Page:Evolution of the thermometer.djvu/11

 and the air begins to cool, then the air in the retort gets thicker and heavier, so that the retort fills with water, and if the glass was made very hot the water will completely fill it."

This simple experiment merely shows the expansion of air by heat and its contraction by cold, and there is no question whatever of measuring the amount of heat; besides, Drebbel had been anticipated by Hero of Alexandria, who described essentially the same phenomena 1,750 years before, and by Drebbel's contemporary della Porta.

Giambattista della Porta of Naples, the precocious author of "Magia Naturalis" (1558), is sometimes credited with the invention of the thermometer, owing to a passage in his book "I tre libri de spiritali," published at Naples in 1606. In this work he describes an experiment devised to measure the expansion of air when heated by a fire; the arrangement described is the same as Drebbel's retort and basin, but the cut accompanying the text shows an inverted matras with its mouth under water; Porta marked on the tube with pen and ink the highest and lowest points of the water-column, but he does not seem to have used the instrument as a heat-measurer.