THE THERMOMETER AND PYROMETER. 



THE ancients were unacquainted with 

 any more certain mode of marking 

 the variations of temperature, than 

 the indications of the senses, and the 

 limited knowledge derived from ob- 

 serving the melting or combustion of 

 different substances. In modern times, 

 .instruments have been invented for 

 noting variable degrees of heat and 

 cold, which, under the designation of 

 thermometers, or thermoscopcs, py- 

 rometers, or pyroscopes, are now in 

 general use in every part of the civi- 

 lized world. Their names are derived 

 from the Greek terms ttyus, M?, sig- 

 nifying heat, fire, and pir^ov, trxoveos, a 

 measure, an investigator. 



The principle on which all such in- 

 struments are constructed, is the change 

 of bulk which every body undergoes by 

 alteration of its temperature. 



All homogeneous bodies, except wa- 

 ter, within a few degrees of its freezing 

 point, expand by heat and contract by 

 cold.* Their expansion, then, may 

 afford a relative, measure of the in- 

 crease of temperature ; and their 

 contraction, of its diminution. This 

 law holds good in gases, liquids, and 

 solids ; and, accordingly, matter in those 

 three states of existence has been em- 

 ployed in the construction of instru- 

 ments for measuring the intensity of 

 heat and cold. 



The changes of volume which gases 

 or aeriform bodies undergo, were first 

 employed for this purpose; liquids, 

 such as spirit of wine, oils, or mercury 

 were next used ; and lastly, the 

 changes in the bulk of solids were ap- 

 plied to measure the variations of 

 higher temperatures, which would have 

 too much expanded gaseous and liquid 

 bodies. 



The designation of thermoscope or 

 pyroscope might be, with most pro- 

 priety, applied to such instruments ; 

 but, in conformity to common usage, it 

 is proposed in this treatise to apply the 



* Clay, a seeming exception, is not a homogeneous 

 substance, of which afterwards. 



general term thermometer to the in- 

 struments depending on the expan- 

 sions of aeriform and liquid bodies, 

 and pyrometer to those in which the 

 expansion of solids is the measure of 

 the elevation of temperature; and the 

 subject will be treated under the fol- 

 lowing heads. 



I. Of the common Thermometer. 



1 . Its history and construction. 



2. The precautions necessary in its 

 construction and graduation. 



II. Of the Pyrometer. 



III. Of Register Thermometers. 



IV. Of the Differential Thermometer, 

 and its modifications. 



V. Of some peculiar applications of 

 the Thermometer. 



VI. Of the imperfections common to 

 all instruments for the indication of 

 heat. 



CHAPTER I. 

 Of the Common Thermometer. 



1. History and Construction of the 

 Thermometer. 



THE invention of the thermometer, like 

 almost every other discovery of great 

 utility, has been claimed for different 

 philosophers ; and national vanity has 

 occasionally been enlisted in sup- 

 port of the pretensions of rival claim- 

 ants. There seem, however, but two 

 whose titles are worthy of notice. 



The Italian writers generally give the 

 honour to their countryman Santorio 

 Saniorio, long a physician at Venice, 

 and afterwards a professor at Padua, 

 who flourished about the beginning of 

 the seventeenth century ; and who had 

 obtained just celebrity by his discovery 

 of the insensible perspiration of the 

 animal frame : the Dutch philosophers 

 as unhesitatingly ascribe it to Cornelius 

 Drebbel, a physician of Alkmaar, who 

 appears to have enjoyed a high reputa- 

 tion as a chemist, a mathematician, 

 and an inventive mechanical genius. 



