THE NEW NATIONAL STANDAED OF LENGTH, AND ITS PEINCIPAL COPIES. 623 
of these operations, the equivalent of the Exchequer end-standard was laid down on the 
Eoyal Society’s bar ; it is distinguished by the word “ Excheq,” 
Philosophical Transactions, 1768, page 324, &c. In Maskelyne’s Account of the 
Maryland degree, there is incidental mention of the comparison of Brno’s scale with the 
Royal Society’s brass standard, and of Bird’s scale with the French toise ; giving, how- 
ever, a result for the relation between the Royal Society’s Standard and the Toise which 
differs sensibly from that found by Graham. The process of laying down the value of 
an end-measm’e (as the toise) upon a flat surface appears rather rude. 
Philosophical Transactions, 1785, page 402, &c. In General Roy’s Account of the 
measure of the Base on Hounslow Heath, there is mention of the exact agreement (as 
ascertained Rom a comparison made by means of Ramsdeh’s beam-compasses) between 
3 feet of the Royal Society’s or Tower Standard and 3 feet of General Roy’s Scale (a 
scale originally the property of Graham, which is said to have been divided by Bird 
when in the employ of Sissox); and it seems that, in reliance on the uniformity of 
the division. General Roy then used 40 inches of his scale without any verification of 
the supplementary 4 inches. There is also a comparison of the Exchequer Yard (as 
marked on the Royal Society’s bar) with the Tower Yard (similarly marked). And 
there are experiments on the expansions of various metals. 
The lengths of the bars and chain to be used in the actual measure of the base were 
laid down by means of beam-compasses. Omitting the tentative measure by deal rods, 
the base was measured in two different ways; namely, by glass rods, and by a steel 
chain. When the glass rods were used, each rod carried a small slider in its end, which 
was brought into contact with the end of the next rod; the position of the slider, in 
respect to its o-wn rod, being noted optically with reference to a graduated scale or 
vernier attached to its own rod. When the chain was used, the position of a mark near 
each end of the chain was noted optically with reference to marks carried by a plate of 
metal attached to a fixed post. The same chain was subsequently used for the measure 
of other bases. 
Philosophical Transactions, 1798, page 135, &c. Sir George Shuckburgh describes 
his Scale as having been laid down by Troughtox from a scale of Bird’s (apparently not 
from Roy’s scale, which is separately mentioned). When in use, it was fixed by three 
screws to a block of mahogany. The comparing instrument was not a beam-compass, 
but a bar carrying adjustible micrometer-microscopes, differing in no important degree 
from those now employed. This appears to be the first genuine instance of optical 
comparison ; and, from this time, beam-compasses have never been used in England for 
comparisons. Shuckburgh’s Scale was compared with the Royal Society’s Tower 
Standard E, with the Exchequer Yard on the Royal Society’s bar, with Roy’s, with two of 
Bird’s, with Henry VH. and with Elizabeth’s. Bird’s Standard of 1758, and appa- 
rently also that of 1760 (both made for the House of Commons), were transferred to the 
scale by beam-compasses, and then compared in the same way. 
Base du Systeme Metrique Decimal, tome troisieme. About the end of the last cen- 
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