48 SEC. 3. - MEASUREMENT. 



207. Standard Gallon, Quart, and Pint of Queen 

 Elizabeth, dated 1601. From the Muniment Room, Winchester. 



208. Standard Winchester Bushel, given to the Cor- 

 poration by Henry VII. in the year 1487. 



209. Standard Winchester Gallon, given to the Cor- 

 poration by Henry VII., in the year 1487. 



SET OF MEASURING INSTRUMENTS CONTRIBUTED BY SIR J. WHIT- 

 WORTH, BART., D.C.L., F.R.S. 



217. True Planes. The true plane is the foundation and 

 source of all truth in mechanism. 



The patent hexagonal surface plate is constructed so as to be supported and 

 suspended from three points, and remains true in either position. The original 

 true planes first exhibited by Sir Joseph Whitworth at the meeting of the 

 British Association at Glasgow, in 1840, were rectangular, and were ribbed, 

 so as to allow of their being supported on three points ; but when large rect- 

 angular surface plates were suspended from the two handles a perceptible 

 alteration took place, and they were no longer as true as when supported on 

 the three points. 



212. Whitworth's Workshop Measuring Machine, for 



making difference gauges from correct cylindrical standards of 



size. 



One division of the micrometer wheel represents l6 ^ 00 of an inch, one 

 quarter of a division, viz., ?0 -^ 00 of an inch can be distinctly felt and gauged. 



No proper size of bearing can be made for an axle to work in without 

 having a difference gauge of such size as experience has proved to be best. 



214. External and Internal Standard Cylinder Gauge, 



1 inch in diameter. 



The standard gauges are usually made from y^th to 2 inches diameter, but 

 they are also made for larger diameters. 



They are a necessary adjunct to the workshop measuring machine when 

 making difference gauges. 



21O. Box of Standard Lengths, of end measure, 1 inch to 

 12 inches. 



Either these standard lengths of end measure or the cylindrical standard 

 gauges are used for adjusting the workshop measuring machine ; for large 

 dimensions these are preferable. 



213. Box of Cylindrical External and Internal Dif- 

 ference Gauges, differing by ^Vo^ 1 of an inch in diameter. 



By means of these a workman can feel his way step by step and so make 

 the bore of any number of barrels or tubes exactly the same diameter. They 

 illustrate the importance of small differences in size ; while one fits, another 

 3-jJ;-^ of an inch less in diameter appears not to fit at all. 



A tight fit is not a proper fit, there must be a certain difference in diameter 

 between an axle and the bearing in which it has to work ; what the difference 

 should be depends on a variety of circumstances which experience alone can 

 determine. 



