THERMOMETER. 



595 



Ing, since they contain descriptions and curious 

 wood cuts of the various forms of the thermometer 

 which he proposed for medical purposes; and the 

 extent to which he carried it in other matters, is 

 shown by his attempt described and figured at folio 

 22 of that work, to compare the heat of the sun's 

 and moon's rays, an attempt which afterwards 

 became one of the most delicate in natural philo- 

 sophy. Perhaps Sanctorius and Drebbel invented 

 much about the same time this instrument, a cir- 

 cumstance by no means improbable considering the 

 then advancing state of physical science. Be this 

 as it may, the instrument contrived by each was 

 precisely similar, in which air was employed as 

 the expanding substance, which, in several respects, 

 is remarkably well fitted for this application. 



The air thermometer, as shown in Plate LXXXIX. 

 fig. 9, consisted of a ball of glass A, with a stem B, 

 nearly filled with air, but having <he lower part of 

 the tube occupied by a coloured liquor, which, 

 when the air in A was expanded, was forced into 

 the recipient C, and hence a scale being applied to 

 the stem, the dilatation of the inclosed air by heat 

 was marked by the descent of the coloured fluid. 

 In this form the instrument was obviously unfit for 

 trying with any ease the temperature of fluids. 

 Boyle, therefore, subsequently proposed to include 

 both the air and the coloured fluid in one bulb, as 

 shown at Fig. 10, where the tube AB, which is 

 open at top, reaches below the surface of the fluid 

 nearly to the bottom of the receptacle C, into the 

 neck of which the tube is hermetically sealed. 

 The same philosopher, however, demonstrated the 

 grand defect of the air thermometer, that by a 

 change of pressure in the atmosphere, as shown by 

 the barometer, the elasticity of the inclosed air is 

 altered independently of temperature, so as to 

 render the indications of the same instrument not 

 comparable at different times. The air thermometer 

 was subsequently modified by the ingenious Hooke, 

 in order to act as a barometer, which it obviously 

 does, if the effects of temperature be corrected, and 

 those of pressure alone shown, just as on the other 

 hand, if the result of variable pressure were neutra- 

 lized, that of temperature would be truly expressed. 

 Hooke attached a mercurial thermometer to the 

 original instrument, the temperature of which thus 

 indicated gave the data for separating the influence 

 of dilatation caused by heat, which was performed 

 by means of a sliding scale. By this elegant modi- 

 fication, Hooke converted the air thermometer into 

 a marine barometer, which, however, was soon 

 abandoned, from the absorption that was found to 

 take place of the excluded air by the coloured fluid. 

 This defect has more lately been in a great measure 

 remedied by the substitution of hydrogen gas instead 

 of the common included air, by Mr Adie, who has 

 revived this instrument under a very elegant and 

 portable form, and under the name of the sympieso- 

 meter. 



The defects of the air thermometer having been 

 duly appreciated by the Florentine Academia del 

 Cimento, that enterprising body published, in the 

 first volume of their Transactions, a description of 

 a new thermometer, in which spirit of wine was 

 used as the expanding substance, which, as it might 

 be hermetically sealed up in a glass tube or bulb, 

 was free from any defect arising from pressure, as 

 well as the possibility of any loss of fluid by 

 evaporation. This instrument was constructed 

 much in the same way as at present, the spirit 

 being dilated till it filled the whole tube, when it 



was quickly sealed, and on cooling, the fluid retired, 

 leaving nearly a vacuum above it. The great 

 defect of the Florentine weather glass, as it was 

 commonly called, was the want of any fixed scale 

 of graduation, on which account no instrument, 

 except those graduated by the original one of the 

 academy, could be comparable with any other, the 

 only direction being that the cold of ice and snow 

 should make it stand at 20 deg. and the greatest 

 summer heats at Florence, at 80 degrees. The 

 spirit thermometer was faulty in several other 

 respects, yet it cannot but be thought fortunate 

 that this fluid, which is esteemed the second best 

 for filling thermometers, should have been so early 

 thought of. 



We now come to mention the greatest improve- 

 ment made upon the thermometer since the period 

 of its invention, the practical introduction of 

 mercury as the expanding fluid. This practical 

 discovery appears, as far as the testimony of con- 

 temporary writers can guide us, to be due to Olans 

 Romer, the ingenious discoverer of the progressive 

 motion of light, who, according io Boerhaave, like- 

 wise proposed the scale now known under the name 

 of Fahrenheit's, and so early as the year 1709, ob- 

 served the mercury to sink by a natural cold to the 

 zero of that scale ; but it is more commonly con- 

 ceived that Fahrenheit himself made that observa- 

 tion, and founded his scale upon it, which was 

 contrived in the year 1720, and described to the 

 Royal Society of London in 1724. From this 

 period the thermometer became of scientific utility, 

 and its indications being founded upon fixed prin- 

 ciples, gave the assurance that all instruments made 

 with equal care would he strictly comparable. 

 There is even now, after the lapse of a century, no 

 prospect of materially improving the plan of the 

 simple thermometer, or of finding any substance 

 fitter than mercury for forming a scale of heat. In 

 fact, it unites the most important qualifications. 



Mercurial thermometers are usually made in 

 this country in a style resembling Fig. 12, the 

 scale being of ivory, and sometimes the bulb and 

 stem project below from the scale for the purpose 

 of greater delicacy ; the bulb, too, is then generally 

 made elongated like the instrument of Fahrenheit. 

 The scale adopted by this philosopher was rather 

 an arbitrary one, and the motive of its adoption we 

 have not been able quite satisfactorily to discover. 



We shall now notice the principal modifications 

 of the simple thermometer; the most interesting of 

 these are termed self-registering, which, by inspec- 

 tion, denote the greatest heat or cold which has 

 occurred since the last observation, or else are em- 

 ployed to note the temperature at any moment in 

 the absence of the observer. Of the former kind, 

 the first, though Bernouilli seems to have the 

 merit of originating the idea, is the thermometer of 

 lord Charles Cavendish. For description, see 

 Phil. Trans., 1757. 



The thermometer which succeeded those of 

 Cavendish was that of Mr Six, described in the 

 Philosophical Transactions for 1782. 



This instrument is not a little complex in. its 

 parts, having two contacts of spirit and mercury, 

 and two indices, all which are detrimental to the 

 practical accuracy of the thermometer. On this 

 account the instrument has usually been made on a 

 large scale, to prevent derangement in working. 

 Mr Six made the bulb no less than sixteen inches 

 long, and half an inch diameter, while the tube bad 

 a bore of .fo inch. But this enormously clumi-y 

 2 p 2 



