Professor Adams on the Theory of the Thermometer. 127 
A mercurial thermometer, in the present improved state 
of thatinstrument, must be constructed according to the a9 
lowing facts. Ist. The tube in which the mercury is 
range between its extreme points, must be divided into ak 
tions of equal capacity ; since the bore of no tube is equal 
in all its parts. Such a division may be most convenient- 
ly made by means of an instrument invented by M. 
Lussac.* 2d. It must be as free as possible from air and 
vapours, by being hermetically seated under the most fa- 
vourable circumstances. It is not de esigned here to enter 
into the details of the practical construction of the ther- 
mometer. 3d. When it is wished to render the instrument 
very sensible, the bulb should be cylindrical or ira 
otherwise, the bulb may be spherical. 4th. must be 
at least two fixed points; viz. that of meltin pies freez- 
ing water) and that of boiling water. 5th. The water used 
in determining the fixed points, must be distilled, and must 
be boiled in a metallic vessel.t 6th. The boiling point 
for water must be determined, when the barometer, after be- 
ing reduced to the level of the sea, and to the souepuieranes “4 
melting ice, stands at the height of thirty inches. If, aft 
the reductions above mentioned are made, the bein: 
ter is not atthirty inches, the boiling — must be seer 
ed by making an allowance at the rate of 1° Fah. for a dif- 
ference of 0.589 of an inch of shied pressu e.* 
It admits of mathematical demonstration, that disdain: 
eters constructed with the ver eneg above specified, are 
strictly comparable with each othe 
tis said above, that the ecaalione viewed as a meas- 
urer of temperature, is, like other standards of measure- 
ment, an instrument of a conventional nature. The length 
of the English — was adjusted by the length of the arm 
of king Henry I . and the original metallic rod is preserved 
* Bict, Traité de Physique, Tom. I. 46—8, 
+ Bict, I. 43. 
* The ex See = contraction of the glass in aoa ing is at least 
theoretically speaking, a cause of error in ‘their r sults. Butas this cause 
of error is common ie ll thermometers, and affec ts them all in very nearly 
88 same degree, and is itself ——s a Lagpie v= effect will be scarcely dis- 
rnible. It would not be diffieu it of any use in practice, to give a 
farmula for the correctien of this ser ae, 
