THE NEW NATIONAL STANDAED OF LENGTH, AND ITS PEINCIPAL COPIES. 691 
the pendulum, and Mr. Baily’s comparisons of the metre a traits with the yard, no 
inconvenience from that cause was found by either observer. It is moreover to be 
remarked, that the whole of the British geodetic bases have been measured by bars con- 
structed on the line-principle ; and a standard which is intended to apply advantageously 
to them must be constructed on the same principle. The end-measure has never, so far 
as we know, been applied to any scientific determination in England. 
“ 7. The construction of the ends of a standard formed on the end-principle (which 
must be made of materials harder than the rest of the bar, so mounted as to admit of a 
slight adjustment), is, in Bessel’s construction, complicated and unsatisfactory ; that of 
the ends of a standard on the line-principle is simple. The tendency of an end-standard 
to alter its length by wearing is in only one direction ; that of a line-standard, so far 
as change can be conjectured, is matter of chance; it will however be practically 
invariable. 
“ 8. A single comparison by end-measure is perhaps more accurate than a single com- 
parison by line-measure. On the other hand, it seems doubtful whether this accuracy 
may not be impaired by the immersion in fluid which has been thought necessary for 
preventing unequal expansion. There has also been recognized in the line-measures 
which have been made in the course of these observations, a personal equation difierent 
with difierent observers ; but this was only discovered after most of the observations 
were made. We have no emlence whether such equation exists in comparisons by end- 
measure, as the whole of the comparisons for the Prussian Standard were made by 
Bessel himself. Still it is scarcely doubted that in this respect the system of end- 
measures has an advantage. On the other hand, there is no doubt that, by repeating 
the measures, and by varying the observers, the errors in line-measures arising from 
recognized causes will be eliminated, and unexceptionable accuracy will be given to the 
results. 
“ 9. After consideration of all these points, as far as they were known at the time ; 
remarking, in particular, the freedom of a line-standard from any tendency to constant 
wear, the applicability of a line-standard to our geodetic measures, the facility of com- 
paring all other measures of a similar construction with a scale carefully subdimled, the 
circumstance that there exist no means of reproducing the old standard length but by 
reference to existing line-measm’es, and the historical fact that the British standard for 
many years past has been a line-standard ; we decided on adopting a line-standard, or 
measure a traits^ for the Parliamentary Standard of Length. 
“ 10. The point which next engaged our attention was the material of which the new 
Standard of Length should be formed. The qualities required in the material are 
invaiiability and durability. Already we had reason to suspect that bars, which have 
received their form by undergoing while cold the violent mechanical operations of ham- 
mering, roUing, or drawing, will not preserve an invariable length ; and these suspi- 
cions were raised to certainty by Mr. Baily’s examination of the standard scale which 
primd facie appeared to be the most trustworthy representative of the old Imperial 
