THERMOMETER. 
Lose of rendering it more visible ; hence 
hey are denominated the mercurial tlier- 
nometer, and the spirit thermometer. Other 
[aids, on account of their clamminess, or of 
heir great irregularity of expansion, are not 
jiseful for thermometers, 
j The most 'proper and the most useful 
Jhape for thermometers, is that of a long 
tube with a narrow bore, and with a globular 
cavity at one extremity (see Plate Miscel. 
fig. 237.). The cavity of the bulb C, and 
part of the tube, as far, for instance, as A, 
are tilled with the fluid : the rest of the tube is 
either partly, or quite, exhausted of air ; and 
the end B of the tube is hermetically sealed ; 
viz. perfectly closed by melting the extremity 
Cf the tube at the flame of a candle or lamp, 
iursml by means of a blowpipe. 
When the bulb C is heated, tiie mercury, 
or the spirit of wine, is expanded ; and not 
being able to extend itseli any other way, 
all the increment of bulk is manifested in the 
tube, viz. the surface A of the fluid will rise 
considerably into the tube. On the other 
hand, when the bulb C is cooled, the fluid 
contracts, and its surface A descends. It is 
evident, that, caderis paribus, the larger the 
bulb is, in proportion to the diameter of the 
cavity of the tube, or the narrower the latter 
is in proportion to the former, the greater 
will the motion of the surface A be in the 
tube. But it must be observed, that when 
the bulb is very large, the thermometer will 
1 l-i ot easily arrive at the precise temperature 
of any place, wherein it may be situated. 
Rome* persons, in order to give the bulb a 
greater surface, and of course to render it 
more capable of readily attaining a given 
temperature, have made it not globular, but 
cylindrical (which shape was adopted by 
Fahrenheit), or flat, or bell-like, &c.; but 
those shapes are improper, because they are 
liable to be altered by the varying gravity of 
j the atmosphere, consequently those ther- 
mometers cannot be accurate. 
If a thermometer is heated suddenly, as 
when the bulb C is immersed in hot water, 
the surface A of the fluid in it will be seen to 
descend a little, and instantly after will be 
seen to rise ; the reason of which is, that the 
j heat of the water enlarges the glass first, and 
is then communicated to the fluid, &c. On 
the contrary, if the bulb of a thermometer 
is cooled suddenly, the surface A of the fluid 
will tirst rise a little, and then will descend ; 
because the cold contracts the glass alone at 
first, and afterwards contracts the fluid. 
Ice is melted by a certain invariable degree 
of temperature ; and water freezes at about 
the same temperature ; therefore, if the bulb 
C of a mercurial thermometer is placed in 
melting ice, or melting snow, and a mark is 
made on the outside of the tube, even with 
the surface of the fluid, as at D; that mark 
is called the freezing-point, though in tact it 
is the melting-point of ice ; the freezing- 
point of water being not so constant. If the 
bulb of the thermometer is placed in boiling 
water, and a mark is made on the glass tube, 
even with the surface of the fluid within, as at 
. E, that mark is called tfie boiling-point ; for 
in an open vessel, and under the same at- 
mospherical pressure, which is indicated by 
the barometer, water constantly boils at 
the same temperature, and an increased fire 
■will force it to evaporate faster, but will not 
raise its temperature. Those points being 
VOL. II. 
ascertained, if the length of the tube from D 
to E is divided into any number of equal 
parts, those parts will be the degrees of the 
thermometer, or the degrees of heat, indi- 
cated by the corresponding expansions of the 
fluid within the thermometer. And the same 
degrees, or equal divisions, may be continued 
belotv D and above E, in order to shew the 
degrees of temperature below the freezing, 
and above the boiling, point. 
Those two unalterable points of tempe- 
rature, viz. the former where ice becomes 
water, and the second where water becomes 
vapour, have been universally adopted by 
the various constructors of thermometers for 
the graduation of those instruments ; but the 
space between them has been divided dif- 
ferently by different persons, and this differ- 
ence gives the different names of thermome- 
ters, or rather of their graduations ; such as 
Reaumur’s thermometer, Fahrenheit’s ther- 
mometer, &c. Reaumur divides the space 
between the abovementioned two points, into 
80 equal parts or degrees ; placing the 0 
at freezing, and the 80th degree at the boil- 
ing point. Fahrenheit divides it into 180 
degrees or equal parts, but he places the 0 
thirty-two degrees below the freezing-point 
D; so that the freezing-point is at 32, and 
the boiling-point E is at 212 degrees. 
