686 
-MR. AIEY’S ACCOrXT OP THE COXSTErCTIOY OF 
no force which it is practicable to apply in the mounting of the bars would tend to cor- 
rect the error. The only way which seems practicable is, to place the bars in the veitical 
position, one bar standing freely upon the end of the other ; but this requires a rather 
inconvenient arrangement of the microscopes. 
These errors and inconveniences are entirely avoided by giving to the ends the form 
of a spherical surface whose centre is the centre of the di'vision-line at the middle of the 
bar’s length. There is a tendency to thrust away particles of dust from the point of 
contact ; and a small error in the elevation, or inclination, or azimuthal position, of 
either bar, produces no error on the distance between the centres of the division-lines of 
the two bars. 
Accordingly, about the beginning of 1854, Mr. Sheepsbaxks ainanged an apparatus 
for grinding the ends of end-bars. The bar was grasped by its middle, in a cramp which 
turned in jimbals or universal-joint; and a plane surface carrying a grinding powder was 
placed at the proper distance. The operation requu’es no explanation. 
It was then necessary to decide on the substance which should be used for the hard 
ends of the bars. The properties required were, hardness, toughness, and susceptibility 
of polish. After several trials, it was found best to use a veiy shghtly conical plug, of 
either agate or hardened steel (the reason for subsequently abandoning the hardened 
steel will be given hereafter), and to fix the plug in the corresponding very shghtly 
conical hole at the end of the bar, by fitting it in when the bar was somewhat heated 
(below the boiling-point of water) and allowing the metal of the bar to shrink upon it. 
Then the metal and the plug were ground at the same time, so as to make a flush joint. 
It was found best to give any final reduction of length by a polishing-touch of the hand. 
Mr. Sheepshanks had commenced the preparation of some end-bars ; but tliis process, 
in which a bar if too much shortened is injured beyond the possibility of restoration 
except by inserting a new plug, is very tedious. This was more particidarly felt, when 
the grinding and polishing were efiected in Mr. Simms’s workshop, and the optical test 
was applied at Somerset House. No eificient progress was therefore made, up to the 
time of Mr. Sheepshanks’s decease. On then revising the condition of the work, and 
after having consulted with Professor Millee, I requested Mr. Siiois to undertake the 
whole operation of testing and adjusting the bars in his omi workshops, and finally of 
making the definitive comparisons with Bronze 28 on Mr. Sheepshanks’s apparatus at 
Somerset House. 
The bars first tried were those prepared by Mr. Sheepshanks, some hawng agate ends, 
some having ends of hardened steel. It happened that an end of agate and an end of 
hardened steel were left in contact for about a day, when it was found that the distance 
between their division-lines had increased about Yb\) oth of an mch. Upon examining 
the end of the steel plug, it was found that the proper point of contact and a disk about 
^th of an inch in diameter was clean ; but an annulus surrounding it, of the breadth 
perhaps of i^th of an inch, was thickly rusted ; and that the rust had acted like a 
wedge, forcing the bars asunder in opposition to a spring-pressure of about 3 lbs. It 
