52 
Mr. Troughton’s letter may not be uninteresting.—‘ The terminat- 
«ing lines of the iron chain were set off from the British brass 
«* standard, at the temperature of 65°. of Farenheit’s thermometer, 
this is 10°. above the temperate point ; the correction for this, ac- 
« cording to the expansion of iron with heat is .07704 of an inch 
“ upon the whole length, and those will be data for reducing that 
« length for any other temperature ; the chain will, I think, require 
«“ three people to carry it forward, for it should not be dragged on 
« the ground: care must be taken in folding it: in the most accu- 
rate operations the pieces with sliding lines should be used for re- 
gistering the end of the chain. In other cases the arrows may be 
“ sufficient, being more expeditious. As the accuracy of the chain 
is extreme, I leave you to judge if it may not supersede the stand- 
“ ard which you ordered of me.”—To convince myself of the ac 
curacy and power of this instrument, I measured on the lake of 
Castlebar, when frozen, a distance of near two miles, and by re- 
peating the measurement it turned out, after making the necessary 
allowances for temperature, to come within one inch: a result I 
scarcely expected, and which proved its excellence for the purposes 
intended. This trial was made by lines of coincidences, using the 
sliding pieces of brass for registering correctly the length of each 
chain, as the surface of the ice was perfectly smooth. It was af- 
terwards proved in several measurements on firm sands, dry at low 
water, and on level meadow-land, not to differ more than two 
‘inches in two miles. From the circumstance before stated, that 
Mayo afforded no flats, on sufficiently favourable ground, of more 
than two miles in length, I determined on having a series of short 
and accurate bases, in preference to long ones, which might be 
doubtful, from having been measured over an irregular surface. 
