192 SCIENTIFIC APPARATUS. 



found so difficult to fit the piston accurately to the cylinder that 

 the anticipated success of the first trial was in a great measure 

 defeated. But it is well known that difficulties of this kind exist 

 no longer. A small case in the collection will make this evident, 

 and will indicate the manner in which it has been accomplished. 

 Here will be seen a splendid specimen of a surface plate, lent by 

 Sir J. Whitworth & Co., and many will examine with interest the 

 mode both of constructing and supporting that primary surface upon 

 which we rely in the construction of machinery, being in effect, 

 the closest approximation to an absolute plane surface which has 

 yet been attained. 



It might be thought that when a so-called " mechanical true 

 plane " had been produced there was nothing more to be done ; 

 but that is only the beginning of the work. By a rapid extension 

 of construction, Sir J. Whitworth made a measuring machine for 

 the workshop which will enable the mechanic to test the accuracy 

 of measurement to the ten-thousandth of an inch, and in the col- 

 lection there will be found a steel cylinder one inch in diameter, 

 used as a standard measure ; and a corresponding collar or 

 internal gauge into which the cylindrical or external gauge 

 exactly fits ; a second cylinder of steel one ten-thousandth of an 

 inch less in diameter is placed side by side with the former, 

 and the difference of mechanical fit due to this ascertained differ- 

 ence of size is made apparent at once. But if it be possible to 

 make two exact cylinders differing by such a minute difference of 

 diameter, and, further, to measure this difference by a machine, 

 it is quite clear that our constructive power must have advanced 

 most wonderfully since the days of Watt. In truth it is now equal 

 to the task of producing a machine by which an interval of one- 

 millionth of an inch is made a measurable quantity. We have 

 not space to discuss this matter fully, or to show how much 

 may be learnt from the examination and handling of apparatus of 

 this kind, but it is needful to point out that the power of con- 

 structing true surfaces and the power of measurement are the two 



