Page:Evolution of the thermometer.djvu/50

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aer calidus, magnus calor;" and the large sphere above the tube was inscribed "Mobile perpetuum."

De Guericke constructed a barometer on similar lines, and gave it the legend "Semper vivum;" he described it in a letter to G. Schott, dated 30 December, 1661.

Still more remarkable was the self-registering meteorological apparatus devised by the great German physicist; it recorded every hour the change in temperature, direction of the wind, the rainfall, and amount of snow or hail. Monconys describes this briefly, with a diagram showing the arrangement, in his Journal des Voyages (Vol. III, 1663).

The differential thermometer was invented by Gaspar Schott, as early as 1657 (Mechan. hydraul-pneumat. II, 231), and afterwards improved by Joh. Christ. Sturm, Professor in Altorf, Bavaria, in 1676. It was a U-tube with arms of uneven length, both tubes being closed; Sturm explained its action quite correctly.

In the latter part of the seventeenth century references in scientific literature to the construction and use of the ordinary thermoscopes multiplied; Le Febure in his admirable treat-