I. On the temperature of the interior of the earth, as indicated by observations made during the construction of the great tunnel through the Alps

It had been arranged from the commencement of the Alpine tunnel (often, though incorrectly, called the Mont-Cenis tunnel) that observations should be taken at intervals of about one kilometre (3281 feet) from both the French and Italian ends, with a view to determine as nearly as possible the law of increment ; and before the completion of the work it had become evident that the ordinary estimate of increased temperature due to the depth below the surface of the earth would by no means apply to this particular case. The actual length of the tunnel being 40,140 feet, and the culmi­nating point of the mountain being 5280 feet vertically above a point 21,156 feet from the Italian end, there was evidently scope for a number of valuable observations.