A THE YARD

could restore the lost measure without sensible
error; and we may take it that what that com-
mittee would call a sensible error would be perhaps
a five-thousandth or a ten-thousandth of an inch—
for bear in mind it was not a merely commercial
measure that they aimed at, but one entitled to
scientific respect and available for scientific pur-
poses requiring the utmost exactitude, such for
instance as geoditic surveys, where an error in the
unit may perhaps be magnified ten thousand times
in the resulting measures. Modern manufacturing
requirements, however, do call for a considerable
degree of accuracy; for instance, in great engi-
neering works, for which large iron castings and
forgings have to be prepared at a distance, and
perhaps in another country, from figured dimen-
sions, it is essential that the scales or measuring
rules be precisely similar in the two places. Occa-
sionally this perfect agreement is only secured by
an exchange of foot-rules.

The Committee reported (in 1841) in favour of
restoring the British yard by the scattered copies
of it ; and their suggestions, which as to details
were numerous, were approved, and another larger
commission was appointed three years after to
execute their recommendations.

Theirs was no easy task. The first duty,” that
of collecting the available standards, was simple
enough. The bars brought together were princi-
pally one that had been made for the Royal Astro-
nomical Society, one in the possession of the Royal
Society, and two which had been used in measuring
the base-line for the Trigonometrical Survey of
Britain. Then a microscopic comparing apparatus
which would defy competition on the ground of ac-
curacy, had to be devised and constructed. Elabo-

rate experiments upon the best metal for a standard
were undertaken. The expansions by temperature of
the various metals had to be accurately determined.
Thermometers of unimpeachable exactitude had to
be specially made for this investigation ; for,
strangely enough, at that time, instruments of this
class were too unreliable for the work in hand.
Experiments, too, had to be made upon the best
form of bar, and upon the relative advaniages of
«« end-bars,”—bars whose total length defined the
yard ; and “line-bars ”—those in which the yard is
defined by dots or lines engraved upon the metal.
The work of directing these executive works was
undertaken by the late Mr. Francis Baily, the
famous amateur astronomer ; but he died almost at
the outset of them, and his place was voluntarily
filled by Mr. Sheepshanks, another amateur astro-.
nomer, whose name is to be mentioned with the
highest praise, for he worked incessantly in this
business for eleven years, directing every detail,

and himself recording thousands upon thousands of
micrometrical comparison-measurements ; and he |

MEASURE. |
did all disinterestedly, and with no prospect of re- |
ward beyond that which followed from the con- |
sciousness of having done a great and enduring |
work in the most perfect manner possible. He
died, just as his labours were closing and with his
thoughts still upon them,—one of those soldiers of
science who fight their battles in silence, and of
whom the world, hearing no trumpet sound of their
victories, often knows but little.

The committee finished their work in 1855.
They had then constructed more than seventy
standards, each representing the British yard at
some definite temperature (near 62° Fahrenheit)
which was engraved upon it. Five of these were
reserved for home use, and nearly all the rest
were distributed to the various governments of the
world. An Act of Parliament was passed in the
same year, a portion of which we quote, as it gives
the best idea of the actual form and structure of
the British Imperial standard yard. After allud-
ing to the destruction of the legal standard by fire,
it proceeds :—‘‘ And whereas there exist bars
which had been accurately compared with the
said standard yard so destroyed as aforesaid which
afforded sufficient means for restoring such original
standard: and whereas scientific men ....
have constructed a standard of length, equivalent
to the Imperial standard yard so destroyed, and
four accurate copies of the standard so constructed ;
and whereas the form adopted for the standard of
length, and for all the copies thereof, is that of a
solid square bar, thirty-eight inches long, and one
inch square in transverse section, the bar being of
bronze or gun-metal ; near to each end a cylindri-
cal hole is sunk (the distance between the centres
of the two holes being thirty-six inches) to the
depth of half an inch ; at the bottom of this hole is
inserted in a smaller hole a gold plug or pin about
one-tenth of an inch in diameter, and upon the sur-
face of this pin there are cut three fine lines at in-
tervals of about the one-hundredth part of an inch
transverse to the axis of the bar, and two lines at
nearly the same interval parallel to the axis of the
bar ; the measure of length is given by the inter-
val between the middle transversal line at one end
and the middle transversal line at the other end,
the part of each line which is employed being the
point midway between the two longitudinal lines;

_, . And whereas the standard of length so
constructed as aforesaid, the bronze bar being
marked ‘Copper, 160z., Tin, 23, Zinc, 1., Mr.
Baily’s metal. No. 1 Standard Yard at 62°-00,
Fahrenheit, cast in 1845. Troughton and Simms,
London’... . [has] been deposited in the office
of the Exchequer at Westminster.” The Act then
goes on to specify one copy of the standard which
is to be deposited with the Royal Society, a second
to be kept at the Royal Mint, a third to be pre-

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