ON THE MEASURES AND THK NATURE OF HEAT. 649 



scale beginning from the cold produced by a freezing mixture, which was 

 supposed by Fahrenheit to be the greatest that would ever occur in nature. 



The expansion, which is observed in a mercurial thermometer, is in reality 

 only the difference of the expansions of mercury and of glass; but this cir- 

 cumstance produces no difference in the accuracy of the results. The 

 separate effects of the expansion of glass are, however, sometimes perceptible; 

 tlms, when a thermometer is plunged suddenly into hot water, the glass, 

 being first heated, expands more rapidly than the mercury, and, for a 

 moment, the thermometer falls. This circumstance would perhaps be still 

 more observable in a thermometer of spirit or of water; for an equal bulk of 

 these liquids would be much longer in acquiring tlie temperature of the sur- 

 rounding: medium than a mercurial thermometer. 



'o 



The expansion of elastic fluids affords in some cases a test of heat, which is- 

 very convenient from its great delicacy, and because a very small quantity of 

 heat is sufficient to raise their temperature very considerably. The thermo- 

 meter first invented by Drebel was an air thermometer; but instruments of 

 this kind, when they are subject to the variations of the pressure of the 

 atmosphere as well as to those of its temperature, are properly called 

 manometers, and require, for enabling us to employ them as thermo- 

 meters, a comparison with the barometer; while on the other hand, they may 

 be used as barometers, if the temperature be otherwise ascertained. They 

 are however, very useful even without this comparison, in delicate experi- 

 ments of short duration, since the changes of th« barometer are seldom very 

 rapid; and they may also.be wholly freed from the effects of the pressure of 

 the atmosphere, in various ways. Bernoulli's method consists in- closing 

 the bulb of a common barometer, so as to leave the column of mercury in equili- 

 brium with the air contained in the bulb at its actual temperature, and capable 

 of indicating, by the changes of its height and of its pressure, any subse- 

 quent changes in the temperature of the air, which must afl'ect both its bulk 

 and its elasticity. Mr. Leslie's photometer, or differential thermometer, has 

 some advantagesover this instrument, but it can only be employed where the 

 changes of temperature can be confined to a part only of the instrument. The 

 elasticity of the air contained in the bulb is here counteracted, not by the pres- 

 sure of a column of mercury, but by the elasticity of another portion of ak 



