THE NATIONAL STANDAED OF LENOTH, AND ITS PEINCIPAL COPIES. 065 
Appendix No. 1. 
“ On the Scales used in the Trigonometrical Surrey, and especially Roy’s Scale. 
“ The Irish Survey depends on a base which is ultimately referred to an iron bar of 
10 feet, and a brass bar of 6 inches. These were laid off, I believe, from the 5-feet brass 
measures of Teoughton or Shuckbukgh, as nearly as could be readily managed, but they 
have no immed:iate relation to any yard, standard, or otherwise. It will be necessary to 
ascertain the value of these units in terms of the new standard, when that is fixed ; but 
there is nothing to be extracted from them which would assist us in restoring the 
Imperial Standard destroyed in 1834. 
“ At first sight, one might have expected some help from the scales employed in the 
English Survey, viz. from Eoy’s scale and Ramsden’s scale, which were, or were intended 
to be, the units of the bases of Hounslow Heath, Romney Marsh, «&c. ; but a careful 
examination of the memoirs relating to those bases, and of Katek’s memoir (Philoso- 
phical Transactions, 1821, p. 75), extmguishes hope of any aid from this quarter. 
“ Roy’s scale (Philosophical Transactions, vol. Ixxv. p. 401) was purchased by him at 
Shokt’s sale. ‘ It was originally the property of the late Mr. Geaham, the celebrated 
watchmaker; has the name of Jonathan Sisson engraved upon it, but is knoAvn to have 
been di\ided by the late Mr. Bied*, who then worked with Sisson.’ Roy, aided by 
Rayisden, found that 36 inches, taken from ‘ Geaham’s Tower Yard E,’ exactly reached 
to 36 inches on his scale, the temperature being 65°. Geaham’s Exchequer Yard was 
formd 0“"0069 shorter. From the 40 inches of Roy’s scale, the deal rods and glass rods 
used by Roy in measuring the HounsloAv Heath base were laid off, not directly, but by 
40-inch spaces marked off on a large plank. By these points laid doAvn on the large 
plank, the chain used in the Romney Marsh base Avas verified. 
“ Ramsden’s scale Avas also compared AAith Geaham’s ToAver Yard E, ‘ Avhen they Avere 
found to be precisely of the same length.’ — Philosophical Transactions, 1795, p. 435. 
This must have been a comparison of 36 inches only. From his OAvn scale, Rayisden laid 
off 40-inch spaces on the 21-foot iron bar Avhich he used for ascertaining the actual lengths 
of the chains A and B, by Avhich the base of PIounsloAv Heath Avas remeasured, &c. 
“ If, then, the operations had been correctly performed, all the base measurements of 
the English Ordnance Survey Avould haA'e been expressed in terms of Geahayi’s ToAA^er 
Yard E. 
“ But it is impossible to trust, implicitly, random expressions that two measures agree 
exactly, this cannot be ascertained by a beam compass. Roy and Rayisden are both 
silent as to the manner in Avhich they ascertained the relation of their 40 inches to 
36 inches ; and Katee’s account of the 21-foot iron bar does not give a A^ery high idea 
of Rayisden’s exactness in transferring his 40-inch spaces to his iron bar. (See his 
memoir. Philosophical Transactions, 1821, p. 71.) Thus, according to Katee, — 
* “ The fundamental length was very probably laid off by Geahaai himself.” 
