12 APPENDIX. 



applied thereto, it is necessary to fix upon two 

 certain points at some distance, of each of which 

 we can judge by unvarying criteria. These ii 

 Fahrenheit's Tliermometer (the one most generally 

 used in England) are the boiling and freezing 

 points of water. With regard to the former of 

 these, it should be recollected as observed in the 

 Twenty-ninth Lesson, that the degree of heat at 

 which water bolls, is varied by the difference 4 of 

 the pressure of the atmosphere. The boiling heat 

 is therefore taken at some certain degree of pres- 

 sure : the degree now used is such as makes the 

 mercury in a barometer stand at 29| inches. As to 

 the latter of these points, it does not appear to be 

 a proper criterion ; or at least, if it be, it has a 

 wrong name affixed to it ; for it is well known 

 that,' though fluids always thaw at the same de- 

 gree of heat, yet the degree at which they freeze 

 is liable to be varied with circumstances. It would 

 therefore be better, if in future this latter point 

 wene :laid aside^ and instead thereof the thawing 

 point made use of. 



The different degrees of heat and cold are gene- 

 rally estimated by their distance either above or 

 below the, freezing point. Those above may be 

 denoted by the affirmative sign : those below by 

 the negative one ; though sometimes the words at 

 length are used ; and at others, the expressions 

 alove or below nothing are made use of j but this 

 latter method seems somewhat absurd. 



Those who purchase thermometers ready made 

 should pay particular attention to the tube and 



corresponding 



