Page:Evolution of the thermometer.djvu/70

 was patterned after it), and was the first that could be used for all liquids.

In the fifth paper Fahrenheit describes his invention of the thermo-barometer, based on the fact that the boiling-point of water is influenced by barometric pressure. Boyle had observed the lowering of the boiling-point under the receiver of the air-pump, but Fahrenheit was the first to discover the principles of hypsometry.

Fahrenheit's publications are few in number and very brief, but they show him to have been an original thinker, and his great mechanical skill in working glass enabled him to carry out his designs. His account of the thermometer is of so great interest that I give it entire.

"The thermometers constructed by me are chiefly of two kinds, one is filled with alcohol and the other with mercury. Their length varies with the use to which they are put, but all the instruments have this in common: the degrees of their scales agree with one another and their variations are between fixed limits. The scales of thermometers used for meteorological observations begin below with 0° and go to 96°. The division of the scale depends upon three fixed points which are obtained in