With magnetic rowing machines, adjusting the resistance means varying the distance between a flywheel and one or more strong magnets. This is done either manually, using mechanical sliders, or digitally by the console controls. Like hydraulic rowers, magnetic rowing machines operate quietly and have a compact design for easy storage. Unlike hydraulic rowers, they can provide a smooth and consistent workout. It's a very different kind of feel to what you'd expect from an air or water rower, though. You still get a good workout, but it doesn't simulate the sensation of rowing in quite the same way.
The table on the left gives the speed of waves of different wave lengths in deep water. “Deep” in this context is not an absolute value, but is relative to wave length. The simple relationship starts to to breakdown when the depth of the water is less than 1/4 th the wave length. At that depth the bottom exerts sufficient drag on the wave to slow its motion and thus decrease the wavelength [equations and more about wave speed].
Head races are time trial / processional races that take place from autumn (fall) to early spring (depending on local conditions). Boats begin with a rolling start at intervals of 10 – 20 seconds, and are timed over a set distance. Head courses usually vary in length from 2,000 metres (1.24 mi) to 12,000 metres (7.46 mi), though there are longer races such as the Boston Rowing Marathon and shorter such as Pairs Head.