Measurement of a Bafe on Hounllow-Heath . 4yj 
mod to contract to the true freezing temperature,, as was eafily 
known by the wires becoming perfectly fixed and fiationary 
with regard to the marks ; the ice was then, removed from the 
copper trough ; and the fame being filled with water nearly on 
the boil, the ebullition was completed, and kept up, by means- 
©f the lamps now lighted for the purpofe, and flipped m 
underneath. 
The expanfion, anfwering to the i8o° between freezing and' 
boiling, was now meafured by working with the micrometer 
fcrew until the bifectioii * of its wire with thofe of the mark 
was- again complete the obferver at the fixed microfcope taking 
alfo efpecial care all the while to keep his bifedtion perfectly 
accurate., The number of revolutions, regifiered by the num- 
ber of entire divifrons that the zero of the circular fcale had 
departed from its dart or index, and alfo the value of any frac- 
tional revolution* regifiered by the divifions on the head in- 
tercepted between zero and its proper dart, were then noted, a.& 
exprefled in the firfi column of the fiibjoined table gf experi- 
ments ; which requires no other explanation than what is 
therein in felted, and which has been extended purpofely to 
fhew at one view, from mfpeftion only, how much the length 
of our bafe would have been affected, if meafured by thefe 
metals refpectively,, in temperatures between 3 2° and 62°. 
All the experiments were repeated at leaft twice, and- fame 
©f them, three times,, except the fiandard fcale and glafs pen- 
* This bifecUon of the wires may always be made to a great degree of preci- 
sion, by one with a tolerably good eye, and aocuftomed to obfervaticns of this fortv 
I have . myfelf repeatedly adjufted the wires eight or ten times running* allowing 
another perfon to read off and unadjuft each time, without the mean, difference 
exceeding one-fourth of -a divifion -of the. head,,, which is only ^.so-oth part of an, 
inch. 
d ulum 
1 
