692 
ME. AIET’S ACCOUNT OF THE COXSTErCTIOX OF 
Standard, namely, the Royal Astronomical Society’s drawn tubular scale. Remarking 
then, among the fusible metals and alloys, that copper is soft, and is not easily cast ; 
that cast brass is considered unsound untd it has been hammered; that gun-metal is 
easily cast, and may be made (by due proportion of its component metals) pretty hard 
and elastic, and is free from liability to corrosion; and that cast steel is perfectly 
manageable; remarking, also, that platinum, although it possesses the advantageous 
properties of being very durable, and little atfected by change of temperature, is incon- 
r eniently heavy and soft ; but that iron, whether cast or forged, is manageable, and 
probably invariable : we considered that our choice of materials was practically limited 
to the following metals and alloys : — 
Gun-metal (in proportions of copper, tin, and zinc to be determined by experiment). 
Cast Steel. 
Cast Iron. 
Forged Iron. 
“II. A series of experiments was made by Mr. Baily on gun-metal formed of different 
proportiorrs of the simple metals. It was found at length that an alloy formed in the 
following proportions, — 
Copper 16 
Tin 2i 
Zinc I 
appeared to possess every desirable property. It is hard ; it is highly elastic, recovering 
its form (as to sense) perfectly after every strain, uirtil the weight placed orr it is suffi- 
cient to break it, when it breaks without bendirrg ; it is also strorrg. 
“ 12. In the guir-metal above described, and in cast steel, arrd hr cast or wrorrght iron, 
we were satisfied that we possessed materials for an invariable measru’e. In considering 
the question of durability^ we could rrot fail to remark that bars of hon or steel are liable 
to rust ; aird that there is great danger of injury or everr total destrrrction frorrr this carrse 
if the bars are inaccessible for many years or for ceirtrrries, or if, although accessible, 
they are very rarely examined. We at length decided orr adopting, for the rrraterial of 
the Standards of Length, gun-metal composed in the proportiorrs merrtiorred above. 
“13. The form adopted for the Standard of Lerrgth arrd for all its copies is that of a 
solid square bar, 38 inches long and 1 inch square irr transverse sectiorr. Near to each 
end a cyliirdrical hole is sunk (the distance betweeir the ceirtres of the two holes being 
36 niches) to the depth of 0'5 inch. At the bottom of this hole is inserted hr a smaller 
hole a gold plug or pin about 01 in diameter, and upoir the surface of this pin there 
are cut three fine lines at intervals of about O'Ol inch trarrsverse to the axis of the bar’, 
and two lines at nearly the same interval parallel to the axis of the bar. The rneasui’e 
of length is given by the interval between the middle transi’ersal lure at one errd and the 
middle transversal line at the other end ; the part of each line which is employed being 
the point midway between the longitudinal lines. The other trarrsversal lines were 
used, in the operations of comparison, only for assigning the scales of the irricrorrreters. 
