Phisique. Thermobatometre ou Appareil pour mesurer la temperature de la mer a de grandes Profondeurs

Scarce engraving of the thermometer that Peron used on the Baudin voyage to record sea temperatures at various depths. (Thermometer or Apparatus for measuring sea temperature at great depths)

Peron developed a new “less sensitive as possible” thermometer so that the depth measurement varied as little as possible with the ascent of the measuring device attached to a probe line. On the strength of his invention, Péron guaranteed “the superiority of [my] device over all those used to date“, and this even if the instrument in question was often reassembled flooded or broken due to pressure: “if the water pressure, always stronger than my means, would not have allowed it to still enter the interior of our device. Despite these serious disadvantages [sic], always the same result, always the sea temperature decreasing as we sink into the abyss … “. 

If the heart of the device consisted of “a mercury thermometer on an ivory ruler”, the naturalist transformed, for his measurements, his thermometers into “thermobatometers” by covering them with different layers of insulators supposed to prevent variation of temperature during the ascent to the surface 181. To make these layers, he successively used glass, wood, coal, tallow and tar. Such an insulated thermometer required prolonged immersion at the measured depth. However, the author did not detail the method used to do this. Likewise, he did not expand on the type of thermometer he used to develop his thermobatometers.

Ultimately, in his summary table of “general results of all the experiments made to date on the temperature of sea water either on its surface or at various [sic] depths”, Péron analysed the temperature of the seas as colder in depth than on the surface: “this cooling seems to be in some relation to the depth itself since it is all the greater, as the experiments were made by considerable depths”. Then he synthesised knowledge using the analogy with mountains: “all the results of observations made to date on this object come together to prove that the deepest abysses of the seas, as well as the summits of our highest mountains, are eternally frozen, even under the equator “

From Peron, Voyage de decouvertes aux terres australes. Paris