THERMOMETER. 
EcsrATiON OF The boiling point. 
Barometer. 
Equation. 
Difference. 
31.0 
-f 1.57 
0.78 
30.5 
- 4 - 0.79 
0.79 
30.0 
0.00 
0.80 
29.5 
— 0.80 
0.82 
29.0 
— 1.62 
0,83 
28.5 
— 2.45 
0.85 
28.0 
— 3.31 
0.86 
27.5 
— 4.16 
0.88 
27.0 
— 5.04 
The numtiers in the first column of this 
table express heights of the quicksilver in 
the barometer, in English inches and deci- 
mal parts : the second column shows the 
equation to be applied, according to the 
sign prefixed, to 212" of Bird’s Fahrenheit, 
to find the true boiling point for every such 
state of the barometer. The boiling point, 
for all intermediate states of the barometer, 
may be had, with sufficient accuracy, by 
taking proportional parts, by means of the 
third column of differences of the equa- 
tions. 
The method of constructing Fahrenheit’s 
thermometer, which is now in general use 
in this country, is the following: a small 
ball is blown on the end of a glass tube, of 
an uniform width throughout. The ball and 
part of the tube are then to be filled with 
quicksilver, which has been previously boil- 
ed to expel the air. The open end of the 
tube is then to be hermetically sealed. The 
next object is to construct the scale. It is 
found, by experiment, that melting snow, 
or freezing water, is always at the same 
temperature. If, therefore, a thermometer 
be immersed in the one or the other, the 
quicksilver will always stand at the same 
point. It has been observed, too, that wa- 
ter boils under the same pressure of the at- 
mosphere at the same temperature. A ther- 
mometer, therefore, immersed in boiling 
water, will uniformly stand at the same 
point. Here, then, are two fixed points, 
from which a scale may be constructed, by 
dividing the, intermediate space into equal 
parts, and carrying the same divisions as 
far above and below the two fixed points 
as may be wanted. Tims, thermometers 
constructed in this way may be compared 
together ; for if they are accurately made, 
and placed in the same temperature, they 
will always point to the same degree on the 
scale. The fluid, as we have seen, em- 
ployed is quicksilver, and it is found to an- 
swer best, because its expansions are most 
equable. The freezing point of Fahren- 
heit’s thermometer is marked 32", and the 
/ 
reason of this is said to have been, that this 
artist thought that he had produced the 
greatest degree of cold, by a mixture of 
snow and salt ; and the point at which the 
thermometer then stood, in this tempera- 
ture, was marked Zero. The boiling point, 
in this thermometer, is 212", and the inter- 
mediate space, between the boiling and 
freezing points,- is therefore divided into 
180". This is the thermometer that is com- 
monly used in Britain. 
There are three other thermometers em- 
ployed in different countries of Europe, 
which differ from each other in the number 
of degrees between the freezing and boiling 
points. Reaumur’s thermometer was gene- 
rally used in France before the revolution, 
and is still employed in different countries 
on the Continent. The freezing point, in 
this thermometer, is marked Zero, and the 
boiling point 80°. To convert the degrees 
of Reaumur’s thermometer to those of 
Fahrenheit, the following is the formula. 
X 9 
Reanm. — — [- 32 = Fahr. that is, multi- 
4 
ply the degrees of Reaumur by 9, divide by 
4, and add 32. This gives the corre- 
sponding degress on Fahrenheit’s scale. 
The thermometer of Celsus has the 
space between the freezing and boiling 
points divided into 100°. The boiling 
point is 100°, and the freezing point Zero. 
This thermometer is used in Sweden. 
The “ thermometre centigrade,” now used 
in France, has the scale divided in the 
same way. To convert the degrees of this 
thermometer into those of Fahrenheit ; 
Cel. -j- 32 = Fahr. In Delisle’s ther- 
mometer, which is used in Russia, the space 
between the boiling and freezing points is 
divided into 150° ; but the degrees are reck- 
oned downwards. The boiling point is 
marked Zero, and the freezing point 150°. 
To reduce the degrees of this thermometer 
under the boiling point to those of Fahren- 
heit ; Del. — 212 = Fahr. And above 
o 
X 6 
the boiling point. Del. + 212 = Fahr. 
b ' 
Such, then, are the principles and mode 
of construction of the thermometer ; an in- 
strument which has been of (he utmost im- 
portance in enabling ns to discover many of 
the properties and effects of caloric, as by 
it only we can ascertain, with accuracy, the 
relative temperatures. 
, In meteorological observations, it is ne- 
cessai 7 to attend to the greatest rise and 
