The authors herein describe a means for carrying out experimentally Kelvin’s hydrodynamic analogy. A real liquid is used instead of the “ideal fluid” required by the analogy, the eddying flow caused thereby which violates the mathematical identity between the two cases being eliminated by taking photographs of the real liquid before eddying flow begins. The apparatus makes it possible to obtain photographs representing stress lines in shafts and also pictures of warped cross-sections.