THE 



THE 



than the most delicate mercurial, or air 

 thermometer. 



THERMOMETER (^ep/in, heat, /ue- 

 Tpov, a measure). Literally, a measurer 

 of heat; an instrument for comparing 

 the degree of active heat existing in 

 other bodies, by its effect in expanding 

 a column of mercury. 



1. Fahrenheit's Thermometer. That 

 arrangement of the scale of the instru- 

 ment, in which the space between the 

 freezing and the boiling points of water, 

 under a medium pressure of the atmo- 

 sphere, is divided into 180 parts, or de- 

 grees, the freezing being marked 32°, 

 and the boiling 212°. This scale was 

 adopted by Fahrenheit, because he sup- 

 posed, erroneously, that 32 of those divi- 

 sions below the freezing point of water 

 (which was therefore on his scale) was 

 the zero, or greatest degree of cold. 



2. Centigrade Thermometer. This Is 

 the thermometer of Celsius, which is 

 used in France, and is the most conve- 

 nient in practice : it consists in that ar- 

 rangement of the scale, in which the 

 freezing point is marked 0, or zero ; and 

 the boiling point 100. 



3. Reaumur's Thermometer. In this 

 scale the freezing point is marked 0, or 

 zero, and the boiling point 80°. The 

 degrees are continued of the same size, 

 below and above these points, those 

 below being reckoned negative. 



4. These different modes of graduation 

 are easily convertible: the Centigrade 

 scale is reduced to that of Fahrenheit 

 by multiplying by nine and dividing by 

 five ; that of Reaumur to that of Fahren- 

 heit by dividing by four instead of five : 

 or that of Fahrenheit to either of these, 

 by reversing the process. Thus — 



C. 100°x9 = 900-h5 = 180 4-32°=212°F. 

 R. 80Ox9 = 720-^4=180 + 32°=212°F. 

 Or, by reversing the order — 

 F. 212°-32 = 180x5 = 900-^9=100° C. 

 F. 212°-32=180x4=?20-i-9= 80° R. 



5. A Table is added, showing the cor- 

 respondence of the three thermometers. 

 Fahrenheit. Centigrade. Reaumur. 



212 100- 80- 



200 ..., 93-33 74-66 



190 87-77 70-22 



180 82-22 6.5-77 



170 76-66 61 33 



160 71-11 56-88 



150 65-55 52 33 



140 60- 48- 



130 55-55 43-55 



120 48-88 39-11 



110 43-33 34-66 



337 



Fahrenheit. Centigrade. Reaumur. 



100 37-77 3022 



90 32 22 25-77 



80 26-66 21-33 



70 2111 16-88 



60 15 55 12-44 



50 10- 8- 



40 4-44 3-55 



32 0- 0- 



20 - 6-66 - 5-33 



10 -12-22 - 9-77 



-17-77 -14-22 



6. Thermometer, Differential. This 

 instrument consists of two glass bulbs, 

 connected by a glass tube bent in the 

 form of the letter U. The tube is partly 

 filled with a coloured liquid. On ex- 

 posing one bulb to heat, the expansion of 

 the air forces the liquid down, and causes 

 it to rise in the opposite part of the tube. 

 This instrument is not intended to indi- 

 cate the temperature of the atmosphere, 

 as the application of cold or heat to both 

 bulbs at the same time produces no alte- 

 ration in the level of the liquid ; it 

 merely indicates the difference of tem- 

 perature between the two bulbs. 



7. Thermometer, Register. An instru- 

 ment by which the highest and lowest 

 temperatures which occur within a given 

 time, are indicated and made to register 

 themselves. It consists of two thermo- 

 meters placed with their tubes in a hori- 

 zontal position. The one contains spirit 

 of wine, the other contains mercury. In 

 the stem of the former a small iron wire, 

 placed at the surface of the mercury, acts 

 the part of an index, being propelled 

 forward as the mercury expands, and 

 being left at the point of greatest expan- 

 sion, when the mercury contracts ; it 

 then indicates the highest temperature 

 which has occurred during an observa- 

 tion. In the stem of the latter a small 

 piece of ivory is immersed in the spirit, 

 and by a slight inclination of the instru- 

 ment is brought to the surface of the 

 liquid. When the temperature falls, the 

 ivory is carried back with the spirit ; but, 

 when the temperature rises, the spirit 

 only advances, the ivory being left be- 

 hind, thus indicating the lowest tem- 

 perature which has occurred. By in- 

 verting the thermometer, the ivory is 

 brought again to the surface for a new 

 observation. 



8. Thermometer, Magnetic. An appa- 

 ratus for employing the thermo-electric 

 current as a measurer of temperature in 

 cases in which other instruments are not 

 sufficiently delicate ; or in cases in which 

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