78 SCIENTIFIC RECREATIONS. 



has always the same volume ; and although the attributes of the liquid 

 vary under different circumstances, under the same conditions its properties 

 are exactly the same. Now, water expands very much when under the. 

 influence of heat, like all liquids ; solids and gases also expand upon the 

 application of heat. 



We can easily establish these statements. A metallic ring when 

 heated is larger than when cool. A small quantity of air in a bladder 

 when heated will fill the bladder, and water will boil over the vessel, or 

 expand into steam, and perhaps burst the boiler. So expansion is the 

 tendency of what we term heat. 



We make use of this quality of heat in the thermometer, by wktch 

 we can measure the temperature not only of liquids or solids, but of the 

 atmosphere. The reading of the thermometer varies in different countries,, 

 for the degrees are differently marked, but the construction of the instru- 

 ment is the same. It is called thermometer from two Greek words 

 signifying the measure of heat. It is a notable fact that Castelli, writing 

 in 1638, says to Ferdinand Caesarina : "I remembered an experiment which 

 Signer Galileo had shown me more than thirty-five years ago. He took 

 a glass bottle about the size of a hen's egg, the neck of which was two- 

 palms long, and as narrow as a straw. Having well heated the bulb in 

 his hands, he placed its mouth in a vessel containing water, and with- 

 drawing the heat of his hand from the bulb, the water instantly rose in 

 the neck more than a palm above the level of the water in the 

 vessel." 



Here, then, we have an air- thermometer, but as it was affected by the 

 pressure as well as the temperature of the atmosphere, it could not be relied 

 upon as a " measurer of heat." Until Torricelli propounded the principle 

 of the barometer, this " weather-glass " of Galileo was used, for the philoso- 

 pher divided the stem into divisions, and the air-thermometer served the 

 purpose of our modern instruments. 



The actual inventor of the thermometer is not known. It has been- 

 attributed to Galileo, to Drebbel, and to Robert Fludd. There is little 

 doubt, however, that Galileo and Drebbel were both acquainted with it, 

 but whether either claimed the honour of the invention, whether they 

 discovered it independently, or together, we cannot say. Sanctorio, of 

 Padua, and Drebbel have also been credited with the invention. We may- 

 add that the spirit thermometer was invented in 1655-1656. It was a 

 rough form of our present thermometer, and roughly graduated. But it 

 was hermetically closed to the air, and a great improvement on the old 

 " weather-glass." Edward Halley introduced mercury as the liquid for the 

 instrument in 1680. Otto von Guerike first suggested the freezing point 

 of water as the lowest limit, and Renaldini, in 1694, proposed that the 

 boiling and freezing points of water should be the limit of the scale. 



Let us now explain the construction and varied markings of the three 

 kinds of thermometers in use. By noting the differences between the scales 



