LINEAR MEASUREMENT. 221 



so that all shot alike. There was no high gauge and low gauge as it 

 is termed at Enfield. By the use of these difference gauges we were 

 able to work up to a certain point, and they were all made alike. 

 Then I have said that 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. I may mention that in making these cylindrical 

 gauges we do not go below one-tenth of an inch, and when we want a 

 gauge less than that we make this class of gauge flat gauges ; they 

 are made with a tolerably good surface on each side, of steel and 

 tempered, and they last pretty well. This is one-hundredth of an 

 inch, and we can easily make this from the measuring machine. The 

 largest of these gauges is one-tenth and the smallest is one-hundredth, 

 and there are regular differences of sizes between. They serve the 

 purpose of wire gauges. It is very important when you get so smalt 

 a size that it should represent accurately what it professes to be. 

 This is a little apparatus by which it is demonstrable that we can 

 make the ends of this bar at right angles. It was once objected 

 that we could not make the end of the bar at right angles to the faces. 

 It is done by placing the standard in a solid block, with a V shaped 

 channel to receive it, and the end of the standard and of the block is 

 made a trueplane. The standard is then turned round in its resting place, 

 and again tried with the true plane. If it is not true it is again faced 

 up until it becomes so, and when it corresponds to the true plane in 

 each of the four positions it is evident that the end is at right angles 

 to the four sides. This is a rod which is another application showing 

 the sense of touch. It was made for ascertaining after a gun had 

 been proved or fired a certain number of times whether there was any 

 appreciable wear or whether one part expanded more than another. 

 At one end there is an inclined plane, which moves out three little 

 feeling-pieces. The bar is introduced into the bore of the gun and is 

 moved endways, and there is no difficulty at all in feeling the ten- 

 thousandth part of an inch. There is a little roller fixed on the 

 muzzle of the gun on which it works, and the attendant can feel quite 

 easily a difference of the ten-thousandth part of an inch. 



M. TRESCA recognised the fact, that, in the actual state of things, 

 the exactness of one-millionth of an inch is the utmost limit which it is 



