ANNUAL ADDRESS OF THE PRESIDENT. XLVII 
on June 24, 1819, and therein recommended the adoption of the 
standard of length which had been used by General Roy in measur- 
ing the base on Hounslow Heath ;* but in a second report made 
July 13, 1820, they wrote,“We * * * have examined, since 
our last report, the relation of the best authenticated standards of 
length at present in existence, to the instruments employed for 
measuring the base on Hounslow Heath, and in the late trigonome- 
trical operations—But we have very unexpectedly discovered, that 
an error has been committed in the construction of some of these 
instruments:? We aretherefore obliged to recur to the originals which 
they were intended to represent, and we have found reason to prefer 
the Parliamentary standard executed by Bird in 1760, which we 
had not before received, both as being laid down in the most ac- 
curate manner, and as the best agreeing with the most extensive 
comparisons, which have been hitherto executed by various observers, 
and circulated through Europe; and in particular with the scale 
employed by the late Sir George Shuckburgh.”* 
Accordingly, when in 1824 Parliament at length took action, 
Bird’s standard of 1760 was adopted instead of that of 1758. The 
former being a copy of a copy, its selection as a national standard 
of length seems so singular that the circumstances which brought 
about that result should scarcely be passed over in silence. Bird 
had a very accurate brass scale 90 inches long, which he used in 
all his dividing operations, whether upon circles or straight lines, 
and which Dr. Maskelyne said was 0°001 of an inch shorter on 
three feet than Graham’s Royal Society yard E.* In the year 1792, 
or 1793, the celebrated Edward Troughton made for himself a five 
_ foot scale, which conformed to Bird’s, and which he afterwards used 
in laying down the divisions of the various instruments that passed 
through his hands. This was the original of all the standard scales 
he ever made, and at the beginning of the present century he be- 
lieved these copies, which were made by the aid of micrometer 
microscopes, to be so exact that no variations could possibly be de- 
tected in them, either from the original or from each other. Among 
the earliest of the scales so made by Troughton was the one used by 
Sir George Shuckburgh in 1796-8 in his important scientific opera- 
tions for the improvement of the standards. Subsequently, the 
length of the meter was determined by comparison with this scale 
123, p. 4. 327, p. 92. $24, p. 3; also, 25 and 26. #13, p. 326. 