Other persons have adopted other divi- 
sions, which have been suggested by supposed 
advantages or fanciful ideas. 
Most of those graduations are at present 
out of use, but they are to be met with in 
various, not very recent, publications ; we 
have therefore thought it necessary to set them 
down in the following table, which contains : 
1st. The name of the person or society that 
has used each particular division ; 2dly. The 
degree which lias been placed, by each of 
them, against the freezing-point; 3dly. The 
degree which lias been placed against the 
boiling-point ; and, 4thly. The number of 
degrees lying between those two points. 
Freez- 
Boil- 
fl 8x0 
9 G 
QJ . 
> -O <2 
o o a 
Xl £ o 
mg- 
ing- 
bp fl o 
point. 
point. 
p-5 B 
Fahrenheit’s, which is 
generally used in Great 
Britain. It is also used 
throughout this work, un- 
less some other is men- 
tioned - - - 
32 
212 
130 
Reaumur’s, which is ge- 
nerally used in France 
and other parts of the 
Continent 
O 
80 
80 
Celsius’s, which has 
been used chiefly in Swe- 
den, hence it is also called 
the Swedish thermometer. 
It has been lately adopted 
bv the French chemists, 
under the name of cen- 
tigrade thermometer 
The Florentine ther- 
mometers, which were 
made and used by the 
members of the famous 
academy del' Cimento, being 
some of the first instru- 
ments of the sort, were 
vaguely graduated, some 
having a great many more 
0 
100 
100 
degrees than others. But 
two of their most common 
C20 
graduations seem to be 
) lfji 
The Parisian thermo- 
C 1 
meter, viz. the ancicnnc 
thermomeire of the Acade- 
my of Sciences, seems to 
havebeen graduated near- 
ly thus, 
' De la Hire's thermo- 
meter, which stood in the 
25 
observatory at Paris a- 
bove 60 years, was gra- 
duated thus, 
28 
Amanton’s 
51} 
Poleni’s - - - 
I)e L’Isle’s thermome- 
47 JL 
i O 
ter is graduated in an in- 
verted order 
150 
Sir Isaac Newton’s - 
0 
Hales’s - - - 
The Edinburgh ther- 
0 
mometer, formerly used, 
seems to have been gra- 
duated thus, 
8} 
174 
812 
239 
199} 
73 
62 9 
i a 
(?) 
34 
163 
47 
154 
68 * 
214 
171} 
21 } 
15 * 
T4> 
1.50 
34 
163 
38-S 
These are the chief thermometers that have 
been used in Europe; and the temperatures in- 
dicated by the principal of them may l>e redu- 
ced into the corresponding degrees on any of 
the others, by means of the following simple; 
theorems ; in which R signifies the degrees on 
the scale of Reaumur, F those of Fahrenheit, 
and S those of the Swedish thermometer. 
1. To convert the degrees of Reaumur int» 
those of Fahrenheit ; * ■- -{- 32 == F. 
4 
2. To convert the degrees of Fahrenheit into 
those of Reaumur ; 
( F 
32 ) X 4 R 
9 
3. To convert the Swedish degrees into those 
of Fahrenheit ; — -4- 32 r= F. 
5 
4. To convert Fahrenheit’s into Swedish ; 
(f - 32) X 3 
9 
5. To conveit Swedish degrees into those of 
Reaumur ; 
s X 4 
. R. 
6. To convert Reaumur's degrees into Swe- 
dish ; E'li — s. 
To such readers as are unacquainted with the 
algebraic expression of arithmetical formula:, it 
will be sufficient to express one or two of these 
in words, to explain their use: 1. Multiply the 
degree of Reaumur by 9, divide the product 
by 4, and to the quotient add 32, the sum ex- 
presses the degree on the scale of Fahrenheit. 
2. From the degree ot Fahrenheit subtract 32, 
multiply the remainder by 4, and divide the 
product by 9, the quotient is the degree accord- 
ing to the scale of Reaumur, &c. 
Thermometers have been made of a great 
variety of shapes and sizes, suitable to the 
different purposes for which they were in- 
tended. 
Thermometers for shewing the tempera- 
ture of the atmosphere, need not have their 
scales much extended ; it is more than suf- 
ficient if they go as high as 120’. The lower 
degrees may be carried down as low as may 
be necessary for the cold of any particular 
climate. The mercurial thermometer needs 
not to be graduated lower than 40° below 0, 
because at about that degree mercury ceases 
to be a fluid. 
