GEODETICAL STANDARDS WITH THE ENGLISH STANDARD YARD. 
163 
On the Comparison of English and Foreign Geodetical Standards with the English 
Standard Yard. By Captain A. B. Clarke, B.E., F.B.S., &c. 
In the Philosophical Transactions, Part III., 1857, is the Astronomer Eoyal’s “Ac- 
count of the Construction of the New National Standard of Length and of its Principal 
Copies.” Those who have looked carefully into this paper must have perceived that 
the difficulties attending the comparisons of standards, where results of a high order of 
precision are aimed at, are considerable ; requiring the very best workmanship in nearly 
every part of the apparatus, and demanding the greatest patience and circumspection on 
the part of the observer. But the difficulties which were encountered and so success- 
fully overcome by Mr. Sheepshanks are considerably enhanced, when, as in the opera- 
tions which have been recently conducted at the Ordnance Survey Office, Southampton, 
the bars to be compared are of different and incommensurable lengths. It was therefore 
foreseen that without building a room especially for the purpose, and devising an appa- 
ratus that could be adapted to the measurements of all kinds of lengths up to 13 or 14 
feet, the comparison of the geodetical standards with the yard could not be undertaken 
with any prospect of success. 
The bar-room is 20 feet in length by 11 in breadth; the walls are double, the outer 
2 feet thick, and the inner 4j inches with an interval of 3 inches. The foundations are 
very strongly built. The roof is fiat, the walls being spanned by iron girders whose 
lower flanges support large slates, which again are covered uniformly by 9 inches of 
concrete. The direction of the length of the room is nearly east and Avest ; in the north 
face are two small windows, sufficient to admit a moderate amount of daylight. The 
sash frames, which slide in the interval betAveen the two Avails, are further protected by 
wooden shutters, 3 inches in thickness, on the outside. The doonvay is at the east end 
of the room and is closed by double doors, one on the outside of the room, the other 
on the inside, so that any one entering the room may close the first or outer door before 
he opens the second or inner door. 
An outer building encloses this room, and so protects it from the variations of tempe- 
rature of the external air. The outer building is of brick, and is 40 feet in length by 
20 in breadth, Avith an ordinary slate roof. Thus shielded from external influences, the 
•temperature of the inner room is exceedingly steady, leaving nothing to be desired in 
this respect. 
Along the southern wall of the room are three stone piers for supporting the micro- 
meter-microscopes. The centre pier, or block, measures on its upper surface 4 feet by 
16 inches ; the outer blocks are of the same breadth, but only 3 feet 6 inches in length ; 
they are distant 5 feet from centre to centre from the middle block. These stones are 
close to the wall of the room, but are not actually in contact with it ; they have separate 
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