Skeptical observers, from Barett on, have noted that the movement of stick, rods or plumb line probably results from involuntary muscle contractions in the dowser's arms and hands. At least one form of dowsing can be explained only in terms of extrasensory perception. This is map dowsing- a procedure in which the dowser holds a plumb line or some other indicator over a map of the terrain to be explored.
The American Society of Dowsers, which boasts more than 2,000 dues-paying members, estimates that there are at least 25,000 practicing doswers in the United States and in many cases they have been successful.
Are Dowswers something more than natural-born geologists? This and similar questions continue to be hotly debated. Yet one thing is certain: on the basis of performance, no other form of divination has so severely challenged its scientific doubters.