24 TRUE SURFACES AND ACCURATE MEASUREMENTS—ROBB. 
and to fit closely. The bar fitted the bush so well that while it 
could be shoved into its place when free and clean from oil, when 
oil was applied it refused to move except by using considerable 
force, showing that there was not room for the oil. The shaft 
required to be reduced 2/1000" in size to make room for the 
lubricant. Heating of journals is frequently caused by neglect 
of this point. In many kinds of machinery no attempt is made 
to true the surfaces carefully, or indeed to do more than get them 
approximately round or flat, as the case may be, or to make 
them to any exact size nearer than can be measured by the eye 
or an ordinary box wood rule. The result is that journals and 
seats have to work out their own salvation or destruction, by 
wearing the high parts down until the low parts approach near 
enough for the oil to support the whole journal, consequently, 
much care and patience must be exercised in working new 
machinery until itis worn to a bearing, otherwise the metals will 
abrade and heat by friction until the surfaces are completely . 
destroyed, 
The foregoing will emphasize the importance of being able 
to make minute and accurate measurements, as much depends 
upon the certainty with which the mechanic can measure the 
inaccuracies of his work in order to bring it to the necessary 
state of perfection. The ordinary system of measuring by a 
rule graduated to 16ths, 32nds or perhaps the 64ths of an inch, 
the use of which leaves room for an error which is too great for 
the production of good machinery, has been superseded in 
machine shops, where accuracy is aimed at, by the “ Micrometer 
Caliper” and hardened steel gauges of various kinds, by means 
of which measurements of 1/4000 of an inch can be made as 
easily as 1/16 inch can be measured by the ordinary rule. 
“The Interchangeable System,” first used by American work- 
men in the production of watches, fire arms, and other machines, 
having a number of small parts, any one of which should fit any 
other, has done much to introduce refined and rapid methods of 
measurements. Steel gauges, which are hardened and _ then 
ground, are now produced by manufacturers of tools which are 
