TUK THERMOMETER. 19 



placed in the mouth of the tube above the iron bar, and so 

 secured by a strap of platinum foil and a little wedge, that it 

 slides with difficulty in the tube. By the expansion of the 

 metallic bar, the plug of earthenware is pushed outwards, and 

 remains in its new position after the contraction of the metallic 

 bar on cooling. The expansion of the iron bar thus obtained,, 

 is measured by adapting to the instrument an index, c, which 

 traverses a circular scale, before and after the earthenware plug 

 has been moved outwards by the expansion of the metallic bar. 

 The degrees marked on the scale are in each instrument com- 

 pared experimentally with those of the mercurial scale, and 

 the ratio marked on the instrument, so that its degrees are 

 convertible into those of Fahrenheit, (Philosophical Trans- 

 actions, 1830-31.) An air thermometer, of which the bulb and 

 tube were of metal, has also been employed to explore high 

 temperatures. In the old pyrometer of Wedgwood, the degree 

 of heat was estimated by the permanent contraction which is 

 produced upon a pellet of pipe-clay ; but the indications of this 

 instrument are fallacious, and it has long gone out of use. 



The applicability of the mercurial thermometer, to measure 

 degrees of heat, depends upon two important circumstances, 

 which involve the whole theory of the instrument : 



1st. The hollow glass ball, with its fine tube of uniform bore, 

 is a nice fluid measure. The ball and part of the stem being 

 filled with a fluid, the slightest change in the bulk of the fluid, 

 which may arise from the application of heat or of cold to it, is 

 conspicuously exhibited by the rise or fall of the fluid column in 

 the stem. No more delicate measure of the bulk of an included 

 fluid could be devised. 



2nd. It fortunately happens that the expansions of the fluid 

 metal, mercury, which we can thus measure so accurately, are 

 proportional to the quantities of heat which produce them. 

 But the mode in which this is proved, requires a little atten- 

 tion. Suppose we had two reservoirs, one containing cold, and 

 the other hot water. Plunge a thermometric bulb containing 

 mercury first into the cold water, and mark at what point 

 in the stem the mercury stands. Then plunge it into the hot 

 water, and mark also the point to which the mercury now rises 

 in the stem. We can obviously make a heat which will be half 

 way exactly between the hot and cold water, by taking the same 

 quantity of the hot and cold water and mixing them together. 



c2 



