THE THERMOMETER. 



17 



Fahrenheit's 

 scale. 



Centigrade 

 scale. 



100 



Reaumur's 

 scale. 



point is arbitrarily called 32o, or the 32nd degree ; and conse- 

 quently the boiling point is 32 added 180, or the 212th degree.* 

 The scale can easily be prolonged to any extent, above or 

 below these points, by marking off equal lengths of the tube 

 for 180 degrees, either above or below the space first marked. 

 The degrees of contraction below zero, or 0, are marked by 

 the minus sign ( ), a"nd called negative degrees, in order to 

 distinguish them from degrees of the same name above zero, 

 or positive degrees. Thus, 47 means the 47th degree above 

 zero, 47, the 47th degree under zero. 



The only other scale in use is that of Reaumur, in the north 

 of Germany. The expansion between the freezing and boiling of 

 water is divided into 80 parts in this thermometer. The relation 

 between the three scales is illustrated in the following diagram. 



The zero of our scale 

 is 32 degrees below 

 the freezing point of 

 water, and the expan- 

 sions of mercury are 

 available in the ther- 

 80 mometer from 39 

 __ 60 to 600o ; but about the 



latter degree, mercury 



rises in the tube in the 

 20 state of vapour, so as 

 to derange the indica- 

 tions, and at about 660 

 it boils, and can no 

 longer be retained in 

 the glass vessel ; while 

 at the former low point 

 it freezes or becomes 



* A simple rule may be given for converting centigrade degrees into degrees 

 Fahrenheit. 100 degrees Centigrade being equal to 180 degrees Fahrenheit, 

 10 degrees C. = 18 degrees F., or 5 degrees C. = 9 degrees F. ; multiply the 

 Centigrade degrees by 9, and divide by 5, and add 32. Thus to find the degree F. 

 corresponding with 50 C. J50 



9 



5)450 



90 

 add 32 



Or the 50 C. corresponds with the 122 F. 



