Etymology

The Harmonic Series

1 + 1/2 + 1/3 + 1/4 + 1/5 + ...

The name from this series is derived from the study of standing waves. For instance, consider a standing wave on a string. The fundamental resonance frequency of the string occurs when the string is vibrating with both ends fixed an and amplitude maximum in the centre. The first harmonic occurs when there are two amplitude maxima (at the ends of the string0. The second harmonic occurs when there are three amplitude maxima, the third when there are four etc. At each harmonic, the wavelength is given by 2*1/k where k is 1 for the fundamental or one more than the number of each harmonic. Hence 1/k for k = 1, 2, 3,... is called the harmonic series.

Plumbing
Our name for people who work with pipes and water, i.e. plumbers, dervies from the latin word for lead: plumbium. This is because the ancient Romans, who spoke Latin, used to make water pipes out of lead. One of the many factors contributing to the fall of the Roman Empire was a high prevalence of lead poisoning in the population.