first truck, 

 which acquires the velocity V ; engine and truck No. i 

 move together at this velocity till the second chain, an 

 elastic body, is pulled tight, and, on the principle of the 

 elastic blow, the second truck at once attains the velocity 

 V; truck No i would at once come to rest were it not 

 being moved on steadily by the locomotive. Then, with 

 a succession of bangs (elastic blows), the remaining trucks^ 

 one after another, acquire the velocity V until the whole 

 train moves with this velocity. Now, it will be noticed 

 that the wave of percussion travels along the train at a 

 much greater velocity ( W) than the locomotive or the 

 trucks are moving. The reverse phenomenon always 



