Instruments measure or sample various physical, chemical and biological quantities in the water column, such as Nansen bottle which is a device for obtaining samples of seawater at a specific depth. It was designed in 1894 by the early 19th to 20th-century explorer and oceanographer Fridtjof Nansen and further developed by Shale Niskin in 1910. The bottle, more precisely a metal or plastic cylinder, is lowered on a cable into the ocean, and when it has reached the required depth, a brass weight called a "messenger" is dropped down the cable. When the weight reaches the bottle, the impact tips the bottle upside down and trips a spring-loaded valve at the end, trapping the water sample inside. The bottle and sample are then retrieved by hauling in the cable. A second messenger can be arranged to be released by the inverting mechanism, and slide down the cable until it reaches another Nansen bottle. By fixing a sequence of bottles and messengers at intervals along the cable, a series of samples at increasing depth can be taken from a single action.