Editing Challenger expedition (section)

== Scientific objectives ==
The [[Royal Society]] stated the voyage's scientific goals were:<ref name="HMS Challenger: Science">{{cite web|url=http://aquarium.ucsd.edu/Education/Learning_Resources/Challenger/science.php|title=HMS Challenger: Science|publisher=Birch Aquarium|access-date=2013-12-03|url-status=dead|archive-url=https://web.archive.org/web/20130126080934/http://aquarium.ucsd.edu/Education/Learning_Resources/Challenger/science.php|archive-date=2013-01-26|df=dmy-all}}</ref><ref name="Aitken_t1" />
# To investigate the physical conditions of the deep sea in the great ocean basins—as far as the neighborhood of the Great Southern Ice Barrier—in regard to depth, temperature, circulation, [[specific gravity]] and penetration of light.
# To determine the [[chemical composition]] of seawater at various depths from the surface to the bottom, the organic matter in solution and the particles in suspension.
# To ascertain the physical and chemical character of deep-sea deposits and the sources of these deposits.
# To investigate the distribution of organic life at different depths and on the deep seafloor.

[[File:Examining a haul on board the Challenger.jpg|thumb|upright|Examination of caught specimen]]
One of the goals of the physical measurements for HMS Challenger was to be able to verify the hypothesis put forward by Carpenter on the link between temperature mapping and global ocean circulation in order to provide some answers on the phenomena involved in the major oceanic mixing. This study is a continuation of the preliminary exploratory missions of [[HMS Lightning (1823)]] and [[HMS Porcupine (1844)]].<ref name="Aitken_t1" /> These results are important for Carpenter because his explanation differed from that of another renowned oceanographer at the time, the American [[Matthew Fontaine Maury]].<ref name="Aitken_t2" /> All these results of physical measurements were synthesized by [[John James Wild]] (i.e. the expedition's secretary-artist) in his doctoral thesis at the University of Zurich.<ref>{{cite book |last1=Wild |first1=John James |title=Thalassa: An Essay on the Depth, Temperature, and Currents of the Ocean |date=1877 |publisher=Marcus Ward and Co. |location=London, UK}}</ref>

A second important issue concerning the collection of different kinds of physical data on the ocean floor was the laying of submarine telegraph cables. Many transoceanic cables were being laid in the 1860s and 1870s and their efficient laying and operation were matters of great strategic and commercial importance.<ref name="Aitken_t1" />

At each of the 360 stations the crew measured the bottom depth and temperature at different depths, observed weather and surface ocean conditions, and collected seafloor, water, and biota samples. ''Challenger''{{'s}} crew used methods that were developed in prior small-scale expeditions to make observations. To measure depth, they would lower a line with a weight attached to it until it reached the sea floor. The line was marked in {{convert|25|fathom|adj=on}} intervals with flags denoting depth. Because of this, the depth measurements from ''Challenger'' were, at best, accurate to the nearest {{convert|25|fathom|adj=on||}} demarcation. The sinker often had a small container attached to it that would allow for the collection of bottom sediment samples.<ref name="Aitken_t1" />

The crew used a variety of dredges and [[trawl]]s to collect biological samples. The dredges consisted of metal nets attached to a wooden plank and dragged across the sea floor. Mop heads attached to the wooden plank would sweep across the sea floor and release organisms from the ocean bottom to be caught in the nets. Trawls were large metal nets towed behind the ship to collect organisms at different depths of water.<ref>{{cite web|url=http://www.aquarium.ucsd.edu/Education/Learning_Resources/Challenger/science2b.php|title=HMS Challenger – The science: dredging and trawling|publisher=University of California, San Diego|work=Aquarium|access-date=2014-10-13|url-status=dead|archive-url=https://web.archive.org/web/20150219021951/http://aquarium.ucsd.edu/Education/Learning_Resources/Challenger/science2b.php|archive-date=2015-02-19|df=dmy-all}}</ref> Upon the retrieval of a dredge or trawl, ''Challenger'' crew would sort, rinse, and store the specimens for examination upon return. The specimens were often preserved in either brine or alcohol.{{citation needed|date=December 2020}}<ref>{{Cite book |last1=Buchan |first1=Alexander |title=Report on the Scientific Results of the Voyage of H.M.S. Challenger during the Years 1873-76 under the Command of Captain George S. Nares ... and the Late Captain Frank Tourle Thomson, R.N. |last2=Huxley |first2=Thomas Henry |last3=Murray |first3=John, Sir |last4=Nares |first4=Goerge S. Sir (George Strong) |last5=Nares |first5=George Strong, Sir |last6=Pelseneer |first6=Paul |last7=Thomson |first7=C. Wyville, Sir, (Charles Wyville) |last8=Thomson |first8=Frank Tourle |publisher=Edinburgh, Neil, 1880-1895 |year=1880 |edition= |volume=Zoology.–Vol. 1. |pages=23–24 |language= |chapter=GENERAL INTRODUCTION TO THE ZOOLOGICAL SERIES OF REPORTS. By Sir C. Wyville Thomson, F.R.S.}}</ref>  [[Manganese nodules]] and sediment, which was later found to contain [[micrometeorites]], was collected from the ocean floor.<ref>{{cite web | url=https://www.nhm.ac.uk/discover/news/2022/september/hms-challenger-how-150-year-old-expedition-still-influences-scientific-discoveries-today.html | title=HMS Challenger: How a 150-year-old expedition still influences scientific discoveries today &#124; Natural History Museum }}</ref> 

