The first of the Pyrenean stages takes us through two bath towns, Bagnères-de-Luchon, just before the climbing started today, and Bagnères-de-Bigorre at the finish line.
In 1835, the Scottish physicist James David Forbes took a trip to the Pyrenees and visited hot springs across the mountain range, including those at Luchon and Bigorre. In his travel narrative/geological survey/treatise on thermometers, Forbes had this to say about the springs of Bagnères-de-Luchon:
It is a curious fact that all the chief springs of Bagnères de Luchon issue from the granite within a few feet of one another, although their properties are believed to differ considerably, and their temperatures certainly do. They are kept separate by partitions connected with a vertical wall, into which slabs of stone (which may be removed) are cemented. The springs are called La Reine, La Grotte Supérieure, La Source aux Yeux, La Blanche, and La Froide; but all of these, excepting the two first, are apt to mix with one another; and I even learned that such a mixture was practised in order to give a greater apparent supply to some of the more esteemed of the springs. It is quite certain too that rain water mixes with some of these, which with other facts immediately to be noticed, render observations of temperature here of little avail. My observations were made on the springs as they flowed from beneath the wall just mentioned. These springs are highly sulphureous, and the two whose temperature I measured were copious.
Forbes’s main interest in the springs was their temperature. He would later devote a good portion of his career to the study of heat conduction, so the apparent warming of the springs at Bagnères-de-Luchon was absolutely fascinating:
The springs of Luchon have undergone most surprising changes. Camperdon, a writer of credit, and himself physician for thirty years at this place, assures us that the Source de la Reine was cold until 1755, when (on occasion of the great earthquake of Lisbon) it assumed a temperature of 41° Reaumur. The hot springs of many parts of Europe were affected by the same event. What shows that these springs are much connected with the sources of ordinary springs, is a curious fact mentioned to me by M. Barrau, one of the Inspecting Physicians of Luchon. In 1835, after great rains (in the month of May if my memory serves me rightly), the same spring, La Reine, delivered four times as much water as usual, and its temperature fell 16° Reaumur. This continued for twenty-five days, when it resumed its former state.
Forbes also cared a lot about the accuracy of thermometers. Thus, he (and I) appreciated his correspondent’s method of checking his thermometer before taking the temperature of the springs.
Boisgiraud’s observations were made with great care; I had an opportunity of verifying the accuracy of the copy I obtained of them by the originals. M. Boisgiraud informed me that he had verified the zero of his thermometer by frequently plunging it in melting snow.