u8 SEVENTEENTH CENTURY. FT. in. 



mercury (B, Fig. 15) and the end of the tube is a vacuum, 

 or a space with scarcely any air in it, and is still called a 

 Torricellian vacuum, 



Invention of the Thermometer. The date of the 

 invention of the thermometer (or instrument to measure 

 heat) is so uncertain that it will be best to speak of it here 

 in connection with the barometer. Galileo is said to have 

 Vnade the first thermometer, which was simply a tube with 

 a bulb at the end standing upside down in a basin of water. 

 The bulb was filled with air, and when heat was applied to 

 it, it expanded and drove back the water in the tube. A 

 few years afterwards a Dutchman named Drebbel made 

 thermometers with spirits of wine in them, and finally, in 

 1670, mercury was used. Mercurial thermometers have 

 the bulb and part of the tube filled with mercury, and the 

 rest of the tube is quite empty, all the air being driven out 

 by heating the mercury till it completely fills the tube, and 

 then melting the end so as to close it. When the mer- 

 cury cools it contracts and a vacuum is left above it. After- 

 wards, when the bulb of this therrhometer is heated, the 

 mercury expands and rises in the tube ; when it is chilled 

 it contracts and falls. 



The thermometer was not of any great use till early in 

 the eighteenth century, when three men, Fahrenheit, Celsius, 

 and Reaumur, measured off the tube into degrees, so that 

 the exact rise and fall could be known. Celsius and 

 Reaumur took the melting point of ice as zero or o of their 

 scale, but Fahrenheit took his from a mixture of snow and 

 salt, which was the greatest cold he knew how to obtain. 

 For this reason 32 is the freezing point of water in a 

 Fahrenheit thermometer, and his other divisions are different 

 from those of Celsius or Re'aumur. Celsius's scale is the 

 one now used all over the Continent, and scientific men 



