Meafwement of a Bcife on Houiiflow-Heath. 6^*^^'^ 



at fii'fl added ; it having been found in thefe experiments, that 

 ice water only, fuch as drains from the ice itfelf, is that which 

 ihonld 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, efpecially when of fo large a fize as 

 the ft^ndard prifms, to acquire the jufl temperature of freezing, 

 at firft the ice w^as 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 

 iieedlefs ; 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 progreffive, and not equable ; there- 

 fore its rate was attempted to be afcertained by noting the pro- 

 greflion anfwering to 60°, 120°, and 180'' above freezing. But 

 when the inftrument was rendered perfe(fl, and that no fenfiblc 

 difFeren-ce 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- 

 cifely to the fame intermediate heat ; then this tedious mode of 

 conducing the experiments was given up, and the expanfion 

 for 1 80° 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 flill continued to 

 do throughout each experiment, care being taken to have a 

 fupply of pounded ice always ready to keep thefe two troughs 

 iquite full. 



Q q q 2 Two 



