roL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 273 



notch in it, so as to receive the tube of the thermometer, and to suffer the bent 

 part to rest on the bottom of the loop ; this piece must slide in a slit Kk, cut in 

 the plate lInh, and be tightened at any height by the screw t. 



4thly. It is best making the water boil pretty briskly, as otherwise the ther- 

 mometer is apt to be a great while before it acquires its full heat, especially if the 

 vessel is very deep. The observer too should wait at least a minute or 1 after 

 the thermometer appears to be stationary, before he concludes that it has ac- 

 quired its full height. 



5thly. Though this appears to be the most accurate way of adjusting the 

 boiling point ; yet, if the operator was to suffer the air to have any access to the 

 inside of the vessel, he would be liable to a very great error : for this reason we 

 strongly recommend it to all those who use this method, not to deviate at all 

 from the rules laid down without assuring themselves, by repeated trials with a 

 pretty sensible thermometer, that such alteration may be used with safety. But 

 the covering the chimney witii the tin plate ought by no means to be omitted ; 

 for though, if the cover of the pot fits close, it seldom signifies whether the 

 plate is laid on or not, yet, if by accident the cover was not to fit close, the 

 omitting the tin plate would make a very great error. Making the chimney 

 very narrow would not answer the end properly ; for if it was made so small as to 

 make the vessel sufiiciently close when the water boiled gently, it would not 

 leave sufficient passage for the escape of the steam when it boiled fast. 



Anotlier way of adjusting the boiling point is, to try it in a vessel of the same 

 kind as the former, only with the water rising a little way, namely from ] to 3 

 or 4 inches above the ball, taking care that the boiling point shall rise very little 

 above the cover, as in the former method. In this way there is no need to cover 

 the chimney with the tin plate ; and there is less need to make the cover fit close, 

 only it must be observed, that the closer the cover fits, the less the operator will 

 be mcommoded by the steam. The height of the barometer at which the boiling 

 point should be adjusted, when this method is used, is 29^ inches, or -^^ of an 

 inch less than when the former method is used. 



It will be convenient to have 2 or 3 pots of different depths ; for if a short 

 thermometer is to be adjusted in the same pot which is used for a long one, it 

 will require a great depth of water, which, besides taking up more time before 

 it boils, makes the observation rather less accurate, as the heat seems to be less 

 regular when the depth of water in the pot is very great, than when it is less. 

 Perhaps some persons, for the sake of heating the water more expeditiously, 

 may be inclined to use an apparatus of such kind, that the fire shall be applied 

 to a considerable part of the sides of the pot, as well as to the bottom ; we 

 would however caution them against any thing of that kind, as the observations 

 are considerably less regular than when little more than the bottom of the pot is 



VOL. XIV. N N 



