120 SEVENTEENTH CENTURY. PT. in. 



Invention of the Thermometer. The date of the in- 

 vention of the thermometer (or instrument to measure heat) 

 is so uncertain that it will be best to speak of it here in con- 

 nection with the barometer. Galileo is said to have made 

 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 thermo- 

 meters 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 thermometer 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 Re'aumur 

 took the freezing-point of water as their lowest point ; but 

 Fahrenheit took the greatest cold he could obtain by a mix- 

 ture of snow and salt. 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 Reaumur. 

 Celsius's scale is now the one used all over the Continent, 

 and scientific men wished to introduce it into England, be- 

 cause it is so much more simple than Fahrenheit's. It is 

 called ' centigrade,' or a hundred steps, because the freezing- 



