64 DETERMINATION OF THE FIXED POINTS. 



or = - 36 x ~ = - 20 d To convert Reaumur or Gen- 

 y 



tigrade degrees into Fahrenheit degrees, their number 



9 9 

 is multiplied by - or - respectively, and 32 is 



then added to the product ; for example, 12 R. 



= 12 x 9 + 32 = 27 + 32 = 59 F.; - .25 C. 

 4 



= - 25 x 9 4- 32 = - 45 + 32 = - 17 F.; - 5 C. 

 5 



= - 5 x 9 + 32 = - 9 + 32 = 23 F. 



D 



The correct determination of the two fixed points of 

 a thermometer requires various important precautions, 

 and if a thermometer has been purchased of a dealer, 

 it is in all cases necessary to repeat the determination, 

 in order to test the accuracy of the scale. In order to 

 determine the lower fixed point, a vessel is nearly filled 

 with snow or pounded ice and the thermometer im- 

 mersed in it in a vertical position, so that the snow 

 or ice reaches very nearly up to the point marked with 

 zero on the scale. The snow or ice is cautiously 

 stirred with a splinter of wood, and the point is ob- 

 served at which the top of the thread of mercury 

 remains stationary. 



If the determination of the freezing point is made in 

 the summer, and the ice is obtained from an ice-well, 

 the mercury of the thermometer falls at first rapidly, 

 then more and more slowly, and finally it remains 

 stationary until a great portion of the ice is melted. 

 The same happens if the experiment is made in the 

 winter on a day when the temperature of the air is not 



