274 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1777. 



heated. If the pot is heated over a chafing-dish or common fire, we apprehend 

 that there can seldom be any danger of too mucli of tlie sides being heated ; 

 but if the operator should be apprehensive that there is, it is easily prevented by 

 fastening an iron ring an inch or 1 broad round the pot near the bottom. I'his 

 precaution is equally necessary when the thermometer is adjusted in steam, es- 

 pecially when there is not much water in the pot. The greatest inconvenience 

 of this method of adjusting the boiling point, is the trouble of keeping a 

 proper depth of water in the pot ; as to do this, it is necessary first to find the 

 height of the boiling point coarsely by trying it in an open vessel, and then to 

 put such a quantity of water into the pot, that it shall rise from 1 to 3 or 4 

 inches above the ball, when the thermometer is placed at such a depth within 

 the pot, that the boiling point shall rise very little above the cover. The ope- 

 rator must be very careful that the quantity of water in the pot be not so small 

 as not entirely to cover the ball. 



A 3d way of adjusting the boiling point, is, to wrap several folds of linen 

 rags or flannel round the tube of the thermometer, and to try it in an open 

 vessel, taking care to pour boiling water on the rags, in order to keep the quick- 

 silver in the tube as nearly of the heat of boiling water as possible. The best 

 way is to pour boiling water on the rags 3 or 4 times, waiting a {q\v seconds be- 

 tween each time, and to wait some seconds after the last time of pouring on 

 water before the boiling point is marked, in order that the water may recover its 

 full strength of boiling, which is in good measure checked by pouring on the 

 boiling water. In this method 

 the boiling point should be ad- 

 justed when the barometer is at 

 29.8 inches, that is, the same 

 as when the 1st method is used ; 

 the water should boil fast, and the 

 tliermometer should be held up- 

 right, with its ball 2 or 3 inches 

 under water, and in that part of 

 the vessel where the current of 

 water ascends.* 



Whichever of these methods 

 of adjusting the boiling point is 

 used, it is not necessary to wait till the barometer is at the proper height, 

 provided the operator will take care to correct the observed height according to 

 the above table. 



* In a vessel of boiling water one may almost always perceive the current of water to ascend on 

 one side of the vessel, and to descend on the other. — Orig. 



