By the Constitution of the United States Congress is charged with fixing
the standard of measures (Art. 1, sec. 8); but as no enactment has been
made by Congress, the standard yard in England, which was legal previous
to 1776 in the Colonies, is the standard yard of the United States, and
does not differ with the English standard yard.
Under resolution of Congress in 1830, Mr. Hassler was employed to examine
the standards in use.
Considerable discrepancies were found, but the mean of all examined
corresponded very nearly with the English standard, and in 1832 the
recommendation of Mr. Hassler was adopted, and the standard yard defined
as the distance between the 27th and 63d inch marks, at the temperature
of 62° F., on the brass scale 82 inches long, being an exact copy of Sir
George Schuckburgh’s standard, made by Troughton, of London, for the
Coast Survey, and deposited in the Office of Weights and Measures at
Washington.
In 1836 an Act of Congress ordered standards to be sent to each Governor
of a State, and the work was done under direction of Mr. Hassler.
In 1856, two copies of the English standard yard, as restored after
destruction of the original standard by fire in 1834, No. 11 of bronze,
and No. 57 of Low Moor wrought iron, were presented to the United States
by Airy.
The United States Troughton standard bar being compared with No. 11
was found to be longer by 0·00085 inch, or in proportion of 1 to
1·0000237216, about 1½ inches in a mile, according to Report of Secretary
of Treasury in 1857.
Later comparisons made by J. E. Hilgard, of the Coast Survey, at the
British Standards Office, between No. 11 and the standard imperial yard,
give No. 11 as 0·000088 inch shorter, or it would be of standard length
at temperature of 62·25° F.
We may infer that the Troughton standard is too long by 0·000762 inch, or
would be standard length at temperature 59·77° F. instead of at 62° by
making expansion reduction with Airy’s coefficient for the bronze of the
imperial standards, 0·000342 inch per yard for 1° F.
The mètre was made a legal standard permissively in 1866; the United
States mètre standard being one of the 12 iron mètre bars made and
verified for the French Government in 1799 on the adoption of the
metrical system, and brought to America by Mr. Hassler in 1800, the
relative value being fixed by Act of Congress at 39·37 inches.
The relative value of 39·36850154 United States inches, as obtained by
Mr. Hassler, corrected to 62° F., was used by the Coast Survey till
1868, when it was found advisable to use the relative value of 39·3704
as deduced by Clarke. Since 1800 several standard mètre bars were sent
to the United States by the French Government, and on comparison, there
appearing to be a slight discrepancy, the original iron standard mètre
bar was sent to Dr. F. A. P. Barnard in Paris, and in 1867 it was
compared with the French platinum standard, which is only used once in
ten years to verify other standards.
A difference was found by this comparison of only ·00017 millimètre
or 1/160000 inch, which being only 1/100 of an inch in a mile is
inappreciable.