1833.] ‘Expansion of Metals. 131 
It will be remembered, that the measurement of the base for the 
great Trigonometrical Survey of India, on the Barrackptir Road, was 
conducted with compensation bars of a peculiar construction, each of 
them ten feet in length, or, bearing near their extremities two minute 
points, intended to represent that distance without liability to alter 
by change of temperature. Their construction has been accurately 
described by Major Everest in the 18th volume of the Asiatic 
Researches. To prevent the possibility of derangement in all or 
any of these compound bars, and to serve as a term of compa- 
rison for the whole, a standard iron bar was furnished along with 
them from England, upon which was laid off at a eertain temperature 
with all imaginable accuracy, the measure of the English parliamentary 
standard, to which all the measures of the Indian meridian line should 
be thus reducible. 
After the completion of the Barrackpir base, the compensation bars 
underwent a most rigid comparison with THE STANDARD; as did also 
the steel chains used in measuring the several bases of Col. Lambton’s 
Survey in the peninsula. The particulars of these comparisons, conduct- 
ed with that most elaborate care and precaution, which has distinguished 
all the operations of the new survey, will be described by Major Ever- 
est himself, when he shall favour the public with the result of his labours. 
At present it is but one item of these precautionary measures which 
will come under our review. 
The comparisons with the standard bar were made at a temperature 
differing by many degrees from that at which the latter had been 
proved in England. It became therefore necessary to apply a correc- 
tion for its dilatation by heat: but to do this a question naturally arose 
as to what constant should be employed? The expressions given by 
different experimenters vary from 1.00144 (Troughton) to 1.00118 
(Dulong et Petit), or one-sixth of the whole quantity,—a variation either 
to be attributed to imperfections in the mode of experimenting, or to 
difference of quality in the metal,—but in either way rendering it ad- 
visable to have recourse to a new set of experiments, to obtain the 
individual expansion of the standard bar itself. The experiments 
made by myself in Dec. 1831 upon a rod of iron twenty-five feet in 
length, though nearly agreeing with the results of Lavoisier and 
Smeaton*, were for the same reason inapplicable to a metal which 
might be of different quality. It was therefore determined by Major 
* Expansion of wire-drawn iron by Lavoisier, .......4-0++-008 1.001235 
SSINGAUGN, jaraetuolelsiaaiae es’ sie 1.001258 
IPEHISED,, % cfuie ate s10' wuwree ale 1.001256 
s 2 
