From Kook Science

Woodcut of a dowser holding a forked branch from La Physique Occulte (1693)

Dowsing is the practice of detecting the location of substances or entities — e.g. underground water sources, mineral veins, oil and natural gas, human beings, concealed locations, etc. — using the unconscious or psychic senses of the individual performing the dowsing operation (the dowser), whether operating with or without an apparatus such as a rod or pendulum. The French method of radiésthesie was originated as an ostensibly more scientific approach to the phenomena dealt with by dowsing, though the terms are commonly used interchangeably today.