How do Nansen bottles collect seawater samples from a chosen depth?

The Nansen bottle is a metal device used to collect seawater samples at specific depths. It is attached to a winch wire with its plug valves held open and lowered to the desired sampling depth.1

Once the bottle reaches the target depth, a weight known as a "messenger" is released from the surface and slides down the cable. Upon impact with the bottle, the messenger disengages the upper attachment, causing the device to reverse its orientation end-over-end. This movement triggers the valves to close, effectively trapping the water sample inside.1

During a single hydrographic cast, multiple Nansen bottles can be used in sequence; as each bottle is tripped, it releases another messenger to trigger the next bottle at a deeper level. While historically significant, the Nansen bottle has largely been superseded in modern oceanography by the Niskin bottle, which uses plastic construction and does not require the end-over-end reversal mechanism to collect samples.1