Page:Evolution of the thermometer.djvu/13

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ment from a very hot place to a very cold one. But without disturbing its position, if you lay your hand gently on the upper bulb, it is so sensitive and the air is so susceptible to every impression, that you will instantly see the water descend, and on removing the hand the water will return to its place. It is still more sensitive if one warms the bulb with his breath, as if one wished to speak a word into its ear to command the water to descend.

"The reason of this motion is that the air heated in the tube rarefies and dilates and wishes to have more room, and therefore presses upon the water and makes it descend. On the other hand when the air is cooled and condenses it begins to occupy less space and fearing to leave nothing but a vacuum, the water ascends at once.

"I say in the second place that by this means one can know the degrees of heat and of cold that are in the air at each hour of the day, the air that is enclosed in the bulb rarefies or condenses, ascends or descends. Thus you see in the morning the water stands quite high, and it descends little by little up to midday; towards vespers it remounts. Thus in winter it ascends so high that it nearly fills the tube;