The Cartesian diver is an autonomous velocity profiler capable of operating to 1 km depth in the ocean. It has a self-controlled buoyancy changer which is used to control the direction of profiling. The buoyancy changer has two states, full positive or full negative buoyancy. With correct ballasting the instrument has roughly equal up and down profiling speeds. A passive compressible volume appended to the pressure case is used to keep the buoyancy constant during an up or down run despite changes of density of the surrounding seawater, thus keeping the profiling speed and sampling intervals constant throughout a profile. A microcomputer system controls the buoyancy changer, data collection, and internal recording. Battery energy storage for the buoyancy changer is sufficient for 200 consecutive dives to 500 meters, thus 400 profiles. Horizontal components of velocity fluctuations with vertical wavelengths greater than 10 m are sensed using the method of geomagnetic induction. Since the instrument cannot accelerate relative to the fluid surrounding it, changes of its vertical velocity can be measured with a pressure gauge and interpreted as oceanic vertical velocity. Temperature, conductivity and microscale conductivity probes are included, in order to investigate relationships between finescale velocity structure, finescale density structure, and diffusive scale conductivity structure.