624 
ME. AIEY’S ACCOUNT OP THE COXSTEUCTIOX OF 
tury and the beginning of the present century (it is difficult to give a better reference, 
where a work necessarily occupied a long time, and where no dates are affixed to its 
separate memoirs), the operations were effected for measuring the bases in the French 
survey, for ascertaining the length of the seconds’ pendulum, and for comparing the 
adopted metre with English measure. For the bases : two separate toise-measures were 
prepared, each equivalent to the toise of Peru, being, like the toise of Penr, end- 
measures ; but there is no description of the process of veiif}ing the equivalence. 
Then the two toise-measures were placed end to end, so as to give the length of two 
toises from a fixed block ; and a slider with a vernier, carried by a second fixed block, 
was made to touch the extremity nearest to that second block ; the same thing was done 
with the two-toise measuring-bars ; the difference of the vernier-readings, ascertained 
optically, gave the difference between the length of the measuring-bar and two toises. 
In the actual use of the measuring-bars, two bars were never permitted to touch : each 
bar carried a slider or languette (as in General Pot’s glass rods), and the ultimate refer- 
ence was to a vernier. An operation, similar to that for the measm-mg-rods, was used 
(except that some subsidiary fractions of the toise were also employed, the sum of their 
lengths being ascertained by mechanical contact) for the end-bar which was necessary, 
in Borda’s method, for measuring the double-seconds’ pendulum ; the ultimate determi- 
nation always resting on optical observation of a vernier. For the length of the metre 
(an end-measure) in comparison Avith the English scale belonging to Professor Pictet of 
Geneva, a subsidiary piece bearing a fine line was provided : first, this piece Avas made 
to touch a fixed block, and a microscope was adjusted on the line ; secondly, the same 
piece Avas made to touch one end of the metre while the other end of the metre touched 
the same block, and another microscope Avas adjusted on the line ; these two microscopes, 
thus adjusted, Avere then used to obsei’A'e the graduations of the scale. 
Philosophical Transactions, 1818, page 33, &c. Rater’s Account of experiments for 
determining the length of the Pendulum Adbrating seconds in the latitude of London. 
The measuring apparatus appears to have been generally similar to Sir George Shuck- 
burgh’s, but with microscopes of greater magnifying poAver. The beam of mahogany 
which carried the microscopes Avas elevated by a foot-screAV at each end, for focal adjust- 
ment of the microscopes. There is no clear account of the Avay in Avhich the micro- 
scope-frame Avas moved from one standard to another, or of the Avay in Avhich standards 
were brought successively under the microscoj^es. The distances 0 — 39-4 and 0 — 42 on 
Shuckburgh’s scale Avere compared AAfith 0 — 39-4 and 0 — 42 on General Pot’s scale, and 
a result was obtained differing considerably from Shuckburgh’s. Bird’s standard of 
1758 (made for the Flouse of Commons) AA^as also compared AA’itli 0 — 36 of Shuckburgh's. 
The measure of the distance betAveen the knife-edges of the pendulum (Avhich is an 
operation whose peculiarities, though in the opposite direction, are similar to those of 
the comparison of end-standards AAdth line-standards) Avas effected in tAvo Avays : one by 
introducing supplementary blocks bearing lines, and using the microscopes to observe 
those lines, both Avhen the blocks touched the knife-edges and AA'lien the blocks 
