The Making of Scales upon Glass 153 



It will be seen that the second is the simpler method, and 

 it has the additional advantage that one may, without calcula- 

 tion, take the nearest 5 c.c. error instead of the previous one, 

 without any calculation. 



(c) A thermometer. The thermometer consisting of a single 

 tube only will be considered, other types being now more rare, 

 the only difference in method being in detaching the measuring 

 thread. 



The stem of the thermometer will have a thread of mercury 

 showing. About 10 (or 3 cms.) from the top of this thread 

 the thermometer is heated with a tiny gas flame, produced from 

 a piece of millimetre tube (thermometer tubing). The mercury 

 thread is carefully watched, and will be found presently to 

 break away from the main thread at the hot point, when the 

 instrument should be immediately withdrawn from the flame. 



By means of a reading microscope, the length of this thread 

 is carefully measured at various positions along the bore, the 

 length varying with the volume of the bore, and though the 

 actual volume cannot be determined, numbers proportional 

 to the volume and in terms of degrees may be found in this 

 way. From these a table of corrections should be drawn up, 

 giving the correction to be applied at each 5. 



XVI. THE MAKING OF SCALES UPON GLASS. 



This process is very simple, and after the previous exercises 

 requires little description. 



(a) A flat linear scale. A piece of glass is obtained and 

 coated with wax, by warming the clean dry glass strip, brush- 

 ing wax upon it with a wide brush, and holding in a slightly 

 inclined position until cool. The back and edges are similarly 

 coated, and the glass cemented to the bench either alongside 

 a steel metre scale or in a line with it. 



The steel straight-edge (or try-square) is used in the former 

 case, and a wooden, needle-pointed beam compass, described 

 above, in the second. 



The transfer of the marks should, however, only take place 

 after ruling four parallel lines upon the strip, with a ground 



