heavy uniform cord for all vibratory motions the wave-length of which embraces several intervals separating the bird-shot, and will transmit these motions much more efficiently from one end of the cord to the other than if the bird-shot were not there. This simple experiment with bird-shot and a long stretched cord can easily be tried; it is a very inexpensive experiment and will convince you even if you know nothing about the mathematical elements of the problem. This is the simple experiment which I had in my head during my tramping along the zigzag roads in Switzerland in 1894. A professor of physics, who often acted as consulting physicist to telephone companies, had such a loaded cord hung up over his lecture-room table for the purpose of explaining transmission of wave-motion from one end of the cord to the other, but he never inferred anything from it regarding loading a telephone-line with inductance-coils. When I called his attention to it, and joked about it, he blamed his hard luck, implying, I thought, that solving a dynamical problem and building upon the foundation of this solution an electrical invention is a question of luck. From the simple apparatus, just referred to, to the elaborate electrical demonstrations which would convince stubborn and hard-headed telephone engineers, was a very long pull. The most embarrassing feature in these demonstrations was that I could not afford large expenditures of money to carry them out on actual telephone conductors; besides, it would have been unwise to disclose to interested parties, the owners of long-distance telephone-lines, a theory for which I had not yet obtained a satisfactory experimental proof. I had to invent laboratory apparatus equivalent to telephone lines or cables of great length, which would enable me to do all the experimenting in my laboratory. That required almost as much thought, inventive effort, and mathematical achievement as the solution of the extended La Grangian problem.