690 
ME. AIEY’S ACCOrXT OF THE COXSTEECTIOX OF 
Construction of the Standard of Length. 
“ 4. In regard to the linear measure to be represented by the Standard of Length, we 
see no reason to depart from the decision in the Eeport of 1841, December 21, Article 
19, to adopt the length of One Yard. 
“ 5. The first important question which was left open to us by Article 20 of the same 
Report, was whether the measure of length should be defined by the whole length of a 
bar, or by the distance between two points or fines marked on it ; and to this question 
we gave our early and careful consideration. 
“6. The most celebrated of the standards constructed in modem times is the new 
Prussian Standard, made by the late astronomer, Bessel. In this standard, the whole 
length of the bar is adopted as the measure of length. The reasons which induced 
Bessel to substitute a standard of this construction (or standard a bouts) for the standard 
in which the measure is defined by the distance between points or lines upon its suiTace 
(or standard d traits)^ which was the principle of construction of the former Pmssian 
Standard, are explained in page 7 of the Introduction to Bessel’s Account of the con- 
struction of the new Prussian StandaixT^. The first of these reasons is, that if a flexible 
bar be supported on two points, the extreme length of the bar from the centre of one 
end to the centre of the other end is not sensibly altered by its fiexiue, but the distance 
between two points or fines upon the upper surface may be considerably altered. Bessel 
has himself, however, remarked, that this objection to line-measirre is removed if the lines 
be engraved on surfaces which are depressed to the middle of the thickness of the bar 
(a principle long since employed in English standard bars); and, moreover, the tendency 
to alter the apparent length, whether at the surface or at the middle of the thickness, 
may be destroyed by proper adjustment of the points of support, and perhaps still more 
surely by supporting the bar at numerous points by lever-frames which vdll ensime equal 
supporting forces at all the points (a construction of which the theory has been treated 
by one of the members of the Committee), or in special experiments, by floating the bar 
in quicksilver. A second reason assigned by Bessel is, that the principle of end-measiu'e 
is more convenient than the line-measure for the production (that is, practically, the com- 
parison) of copies of the standard. It appears probable that this remark is well founded, 
as regards the construction of secondary standards for commercial piu'poses, but it is 
doubtful whether it applies to secondary standards for scientific piu'poses, and it can 
scarcely apply to primary standards, in regard to which the consideration of convenience 
(in the few comparisons that may be made at intervals of many years) has no weight, 
when contrasted with the consideration of conservation of the standard length. Another 
reason is, that it is more convenient for use ; but the members of the Committee, who 
have ^vitnessed the operation of measuring a geodetic base by means of bars carrying the 
line-measure, have been led to form a high estimate of the convenience of the line-prin- 
ciple in that instance. It would appear, too, that in Captain Katee’s measurement of 
* Darstellung der Untersuchungen und Maasregelu, welcbe in den Jaliren 1835 bis 1838 dui’cli die 
Einbeit des Preussiscben Langenmaasses veranlasst worden sind, von F. W. Bessel, Berbn 1839. 
