23 



adjoining Westminster Abbey, which was a monk's refectory 

 in days gone by. All coinage standards and sample coins are 

 located under lock and key in the Pyx Tower, close adjoining. 

 The standard photometer, described further on, is fixed in a 

 small room with groined roof, attached to the old refectory ; 

 this was probably used by the monks of those days as a 

 retiring-room, or possibly as a lavatory. The walls are black- 

 ened, as is usual, with open photometers. All these walls 

 are of immense thickness, and of considerable age, and are so 

 well set that the long measures, such as ten feet, are set out 

 on them, and the results are found to be most accurate upon 

 comparison with other standards. 



When it is remembered that the linear expansion of stone is 

 about -00000652 of its length for one degree of Pahr., it will 

 be seen that a practically permanent distance is maintained 

 between the contact plates of the measure. It must be pointed 

 out that the principal reasons for using those rooms are that 

 being underground they are less liable to changes of tempera- 

 ture, and from their solid construction and age are free from 

 any vibration or settlement. 



Bed measures (or end measures) are those measures whose 

 extreme length indicates the measure, and which are accom- 

 panied by a bed into which they fit between two end blocks ; 

 thus a subsidiary measure is formed by the bed, while the 

 principal measure is generally the rod or bar. The process of 

 verifying an inside measurement in a case in which the bed 

 was the principal measure was accomplished by Captain Kater, 

 the well-known authority on such matters, as follows : — The 

 bed was of brass, with rectangular steel terminations, and the 

 bar of brass, one inch square. Two bars of brass three-quarters 

 of an inch square and a little less than eighteen inches long 

 each were prepared ; their ends formed true planes at right 

 angles to their length. Upon the upper surface of each bar 

 very near the end, a fine transverse line was drawn ; the other 

 ends of the bars were then placed in contact, and kept so by a 

 spring, as shown in drawing. 



The distance between the lines was taken by means of two 

 micrometer microscopes, fixed to a bar of wood, and referred 

 to Sir George Shuckburg's scale, which Captain Kater affirms 

 does not sensibly differ from the Imperial Standard yard 

 (see Plate VIII.). It was found that the error was 919 

 micrometer divisions — each division equal to -^-res of an inch, 

 or '0000428 in. The two bars were then placed as shown in 

 diagram INTo. 2 upon the standard to be examined, their marked 

 ends together, and their opposite ends kept in contact with 

 the steel faces of the standard by means of springs. Therefore 

 the distance between the lines should, if the standard had been 



