138 SCIENTIFIC APPARATUS. 



those of a perfect heat-engine. These will be more appropriately 

 treated in VI. below. 



Practically, temperatures are measured by thermometers ; and 

 there is no difficulty (though there is often considerable trouble) 

 in passing from their indications to the corresponding points of 

 the absolute scale. What is wanted for practical work is some- 

 thing easy to use, and easily reproducible. When great scientific 

 accuracy is required it is necessary to translate the indications of 

 such an instrument into the corresponding absolute temperatures. 



THERMOMETERS. 



It seems now certain that the first inventor of the thermometer 

 was Galileo, before 1597 (see "Memoire sur la Determination de 

 1'Echelle du Thermometre de 1' Academic del Cimento," par G. 

 Libri, Ann. fa Chimie xlv. 1830). His thermometer was an air 

 thermometer, consisting of a bulb with a tube dipping into a 

 vessel of liquid. The first use to which it was applied was to 

 ascertain the temperature of the human body. The patient took 

 the bulb in his mouth, and the air, expanding, forced the liquid 

 down the tube, the liquid descending as the temperature of the 

 bulb rose. From the height at which the liquid finally stood in 

 the tube, the physician could judge whether or not the disease 

 was of the nature of a fever. 



A similar instrument was afterwards used, for a similar purpose, 

 by the physician Sagredo, who, till recently, was regarded as the 

 inventor of the thermometer. 



Air thermometers, however, are affected by changes in the pres- 

 sure of the atmosphere, as well as by changes in the temperature 

 of the enclosed air, and, therefore, unless this disturbing cause is 

 removed or accounted for, the reading of the thermometer is of 

 no value. 



Thermometers, containing a liquid hermetically sealed up in 

 glass, were first made under the direction of Rinieri (died 1647), 



