DIFFERENT SCALES. 



coincide. In that case the pressure of the stearn will be exactly 

 equal to that of the atmosphere. 



25. The same unanimity has not prevailed either as respects 

 the unit or the thermometric zero. In England, Holland, some 

 of the German States, and in North America, the interval between 

 the freezing and boiling points is divided into 180 equal parts, 

 each part representing the thermometric unit. The scale is 

 continued by equal divisions above the boiling and below the 

 freezing points. 



The zero is placed at the thirty-second division below the freez- 

 ing point ; so that, on this scale, the freezing point is 32, and the 

 boiling point 32+lSO D =212. 



This scale is known as Fahrenheit's, and was adopted about 

 1724. 



The reason for fixing the zero of the scale at 32 below the 

 freezing point is, that that point indicated a temperature which 

 was at that time believed to be the natural zero of temperature, 

 or the greatest degree of cold that could exist, being the most 

 intense cold which had been observed in Iceland. 



We shall see hereafter that much lower temperatures, natural 

 and artificial, have been since observed. 



The division of the interval between the freezing and boiling 1 

 points into 180 equal parts was founded upon some inexact suppo- 

 sition connected with the dilatation of mercury. 



.The divisions of this scale are continued in the same manner 

 below zero, such divisions being considered negative, and expressed 

 by the negative sign prefixed to them. Thus, + 32 signifies 32 

 above zero, but 32 signifies 32 below zero. 



26. In France, Sweden, and some other parts of Europe, the 

 Centigrade scale prevails. 



In this scale the interval between the freezing and boiling points 

 is divided into 100 equal parts, and the zero is placed at the 

 freezing point. 



27. In some countries the scale of Reaumur is used, in which the 

 interval between the freezing and boiling points is divided into 

 eighty equal parts, the zero being placed at the freezing point. 



28. It has been ascertained by experiment, that mercury, when 

 raised from 32 to 212, suffers an increment of volume amounting 

 to 2-11 Iths of its volume at 32. Thus, 111 cubic inches of mer- 

 cury at 32 will, if raised to 212, become 113 cubic inches. 

 From this may be deduced the increment of volume which mer- 

 cury receives for each degree of temperature. For, since the 

 increase of volume corresponding to an elevation of 180 is T T of 

 its volume at 32, we shall find the increment of volume corre- 

 sponding to one degree by dividing T f T by 180, or, what is the 



155 