The primary thermometer used throughout the ''Challenger'' expedition was the [[Miller–Casella thermometer]], which contained two markers within a curved mercury tube to record the maximum and minimum temperature through which the instrument traveled.<ref name="Aitken_t2">{{cite book |last1=Aitken |first1=Frédéric |last2=Foulc |first2=Jean-Numa |title=From deep sea to laboratory. 2 : Discovering H.M.S. Challenger's physical measurements relating to ocean circulation |date=2019 |publisher=ISTE-WILEY |location=London, UK |isbn=9781786303752 |url=http://www.iste.co.uk/book.php?id=1499}}</ref> Several of these thermometers would be lowered at various depths for recording. However, this design assumed that the water closer to the surface of the ocean was always warmer than that below. During the voyage, ''Challenger''{{'}}s crew tested the [[reversing thermometer]], which could measure temperature at specified depths. Afterwards, this type of thermometer was used extensively until the second half of the 20th&nbsp;century.<ref name="HMS Challenger: Science"/> After the return of the Challenger, C.W. Thomson asked [[Peter Tait (physicist)|Peter Tait]] to solve a thorny and important question: to evaluate the error in the measurement of the temperature of deep waters caused by the high pressures to which the thermometers were subjected. Tait solved this question and continued his work with a more fundamental study on the compressibility of liquids leading to his famous [[Tait equation]].<ref name="Aitken_t3">{{cite book |last1=Aitken |first1=Frédéric |last2=Foulc |first2=Jean-Numa |title=From deep sea to laboratory. 3, From Tait's work on the compressibility of seawater to equations-of-state for liquids |date=2019 |publisher=ISTE-WILEY |location=London, UK |isbn=9781786303769 |url=http://www.iste.co.uk/book.php?id=1534}}</ref> [[William Dittmar]] of Glasgow University established the composition of seawater. Murray and [[Alphonse François Renard]] mapped oceanic sediments.

Thomson believed, as did many adherents of the then-recent [[theory of evolution]], that the deep sea would be home to "[[living fossils]]" long [[extinct]] in shallower waters, examples of "[[Missing link (human evolution)|missing links]]". They believed that the conditions of constant cold temperature, darkness, and lack of currents, waves, or [[seismic]] events provided such a stable environment that evolution would slow or stop entirely. [[Louis Agassiz]] believed that in the deeps "we should expect to find representatives of earlier [[geological]] periods." [[Thomas Huxley]] stated that he expected to see "[[zoological]] antiquities which in the tranquil and little changed depths of the ocean have escaped the causes of destruction at work in the shallows and represent the predominant population of a past age." Nothing of the sort came to pass, however; though a few organisms previously regarded as extinct were found and cataloged among the many new discoveries, the harvest was typical of what might be found in exploring any equivalent extent of new territory. Furthermore, in the process of preserving specimens in [[Alcohol (chemistry)|alcohol]], Thomson and chemist John Young Buchanan realized that he had inadvertently [[debunked]] Huxley's prior report of ''[[Bathybius haeckelii]]'', an [[acellular]] [[protoplasm]] covering the sea bottoms, which was purported to be the link between non-living matter and living cells. The net effect was a setback for the proponents of evolution.<ref name=eis/>