174 CAPTAIN A. R. CLARKE ON THE COMPARISON OF ENGLISH AND FOREIGN 
compared with the yard Y 55 , and the space [c . d] of 4- 74 inches was compared with the 
very nearly equal space [rf~\ on the foot F : the result of the comparisons is 
[«.$]= Y 55 +5‘95, 
[b.c]= Y 55 +2-34, 
[c -d]=iiWhY 55 +0-16, 
or 
T„=*ffaY M +8-45±0-21, 
both bars being at 62° F. If both be at 61°‘25 F., or 16’25 Centigrade, 
T„=if^Y 55 + 9-17±0-21 (7) 
The yard space on the metre was compared with Y 55 , and the small space with [p . e] 
on F ; with these results, 
[«.q= y 55 — i-20, 
\b . e]=Mu' Y sb - 134-25, 
or 
M„=MfY 55 -135*45±0-16, 
both bars being at 62°. If both be at 61° - 25 F., or 16-25 C., 
M„=ftetY 55 -135-ll±0-16. ....... (8) 
The length of T 0 is the result of 3120 micrometer and 640 thermometer readings; for 
M 0 were made 1680 micrometer and 320 thermometer readings. The observations for 
the former bar occupied thirty-three days, for the latter sixteen days. 
The Toise. 
The unit of length in which by far the greater part of the European geodetical mea- 
surements are expressed, is the toise ; the actual physical representative of this length 
being the Toise of Peru* , at the temperature of 16°*25 C., or 61 0, 25 F. This bar, which 
was constructed in 17 35 for Messrs. Bouguer and Lacondamine as their standard of 
reference in the operations conducted by them in Peru, is an end measure of polished 
iron, P51 inch in width and 0-40 inch in thickness. In order to trace our direct con- 
nexion with this celebrated and important standard, we must refer to two direct copies 
of it made by Fortin, of Paris, one in 1821 for M. Struve, and the other in 1823 for 
M. Bessel. The authority of the former rests on a certificate of the Bureau des Longi- 
tudes , signed by M. ARAGof, in which he states that he has compared the copy with the 
* We have no precise information as to the present state of this har, bnt from report it would appear that 
at least as far back as 1858 it was so far damaged that comparisons with it were worthless. For a description 
of the Toise of Pern see ‘ Base du Systeme Metrique Decimal,’ tome iii. pp. 405, 680, and the work entitled 
‘ Mesure des Trois premiers Degres du Meridien dans l’Hemisphere Austral,’ par M. de la Conbamixe ; a Paris, 
1751, pp. 75, 85. 
t See 4 Arc du Meridien de 25° 20' entre le Danube et la Mer Glaciale mesure depuis 1816 jusqu’en 1855 
’ par F. G. W. Struve, St. Petersbourg, 1860, tome i. Introduction, p. lxxiv. 
