256 AN ACCOUNT’ OF THE _ 
Besides these two, there were other two posts driven firmly mto the 
ground, on which the brass registers were set, and by means of the slider 
with the fine line, the length ef the chain could be accurately laid off. 
The stands were first put accurately im the same plane by means of the 
small cross of wood, and the boning telescope, and any deviation which was 
afterwards observed, owing either to their slides haying slipped or to any 
unevenness in the boards, was corrected by means of thin wedges placed 
underneath the chain. ‘The links being 5 feet long, however were the less 
liable to accomodate themselves to the trifling inequalities of the boards. 
39. Burne laid accurately it was thought advisable to observe its con- 
traction and expansion, and. whether it agreed with the indications of the 
thermometer, allowing for its change of length according to the known law. 
Thus being stretched by a weight of 19 Ibs. and the registers set, the mean of 
4 thermometers was 58-6: on the temperature, rising to 69:3, as shewn by 
‘the mean of the same thermometers, it was found that it overshot the regise 
ters or had expanded -073 inches. 
Now the expansion of a steel chain was found by Colonel Munce’s ex- 
periment, to be very nearly the same as given in General Roy’s table, in the 
ist Vol. Trigonometrical Survey. ‘This is 0075 inches for every 1 of 
Fahrenheit on 100 feet, | : 
Now, -0075 x 107 = +080 inch. 
Observed expansion — °073 
—aeen 
Error, ‘007 
