460 
LIEUT. -COL. A. R. CLARKE ON STANDARDS OE LENGTH. 
l’a ete aussi directement avec la regie No. 1 de Borda deposee, a l’observatoire de Paris. 
De ces comparaisons il resulte que la regie en fer a de longueur a la temperature de 21 0, 935 
centigrades, 
4 m -0006526±0 m -000002. 
“ C. Ibanez, 
Colonel du genie , attache aux 
travaux geodesiques de VEspagne .” 
This bar has been compared in two entirely different manners — first, with two lengths 
of the Ordnance toise, and secondly with four lengths of the Ordnance metre. The 
former comparison was rendered possible by the fortunate circumstance that four metres 
exceed two toises by almost exactly 4 inches, the difference (’013 inch) being easily 
measured by the micrometers. 
Comparisons of the Spanish Bar with the Toise. 
In order to effect this comparison the four micrometer-microscopes H, I, J, K were 
arranged in line at the distances H I = one toise, I J = 4 inches, J K=one toise, so that 
PI K=four metres very approximately. The adjustment into a horizontal straight line 
of the outer foci of these microscopes was a matter of some difficulty, but was finally 
effected with so much precision that not the smallest error in the comparisons could 
arise therefrom. It remains to add that the alignment was frequently examined during 
the course of the observations. 
The method of comparison will be understood from the following Table, where the 
different operations are on successive lines : r, is the mean of the readings of the four 
thermometers of the Spanish bar before the micrometers are read, e and f are the 
readings of the Spanish bar by the micrometers H K, r 2 the temperature of the bar 
immediately after, and so on. The toise and foot were mounted side by side in the 
same box. Every reading was made by two observers independently. 
