Editing Jean-André Deluc (section)

=== Instruments ===
[[File:Whalebone hygrometer-MHS 1085-P4070321-gradient.jpg|thumb|Deluc's whalebone hygrometer, on display at the [[Musée d'histoire des sciences de la ville de Genève]].|left]]
[[File:Deluc-4.jpg|thumb|237x237px|Introduction page of a 1772 copy of volumes 1 of "''Recherches sur les modifications de l'atmosphère''"]]
Deluc dedicated a large part of his activity to perfecting or inventing [[measuring instrument]]s.

He devised a portable barometer for use in geological expeditions.<ref name=barometer/> His ''Recherches sur les modifications de l'atmosphère'' (2 vols. 4to, Geneva, 1772; 2nd&nbsp;ed., 4&nbsp;vols. Paris, 1784) contain experiments on moisture, evaporation and the indications of [[hygrometer]]s and [[thermometer]]s. He applied the [[barometer]] to the determination of heights. The ''[[Philosophical Transactions]]'' published his account of a new hygrometer, which resembled a mercurial thermometer, with an ivory bulb, which expanded by moisture, and caused the mercury to descend.<ref name=hygrometer/> He later devised a whalebone hygrometer which sparked a bitter controversy with [[Horace-Bénédict de Saussure]], himself inventor of a hair hygrometer.<ref>René Sigrist, "Scientific standards in the 1780s: A controversy over hygrometers", in John Heilbron & René Sigrist (eds), ''Jean-André Deluc. Historian of Earth and Man'', Geneva, Slatkine, 2011, p. 147-183.</ref> He gave the first correct rules for measuring heights with the help of a barometer<!-- Please beware of the mention in 1911 Britannica: "in the ''Philosophical Transactions'', 1771, p. 158"-->.<ref name=AltitudeBarometer/>

Based on his experiments in 1772, Deluc advocated the use of [[Mercury (element)|mercury]], instead of alcohol or other fluids, in thermometers, as its volume varies the most linearly with the method of mixtures. In detail, if two portions of water of equal masses A, B were mixed, and let the resulting water be C, and if we immerse a thermometer in A, B, C, we obtain lengths <math>l_A, l_B, l_C</math>. Deluc expected that <math>l_C = \frac 12 (l_A + l_B)</math>, and similarly for other ratios of mixtures. He found that thermometers made using mercury allowed the closest fit to his expectation of linearity.<ref>{{Cite book |last=Chang |first=Hasok |url=https://www.worldcat.org/oclc/457147642 |title=Inventing temperature : measurement and scientific progress |date=2007|page=64 |isbn=978-0-19-533738-9 |location=Oxford |oclc=457147642}}</ref>

In 1809 he sent a long paper to the Royal Society on separating the chemical from the electrical effect of the [[dry pile]], a form of [[Voltaic pile]],<ref name=Nicholson/> with a description of the electric column and aerial electroscope, in which he advanced opinions contradicting the latest discoveries of the day; they were deemed inappropriate to admit into the ''Transactions''.{{sfn|Chisholm|1911|p=976}} The dry column described by Deluc was constructed by various scientists and his improvement of the dry pile has been regarded as his most important work, although he was not in fact its inventor. He was a valued mentor to the young [[Francis Ronalds]], who published several papers on dry piles in 1814–15.<ref>{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location=London}}</ref><ref>{{Cite journal|last=Ronalds|first=B.F.|date=July 2016|title=Francis Ronalds (1788–1873): The First Electrical Engineer?|journal=Proceedings of the IEEE|volume=104|issue=7|pages=1489–1498|doi=10.1109/JPROC.2016.2571358|s2cid=20662894}}</ref><gallery>
File:Deluc-1.jpg|1772 copy of volumes 1 and 2 of "''Recherches sur les modifications de l'atmosphère''"
File:Deluc-2.jpg|Title page of a 1772 copy of volume 1 of "''Recherches sur les modifications de l'atmosphère''"
File:Deluc-3.jpg|Table of contents page of a 1772 copy of volume 1 of "''Recherches sur les modifications de l'atmosphère''"
File:Deluc-7.jpg|Dedication to a 1772 copy of volume 1 of "''Recherches sur les modifications de l'atmosphère''"
File:Deluc-8.jpg|Dedication to a 1772 copy of volume 1 of "''Recherches sur les modifications de l'atmosphère''"
File:Deluc-5.jpg|First page of a 1772 copy of volume 1 of "''Recherches sur les modifications de l'atmosphère''" 
</gallery>