218 SECTION MECHANICS. 



accurate measurement requires to be expressed in decimals of an inch. 



In 1857, when President of the Institution of Mechanical Engineers, 

 I read a paper on standard decimal measures of length, and I am 

 happy to say that since that period the decimal system has been 

 introduced to a certain extent in many engineers' works, but it is still 

 far from being universal. 



In the manufacture of our standard gauges, the workmen measure 

 to the one twenty-thousandth of an inch, and these measures are as 

 familiar and appreciable as those of larger dimensions. 



As an illustration of the importance of very small differences of 

 size, I have here cylindrical standards with a difference of the ten- 

 thousandth of an inch. It is therefore obvious that a difference of 

 one ten-thousandth of an inch is an appreciable and important 

 quantity. 



It will be at once conceded that the only scale of measurement 

 which can be used for such small differences must be a decimal one. 



For many years the decimal system has been in use at our works, 

 taking the inch as the unit, and the workmen think and speak in 

 tenths, hundredths, and thousandths of an inch. 



It is of great importance to the manufacturer to have the means of 

 referring to an accurate fixed measure, as it will enable him, at any 

 time, to reproduce a facsimile of what he has once made, and so 

 preserve a system of sizes of the fitting parts unaltered. 



The great value of the workshop measuring machine is making 

 difference gauges. 



Every external diameter having to work in an internal diameter 

 should have a certain difference of size ; and close observation and 

 experience can alone determine what this difference of size ought 

 to be. 



Take, for instance, a railway axle ; if the bearing in which it has to 

 work be too small the heating of the axle by rapid rotation will be the 

 consequence ; if, on the other hand, the bearing be too large, it will be 

 sooner worn out. 



It is therefore most important when rapid revolutions and great 

 strains have to be undergone, that the proper difference of size, when 

 once ascertained by experience, should be strictly adhered to. 



In the manufacture of axles there should be two gauges used, the 



