30 cALonic. 



To facilitate the observation, the tube is divided 

 into a number of equal parts, called degrees, or 

 there is a divided scale attached to it. 



This scale is graduated in different manners by 

 different nations : Fahrenheit's scale is that al- 

 ways used in this country. 



The standard points are obtained by freezing 

 and boiling water, degrees of heat which are con- 

 stantly the same in nature. The heat at which 

 the mercury stands, when immersed in each, being 

 marked, the distance between them is divided into 

 180 parts, and 32 parts of the same size are con- 

 tinued downwards, so that 32° shows the heat of 

 freezing water, and 212° that of boiling water. 

 AVater cannot be made hotter than this in open ves- 

 sels, because it then becomes converted into steamy 

 or aqueous gas. 



The mercurial thermometer, it is evident, cannot 

 measure degrees of heat above that of boiling mer- 

 cury, nor below that of freezing mercury ; the 

 former is 600°, and the latter 40° below of Fah- 

 renheit's scale. 



For greater degrees of cold, thermometers of 

 spirits of wine, or essential oil, are used ; and to 

 measure those higher degrees of heat to which the 

 thermometer cannot be applied, pyrometers are em- 

 ployed. An instrument of this kind was invented 

 by the late Mr. Wedgewood. It consists of two 

 pieces of brass, fixed so as to form an angle, having 

 the legs divided into equal parts. Pieces of baked 

 clay are prepared for this scale, so as to fit the 

 brass at a certain place. If then the piece of 

 clay be exposed to the heat required to be ex- 

 amined, it will contract in its dimensions, and, 

 when again applied to the brass scale, it will be 

 seen how much it has contracted. By this the in- 



