HEAT APPARATUS. 141 



the electric resistance of these two wires may be compared. The 

 water in the vessel is then heated or cooled till the resistance of 

 the wire immersed in it is equal to that of the wire at the distant 

 station. The temperatures of th~ two wires must then be equal, 

 and by observing with a common thermometer the temperature of 

 the water in the vessel, we obtain the temperature of the distant 

 station. 



(a) Thermometers depending upon the relative expansion of a 

 solid and a liquid. [Florentine academicians, Re'aumur, Celsius, 

 Fahrenheit, &c.] 



Processes of Calibration, and of making, filling, and graduating 

 Mercury, Alcohol, and Ether Thermometers in glass. 



Thermometers with liquid SO 2 or CO 2 . 



Thermometers with protected bulbs to measure temperatures 

 under pressure. [W. Thomson, Miller, Casella, &c.] 



Registering Thermometers. [This class is exceedingly numerous, 

 but the perfect form is the photographic one.] 



Statical Thermometer. [Cavendish, &c.] 



(b) Thermometers depending on the relative expansion of a 

 gas and a solid. 



Ordinary Air Thermometer. [Galileo, Drebbel, &c.] 

 Differential Thermometer. [Leslie, Rumford, &c.] 

 Scientific Air Thermometers specially those at constant pres- 

 sure, to prevent the effects of transpiration at high temperatures. 

 [Regnault, Joule, St. Claire Deville, &c.] 



(<r) Thermometers depending on the actual or relative expan- 

 sion of solids. 



Metallic Thermometers. [Breguet, &c.} 

 Pyrometers. [Wedgwood, Daniell, &c.] 

 Averaging Thermometers. [Stevenson's " Creeper," c.] 

 (d) Thermo-electric Pile and Galvanometer. [Nobili, Schweigger, 

 Melloni, &c.] 



A study of the thermo-electric diagram shows that when the 

 specific heat of electricity is equal in any two metals at all tempe- 



