III. THEBMOMETRY. 261 



the former length. So on all down the tube ; if the length has decreased 

 from the first measurement, it shows that the bore of the tube has increased, 

 and vice vers&. The process is known as calibration. 



1023. Pour Thermometers. Showing the different scales 

 principally in use. Fahrenheit, Celsius or Centigrade, Reaumur, 

 and De Lisle. Dring and Fage. 



Fahrenheit's scale is used principally in Great Britain, its Colonies, and the 

 United States. The zero of this scale is obtained from a mixture of salt and 

 snow ; thirty-two degrees is the point at which ice begins to melt, and 212, or 

 boiling point, from boiling water, when the barometer stands at 29 905. One 

 advantage of this scale is that temperatures may often be expressed in whole 

 degrees, whereas in other scales fractions of degrees are frequently necessary. 



1024. Celsius or Centigrade Scale. Generally used on the 

 continent. Dring and Fagc. 



The zero of this scale is that point at which ice begins to melt, and 100 

 the point at which Avater boils when the barometer stands at 760mm. Celsius 

 is the name of the inventor of this scale ; it is called Centigrade from its 

 being divided centesinially. 



1025. BecquerePs Thermo-Electric Thermometer. 



Conservatoire des Arts et Metiers, Paris. 



1025a. Becquerel's Thermo-Electric Pyrometer. 



Conservatoire des Arts et Mi tiers. 



1026. Hodgkinson's Actinometer. Described in the 

 Proceedings of the Royal Society, vol. XX., p. 328. 



Kew Committee of the Royal Society. 



This is a large thermometer, filled with alcohol coloured blue, and having a 

 bore much contracted for a great part of its length, in order that the scale 

 may be very open ; at its top it opens out into a large chamber, which receives 

 the superfluous fluid at the time of observation. 



A tin case, capped with glass at both ends, prevents the access of extra- 

 neous rays to the bulb of the instrument at the time of observation. 



1028. Drawings, various, partly new, of constructions of 

 Differential Atmospheric Thermometers. 



Dr. Leopold Pfaundlcr, Professor of Physics, Innsbruck. 



This plate presents a general view of all possible forms of construction, 

 which partly appear as modifications of Berthelot's atmospheric thermometers, 

 partly are based on independent principles. 



Fer further details, see Transactions of the Imperial Academy of Sciences at 

 Vienna, Vol.LXXIL, 1875. 



1029. Melloni's Thermo-Electric Apparatus. 



M. Ruhmkorjf. 

 1029a. Line Pile for Spectrum and Galvanometer. 



M. Ruhmkorft\ 



1286. Nobili's Thermo-Electric Pile, of 54 pairs of bis- 

 muth and antimony bars, soldered alternately together ; the smallest 

 temperature between the two faces of the pile develops a current, 

 readily indicated by a suitable galvanometer. Elliott Brothers. 



