In 1817 Mr. Hassler examined the French standards in America, for the
Coast Survey, using the Troughton bronze standard scale, which is
identical with Sir George Schuckburg’s standard, as the reference, with
the following results, all being reduced to temperature of 32° F.
Original Iron Mètre, 1799 39·381022708 inches.
Lenoir Iron Mètre, Coast Survey 39·37972015 “
“ Brass “ “ 39·380247972 “
“ “ “ Eng. Dept. 39·38052739 “
Canivet Iron Toise, 1768 76·74334472 “
Lenoir “ “ 76·74192710 “
In 1814 Troughton had compared with his own scale in London two of the
above.
Lenoir Iron Mètre, C. S. 39·3802506 inches.
“ Brass “ “ 39·3803333 “
In 1832, under resolution of Congress, Hassler again compared the French
standards in the United States, using as before the Troughton scale, and
reducing all to temperature of 32° F. as follows:
Original Iron Mètre, 1799 39·3808643 inches.
Lenoir “ “ C. S. 39·3799120 “
“ Brass Mètre C. S. 39·380447 “
“ “ Eng. Dept. 39·3801714 “
“ “ “ in 1829 39·3807095 “
Fortin “ State Dept. 39·3796084 “
“ Treas. “ 39·3795983 “
Iron Mètre “ “ 39·3807827 “
Gilbert “ Univ. of Virg. 39·365408 “
Platinum Mètre 39·3803278 “
“ (Nicollet) 39·380511 “
Canivet Iron Toise, 1768 76·74290511 “
Lenoir “ 1799 76·74047599 “
From the mean of his comparisons between the United States brass
Troughton standard yard and the authentic French standard mètres used by
the Coast Survey, Hassler, in 1832, deduced the value of the mètre at
39·3809172 inches, at 32° F., and by correction for expansion to United
States standard temperature of 62° F., he made the mètre at 32° equal to
39·36850154 inches at 62° F.
The British imperial standard and the United States Troughton standard
differ by only 0·000762 inch, which applied to the English reduction
of 39·37079, would give 39·36996 as the relative value according to
Troughton standard.
The difference between these reductions is probably to be attributed
to the use of different rates of expansion, in correcting for standard
temperatures, which vary considerably, according to high authority as
follows for brass at 1° F.
Whitworth, 1876 0·00000956 = 0·00034416 in. per yard.
Borda, 1799 0·000009913 = 0·00035687 “
Smeaton, 1750 0·000010417 = 0·00037501 “
Hassler 0·000010508 = 0·0003783 “
Ramsden, 1760 0·000010516 = 0·0003786 “
Faraday, 1830 0·00001059 = 0·00038124 “
And for the bronze of which the British imperial standards are made:
Airy and Sheepshanks 0·0000095 = 0·000342 in. per yard.
Fizeau 0·00000975 = 0·000351 “
The correction at Ramsden’s rate is nearly identical with Hassler’s,
and gives 39·3684933; at Whitworth’s rate it would give 39·36962, very
nearly the same as deduced from the difference between the British
Imperial standard and the United States Troughton standard. The results
of Sir Joseph Whitworth were obtained by use of all late improvements for
scientific precision, and they must be accepted as most reliable.
It would appear preferable to give comparisons at the same temperature in
connection with the corrected result, so that international comparisons
of scientific measurements may not be vitiated by accidental variations.