Meaftirement of a Baje on Hoimflow-Heath. 473 
, at fifft added ; it having been found in thefe experiments, that 
{ ice water only, fuch as drains from the ice itfelf, is that which 
fhould properly be made.ufe of to mix with the pounded ice, 
in order to bring the whole mafs to the true freezing tempera- 
ture. Being at. the commencement uncertain what time might 
be neceffary for the rods, especially when, of fo large a fize as 
the ftandard prifms, to acquire the juft temperature of freezing, 
at firth the ice was put into the troughs over night, to prepare 
for the continuation of the experiment next morning. But 
after many repeated trials, this precaution was found to be 
needlefs ; a quarter of an hour being more than fufficient to 
give to all the freezing temperature, as well as to render the 
lens on the expanding rod ftationary, after the water fupplying 
the place of the ice had been brought fairly to boil. 
The inftrument, in its firft ftate, having in fome cafes made 
the expanfion appear to be progreflive, and not equable ; there-' 
fore its rate was attempted to be afcertained by noting the pro- 
greffion anfwering to 6o°, 120°, and 180° above freezing. But 
when the inftrument was rendered perfedft, and that no fenfible 
difference was found between the expanfion at the lower and 
that at the upper part of the fcale, a fair mean being taken be- 
tween its afcending and defcending rates, and allowing for the 
difficulty of keeping the water, for any length of time, pre- 
cifelyto the fame intermediate heat; then this tedious mode of 
conducting the experiments was given Up, and the expanfion 
for 180 0 was at once determined by bringing the water to boil 
around that rod, which but a little before had been lying in 
melting ice, and which the ftandard prifms ftill continued to 
do throughout each experiment, care being taken to have a 
fupply of pounded ice always ready to keep thefe two troughs 
quite full. 
Q,qq2 , Two 
