THE EARTH. 158 



filled with spirits of wine, tinctured red, so as to 

 be seen when it rises, the ball is plunged into 

 boiling water, which making the spirit within ex- 

 pand and rise in the tube, the water marks the 

 greatest height to which it ascends : at this point 

 the tube is to be broken off, and then hermeti- 

 cally sealed, by melting the glass with a blow- 

 pipe : a scale being placed by the side, completes 

 the thermometer. Now as the fluid expands or 

 condenses with heat or cold, it will rise and fall 

 in the tube in proportion ; and the degree or 

 quantity of the ascent or descent will be seen in 

 the scale. 



No fire, as was said, can make water hotter, 

 after it begins to boil. We can therefore at any 

 time be sure of an equable certain heat ; which 

 is that of boiling water, which is invariably the 

 same. The certainty of such a heat is not less 

 useful than the instrument that measures it. It 

 affords a standard fixed degree of heat over the 

 whole world ; boiling water being as hot in 

 Greenland as upon the coasts of Guinea. One 

 fire is more intense than another ; of heat there 

 are various degrees ; but boiling water is a heat 

 every -where the same, and easily procurable. 



As heat thus expands water, so cold, when it 

 is violent enough to freeze the same, produces 

 exactly the same effect, and expands it likewise. 

 Thus water is acted upon in the same manner 

 by two opposite qualities ; being dilated by both. 

 As a proof that it is dilated by cold, we have 

 only to observe the ice floating on the surface of 

 a pond, which it would not do were it not dilated, 



