120 Description of a Metallic Thermometer. 



contrived by me, for the purpose of exhibiting the dif- 

 ference in temperature, or degrees of heat, which takes place 

 between the mercurial thermometer, the scale of which 

 terminates upwards, at 600°, and that of baked clay, or 

 Wedgwood's thermometer, the scale of which commences 

 at 1077° of Fahrenheit, or red-heat, thus forming an inter- 

 mediate or connecting thermometer between the two above 

 mentioned. 



A metallic composition is formed, not liable to alteration 

 in its quality or quantity by repeated exposure to heat, the 

 melting point of which is at a little below 600° of Fahren- 

 heit, and its boiling point at 1200°. A case resembling 

 in form the glass case for the ordinary thermometer, but 

 somewhat larger, contains the metallic composition, and 

 the scale consists in a slender graduated rod, equal in 

 height at the commencement of the scale, that is when 

 the metallic composition is just liquid to the top of the 

 tube; the graduated rod terminating at the bottom in a 

 ihin, circular, fiat plate, which rests or floats as it were 

 upon the liquid metal; and in proportion as the latter ex r 

 pands and rises in the tube bv heat, the graduated rod is 

 buoyed up, or raised above the top of the tube, passing 

 through a perforated cover to the maximum, or boiling 

 point*. 



The same principle, I might observe, admits of being 

 extended, for the purpose or ascertaining the variation in 

 temperature up to the most intense heat, perhaps, that can 

 be required. 



It is unnecessary to state here, that the influence of the 

 incumbent atmosphere upon, the surface of the liquid me- 

 tal within the open tube is too inconsiderable, even at the 

 commencement of the scale, to deserve notice, and at a 

 higher temperature diminishes to nothing; especially if 

 the whole of the liquid contained in the thermometer, as 

 ought to be the case in the use of every thermometer , be 

 completely immersed or subjected to the temperature, the 

 degree of which it is intended to indicate. 



A method similar to the above, I should think, might 

 be applicable to the purpose of showing in a ready way the 

 degree of expansion in metals bv heat ; but the elongation 

 of a cylinder of any metal, by increase of temperature, is 



* The thermometer case and graduated rod arc at present formed of 

 pipe-makers' clay previously prepared by having been exposed to a suffi- 

 cient degree of heat. 



The scale of this new thermometer is an exact continuation of the scale 

 in the mercurial thermometer; the lower degree of the former correspond- 

 ing with, or indicating like temperatures with, the upper degrees of the 

 mercurial thermometer. much 



