THERMOMETER AND PYROMETER. 



Calendarium ; and the common air 

 thermometer is repeatedly figured in 

 his singular work, De Philosophia Moy- 

 siaca* with its stem equally divided 

 into an ascending and descending series, 

 each of 7 degrees, respectively appro- 

 priated to winter and to summer. It is 

 obvious, that the size of an air thermo- 

 meter, on such principles, is only limited 

 by convenience, and the length of the co- 

 lumn of liquid which the pressure of 

 the atmosphere can sustain in the tube. 

 As originally made, they were unwieldy, 

 they could not be applied to high tem- 

 peratures, and were, besides, liable to 

 two very important objections, as indi- 

 cators of the atmospheric changes of 

 temperature, they were liable to be 

 affected not only by heat and cold, but 

 by the varying pressure of the atmo- 

 sphere ; and the scales adapted to 

 them were arbitrary, and without, fixed 

 points for the comparison of observa- 

 tions made with different instruments. 



The first objection was foreseen and 

 obviated by the scientific members of 

 the Florentine academy del Cimento, 

 assembled under the auspices and pa- 

 tronage of Fernando II., Grand Duke 

 of Tuscany. In the first article in the 

 published transactions of that learned 

 body.f we find a full description and 

 delineation of a thermometer from 

 which the influence of atmospheric 

 pressure is excluded. The expansion 

 of spirit of wine is employed to ascer- 

 tain the temperature, instead of the 

 dilatation of air ; and the instrument is 

 sealed hermetically, as it is termed, or 

 has its orifice closed by melting the 

 glass, after the introduction of as much 

 spirit as fills the bulb and a portion of 

 the stem. The method employed by 

 the Florentine academicians is nearly 

 that still used by the makers of the 

 instrument ; namely, by heating the 

 bulb in the flame of a lamp, to expel 

 the air, and then immersing the open 

 end of the tube in the liquid destined to 

 fill the thermometer. As the ball cools, 

 the atmospheric pressure forces the 

 liquid into the stem and ball, to supply 

 the vacuum ; and the orifice is closed 

 by melting with the blowpipe the end 

 of the tube, from which any excess of 

 the liquid may be previously expelled 

 by again heating the ball. (Fig. 3.) 



The Florentine academicians appear 

 also to have been aware of the neces- 



* Folio, Goydae, 1633. 



f Saggi di Natural! Esperienze. 



sity of adapting some fixed Jftgv, 3 

 scale to the tube ; but their 

 attempts were not very suc- 

 cessful. They described the 

 thermometer as consisting of 

 a ball and tube of such rela- 

 tive size, " that on filling it 

 to a certain mark of its neck 

 with spirit, the cold of snow 

 and ice will not cause it to fall 

 below 20 degrees measured on 

 the stem; nor, on the other 

 hand, the greatest heat of 

 summer expand it more than 

 80 degrees."* This method 

 is undoubtedly erroneous, in- 

 asmuch as the last point could 

 be of no determinate tem- 

 perature; and their method 

 of graduation is in itself 

 rather rude. The tube is 

 directed to be divided by 

 compasses into ten equal parts, these 

 divisions are to be marked " by a little 

 button of white enamel ; and these may 

 be further subdivided by the eye, and 

 the intermediate degrees marked by 

 buttons of glass, or of black enamel." 



This instrument was variously modi- 

 fied by them to suit different purposes. 

 The ball was occasionally enlarged, and 

 the tube reduced in thickness to render 

 the instrument more sensible ; and in 

 the work already quoted, we find a 

 figure of a thermometer of this sort, 

 with the stem spirally twisted to render 

 it more portable, and less liable to 

 accident. 



Another invention of those philoso- 

 phers to indicate changes of tempera- 

 ture may be here noticed. It consisted 

 of hermetically sealed spherules of glass, 

 of different specific gravities, introduced 

 into a wide tube filled with pure spirit. 

 The degree of the Florentine thermome- 

 ter at which each sank was noted, and by 

 hanging this instrument in an apart- 

 ment, it somewhat slowly showed the 

 variations of the temperature of the 

 surrounding air.f Imperfect as these 

 attempts were, they paved the way to 

 very important improvements in ther- 

 mometers. 



The indefatigable Boyle appears early 

 to have turned his attention to the im- 

 provement of the thermometer, and his 

 first attempts were on the air thermo- 

 meter, or the weather-glass, as it was 

 then styled. He rendered the instru- 



* Saggi di Natural! Esperienze, p. 4, 

 f Saggi, p. 10. 



B2 



