070 
ME. AIET’S ACCOrXT OP THE COXSTEUCTIOX OP 
A portion of the comparisons of bars abstracted above was made by Mr. W. .Simms, jun.. 
and a part by Mr. W. Ellis (of the Eoyal Obsen atory). On comparing the results for 
ten bars, it was found that in every instance Mr. Sheepshanks assigned to these bars a 
greater length (in reference to Bronze 12) than was given by Mr. W. Simms. The 
mean value of the discordance was about 0‘“"000054 ; by which, apparently, Mr. Sheep- 
shanks read the length of Bronze 12 shorter than Mr. AY. Simms read it. ^Ir. Sheep- 
shanks and Mr. W. Ellis agreed nearly ; and Mr. Sheepshanks and Mr. W. Simms 
agreed nearly when they observed other bars. It was thus that the suspicion of personal 
equation, in the observation of lines apparently well cut, first arose : and it appeared 
evident that the divisions of Bronze 12 were peculiarly liable to this personal equation. 
Eor this reason, in addition to others mentioned above. Bronze 12 was rejected as Basis. 
Bronze 28 was adopted in its stead, and was retained as Basis in all the subsequent 
observations. 
Section VI. — Operations of impiortant character to the end q/’lSoS: comparisons of a 
small number of bars with Bronze 28; investigation of Personal Ecpiation ; investi- 
gation of the effect of Inside or Outside position of the Bar-, investigaiion of the 
relative thermometric expansion of Steel, Wrought Iron, Cast Iron, Copper, and 
Brass, as coinpared^ with Bronze ; trial and rejection o/'Bailt’s Apparatus. 
Observations made in the year 1850 had led Mr. Sheepshanks to think it possible 
that a very small error might depend on the position of the bars as near to or far from 
the observer. On examining the plans of the comparing apparatus, it will be seen that 
this could arise in no other way tlian the following : that the travelling carriage is not 
exactly in the same position for the observation of the two bars, that the foundation 
stones will therefore be difterently pressed in the two cases, and that tlierefore there 
may be (in some way not easily conceived) a strain of the transom bars which carry the 
microscopes. The first experiments seemed to support this idea ; but the more com- 
plete experiments of 1852 practically negatived it. The following Table contains the 
epitome of the observations made at the end of 1850 and tlie beginning of 1851. Each 
set of observations is the mean of five comparisons. The excess is in every case increased 
by E-OGOO, in order to make it always po.sitive. The computations of means and of 
probable errors were made by Mr. W. Ellis at the Boyal Observatory. 
