VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 575 



one end, and fixed in a long handle at the other, makes a convenient instrument 

 for pushing the piece forward, or drawing it back, again, while red-hot : this in- 

 strument, at every time of using, is heated to the same degree as the piece 

 itself. 



The heat of boiling water is taken without difficulty, by keeping the apparatus 

 in boiling water itself during a sufficient time for the full heat to be communi- 

 cated to it. The water I made use of was a very fine spring water, which on 

 chemical trials appeared very nearly equal in purity to that of rain or snow ; and 

 I had previously satisfied myself, by trials in the cold, that the gage and piece 

 being wet, or under water, made no difference in the measurement. The ex- 

 pansion of the silver by this heat, that is, by an increase of the heat from 50° to 

 2 \2°, or a period containing l6'2°of Fahrenheit, was just 8° of the gage or in- 

 termediate thermometer m ; whence 1 of these degrees, according to this experi- 

 ment, contains just 20°i of Fahrenheit's. The operation was many times re- 

 peated, and the result was always precisely the same. 



For the boiling heat of mercury, it was necessary to proceed in a different 

 manner ; not to convey the heat from the mercury to the instrument, but to 

 convey it equally to them both from another body. I made a small vessel for 

 holding the mercury in the gage itself, seen at i fig. 14, and more distinctly in 

 fig. 15, which is a transverse section of the gage through this vessel. The plate 

 cd, which forms the bottom of the canal, serves also for the bottom of the 

 vessel, which is situated close to the side of the canal, and as near as could be 

 to that part of it, in which both the silver piece, and the divisions required for 

 this particular experiment, are contained. By this arrangement it is presumed, 

 that all the parts concerned in the operation will receive very nearly an equal heat. 

 The gage, with some mercury in the vessel, was laid on a smooth and level bed 

 of sand, on the bottom of an iron muffle kept open at one end ; the fire in- 

 creased very gradually till the mercury boiled, and then continued steady, so as 

 just to keep it boiling, for a considerable time. The boiling heat of mercury 

 was thus found to be 27°4- of the intermediate thermometer, which answering to 

 an interval of 550° of Fahrenheit, makes 1 degree of this equal to just 20° of 

 his ; a result corresponding even beyond my expectations with that which boiling 

 water had given. 



These standard heats of Fahrenheit's thermometer are obtained with little 

 difficulty on a common fire ; but it is far otherwise with the higher ones in which 

 mine begins to apply ; and all the precautions I could take, by using a close 

 muffle, surrounding it as equally as possible with the fuel, varying its position 

 with respect to the draught of air, &c. proved insufficient for securing the 

 necessary equality of heat even through the small space concerned in these ex- 

 periments. Nor had I any idea, before the discovery of this thermometer, of 



