220 SECTION MECHANICS. 



that I can just feel this gauge go through. Now, if I move this wheel 

 half a division, the gauge will not pass through. We get thus half a 

 division, which would be one twenty-thousandth of an inch. No one 

 would believe that so small a difference would produce such an effect 

 as that. When everything is in nice order I can feel perfectly well 

 the fifty-thousandth of an inch. In the case of the millionth machine 

 we introduce a feeling-piece between one end of the bar to be 

 measured, and one end of the machine, and the movement of the 

 micrometer wheel through one division, which is the one-millionth, is 

 sufficient to cause the feeling-piece to be suspended or to fall by its 

 own gravity. This is a standard inch to be measured. We bring the 

 inch bar up to one end, and introduce the feeling-piece. When the 

 machine is adjusted, we find that moving the end one-millionth of an 

 inch is sufficient to cause that feeling-piece to be suspended or to fall. 

 As an illustration of the importance of very small differences of size, 

 I have here cylindrical standards with the difference of ten-thousandths 

 of an inch. This is a standard inch measure, and this is the cor- 

 responding one internal gauge. You see that it is what we call a nice 

 fit. These gauges are made of steel and case-hardened, so that they 

 will last a longtime, and nothing will cut the surface except a diamond 

 or the grinding process by which they are made. This other gauge is 

 one ten-thousandth part of an inch less, and you see the difference 

 which it makes. The internal gauge slides down of its own gravity, 

 and it is therefore obvious that the difference of a ten-thousandth part 

 of an inch is an appreciable and important quantity. I have said that 

 the great value of the workshop measuring machine is making 

 difference gauges. These are a set of gauges which I made for 

 Lord Hardinge in 1855 for the improvement of the Enfield rifle. 

 The size is -577. Each of these gauges are one five-thousandth 

 of an inch less than the other, and you see how much more loosely 

 one fits than the other. The value of these difference gauges is this : 

 If you tell a man to bore a barrel to that size he is sure to make it 

 wrong, he will get it too large ; but if you give him this set of 

 difference gauges he gets the bore first of all to the one-thousandth of 

 an inch less than the standard, and then feels his way up to the 

 standard. In making rifles we never made any allowance. We gave 

 the men these difference gauges, and every barrel was made the same, 



