THE NEW NATIONAL STAND AED OE LENGTH, AND ITS PEINCIPAL COPIES. 659 
“ The observations were made thus. The trough [upon the travelling platform M] 
being wheeled by the hand, so as to brmg the divisions on . one of the bars under the 
micrometers, the trough-adjusting screws were used finally to adjust the division, so that 
the micrometer-readings should be on opposite sides of the zero and nearly to an equal 
amount : three readings were now taken of A [one of the microscopes], and entered 
together ; then three of B [the other microscope] were made and registered ; three more 
of B under these ; and finally, three of A. Mr. Sheepshanks considered this method of 
registering six observations of each division to be a check upon any start which might 
take place in the apparatus ; and three readings were easily kept in memory, so that 
they could be witten down at once. 
“ The trough being wheeled up, and the other bar being similarly adjusted by the 
trough-adjusting screws, the micrometer-readings were taken in precisely the same 
manner. 
“ The lamps were then changed ; that which illuminated micrometer A being applied 
to B, and vice versa. 
“ The trough remaining undisturbed, a similar set of observations were made of the 
same bar, a slight change of reading being frequently found to arise from a change of 
light. Then the first bar was brought up to position and observed. This completed a set. 
“ At an early period of the observations the bars were made to change places in the 
trough, and another set was taken the following day, as Mr. Sheepshanks considered 
that the position of the trough might affect the distance between the micrometers. 
“In Mr. Sheepshanks’ register of observations, the bar marked ‘inwards’ is that 
farthest from the observer. 
“ At a later period the bar of comparison Avas left undisturbed, and a number of other 
bars was compared Avith it ; then that bar Avas placed in the other position, and the com- 
parisons again made ; this was to avoid having to arrange two bars every time. 
“ Mr. Sheepshanks commonly made several sets of observations at one visit, but each 
set was ahvays kept separate.” 
Section V. — Proceedings of Mr. Sheepshanks to the middle of 1850 ; preparation of 
Thermometers ; adoption and rejection of '•'‘Brass 2 ” as Basis for the new Standard 
Yard; adoption and rejection of '•'• Split Plug A •j’’ adoption of '•'• Bronze VI expe- 
riments cm thermometric expansion; comparison of Bronze 12 with several bars; 
first suspicion of personal equation ; rejection of Bronze 12 Basis, and final adop- 
tion of “ Bronze 28.” 
I now commence an account of the measures made by Mr. Sheepshanks Avith the 
various apparatus which I have described. And here I must state generally, that the 
magnitude of the Avork makes it impossible for me to enter into great details. From 
actual numeration of a large portion of the observations, and from estimation of the 
rest, I find that the number of micrometer-readings amounts to nearly two hundred 
thousand. I can therefore only give abstracts or results. 
