tensioned by an elastic rope. The action of the messenger weight is to trip both caps shut and seal the tube. A reversing thermometer may also be carried on a frame fixed to the Niskin bottle. Since there is no rotation of the bottle to fix the temperature measurement, the thermometer has a separate spring-loaded rotating mechanism of its own tripped by the messenger weight. A modern variation of the Niskin bottle uses actuated valves that may be either preset to trip at a specific depth detected by a pressure switch, or remotely controlled to do so via an electrical signal sent from the surface. This arrangement conveniently allows for a large number of Niskin bottles to be mounted together in a circular frame termed a rosette. As many as 36 bottles may be mounted on a single rosette. Thermistor temperature sensors are more commonly employed on Niskin bottle as they are more accurate than mercury thermometers. References External links Niskin Bottles Firing Underwater Bottles Fridtjof Nansen Oceanographic instrumentation