THE F.AUTU. 127 



low ball of glass, with a long tube growing out of it. This 

 being partly 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 expand 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 hermetically sealed, by melt- 

 ing the glass with a blow-pipe : a scale being placed by the side, 

 completes the thermometer. Now as the fluid expands or con. 

 denses with heat or cold, it will rise and fall in the tube in pro- 

 portion : and the degree or quantity of ascent or descent wiU be 

 seen in tlie scale. 



No fire, as was said, can make water hotter, after it begins to 

 boil. We can, therefore, at any time be siu-e of an equable cer- 

 tain 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 coast of Guinea. One fire is more in- 

 tense than another ; of heat there are various degrees ; but 

 boiling water is a heat every where the same, and easily pro- 

 ciu"able. 



As heat thus expands water, so cold, when it is violent enough 

 to freeze the same, produces exactly the same effect, and ex- 

 pands it like\dse. Thus water is acted upon in the same man- 

 ner 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, and grown more bulky, by freezing, than the water 

 which remains unfroze. ]\Ir Eoyle, however, put the matter 

 past a doubt, by a variety of experiments.' Having pomed a 

 proper quantity of water into a strong earthen vessel, he ex- 

 posed it, uncovered, to the open air, in ft-osty nights ; and ob- 

 served, that continually the ice reached higher than the watei 

 before it was frozen. He filled also a tube with water, arid 

 stopped both ends with wax : the water, when frozen, was 

 found to push out the stopples from both ends; and a rod of 

 ice appeared at each end of the tube, which showed how much 

 it was swollen by the cold within. 



I Boyle, vol. J. p. 610. 



