LINEAR MEASUREMENT. 219 



axle being made to the standard gauge, and the bearing bored out to 

 fit a different gauge, which has to be as much larger as experience 

 has found to be necessary, according to the conditions under which 

 the axle has to work. Hence every manufacturer should be in a 

 position to manufacture his own difference gauges. 



Fifty years ago the thousands of spindles in a cotton factory had 

 each to be separately fitted into the bolster in which it had to work. 

 At the present time all these spindles are made to gauge, and are 

 interchangeable. 



It cannot be impressed too forcibly, both upon the student in 

 mechanics and upon the workman, that accuracy of measurement is 

 essential for good and efficient workmanship and that it tends to 

 economy in all branches of manufacture, so as to have the parts inter- 

 changeable. 



I will now endeavour to explain two or three things which I have on 

 the table. I have said that the measuring machines are based on the 

 production of a true plane. I have here a true plane of ten inches 

 square. I will not go into the method by which these true planes 

 were produced, which would take too long, but I may mention that in 

 getting one true plane we have to get three, because if we had only 

 two, one might be a little concave and the other a little convex, and 

 still they might be made to fit, but if the third will fit the two, it must 

 be a true plane. You will see that when I put this plane down on the 

 other, they do not touch each other, but float on the air between them 

 for a short time ; and if I let one fall upon the other, you perceive the 

 metals do not come in contact there is a muffled sound. The plane 

 is equally true when it is suspended as when it is supported. It is 

 suspended by three points, and we also now lift them from those three 

 points, so that when applied to a piece of work it remains true. 



I will now refer to the workshop measuring machine. The divisions 

 on the micrometer wheel represent ten-thousandths of an inch that is 

 if you move this wheel one division, the end of the machine is moved 

 forwards one ten-thousandth of an inch. We find in practice that the 

 movement of a fourth part of a division, being one forty-thousandth 

 of an inch, is distinctly felt and gauged. I have here a small gauge 

 one-fourth of an inch in diameter, and on setting this wheel to zero, I 

 then regulate the machine with the small wheel at the other end, so 



