79 * 
The spirit thermometer may be graduated 
lower if necessary. We shall here just mention, 
that, for reasons which will be noticed here- 
after, if a mercurial thermometer and a spirit 
thermometer are both graduated according to 
the above-mentioned directions, the two 
thermometers will not, in their usual indi- 
cations of the same temperatures, point to 
the same degrees. 
The degrees of thermometers may be de- 
lineated on metal, or wood, or paper, or 
ivory, &c. but such substances should be 
preferred for the scales of thermometers, as 
are not apt to be bent or shortened, or other- 
wise altered by the weather, especially when 
the instruments are not defended by a glass 
case, or by a box with a glass face. 
The bulb of the thermometer must be clean 
and colourless; since coloured surfaces are 
apt to be partially heated by a strong light. 
The ball of the thermometer ought not to be 
iu contact with the substance of the scale, 
lest it should be influenced by the tempe- 
rature of that substance. 
Thermometers which are to be situated in 
the open air out of the house, must be at some 
distance (at least a foot) from the wall, and 
where the light of the sun may not fail di- 
rectly upon them. Fig. 233 represents a ther- 
mometer of the most usual shape independ- 
ant of the case. 
For chemical purposes, the bulb and part 
«f the tube of the thermometer, must pro- 
ject some way below the scales, in order that 
they may be placed in liquids, mixtures, See. 
For other purposes, as for botanical obser- 
vations, hot-houses, brewing-manufactories, 
baths, &c. the thermometers- must be made 
longer, or shorter, or narrower, or particular 
directions must be added to the scales, &c. ; 
but we shall not take any farther notice of those 
fluctuating varieties of shape only. 
It is necessary, however, to describe a sort 
of thermometers which have been con- 
structed for a particular purpose; namely, 
for shewing the highest degree of heat or of 
cold which has taken place during the ab- 
sence of the observer ; as for instance, in the 
course of tire night, or in the hottest part of 
the day, or even during a whole season. 
Thermometers for this purpose have been 
contrived differently by various ingenious 
persons, as by Bernouilli, Kroft, lord Charles 
Cavendish, &c. but the best of them, which 
however is not without faults, and of course 
Is in need of improvements, was contrived by 
Mr. James Six, and is described in the 72nd 
vol. of the Philosophical Transactions. Fig. 
239 exhibits this instrument, but divested of 
the scale and frame ; ub is a tube of thin glass, 
about 16 inches long, and Ag. of an inch in 
diameter; c defgh, a smaller tube with the 
inner diameter, about -J_., joined to the larger 
at the upper end b, and bent down, first on 
the left side, and then, after descending two 
inches below ah, upwards again on the right, 
in the several directions cde,fgh, parallel 
fro, and one inch distant from it. On the 
end of the same tube at ft, the inner diameter 
is enlarged to half an inch from h to i, which 
is two inches in length. This glass is filled 
with highly rectified spirit of wine, to within 
haif an inch of the end i, excepting that part 
of the small tube from d to g, which is filled 
wit! i mercury. From a view of the instru- 
ment in this state, it will readily be conceiv- 
ed, that whefi the spirit in the large tube, 
THERMOMETER. 
which is the bulb of the thermometer, is ex- 
panded by heat, the mercury in the small 
tube on the left side will be pressed down, 
and consequently cause that on the right side 
to rise ; on the contrary, when the spirit is 
condensed by cold, the reverse will happen, 
the mercury on the left side will rise as that on 
the right side descends. The scale, there- 
fore, which is Fahrenheit’s, beginning with 0, 
at the top of the left side, has the degrees 
numbered downwards, while that at the right 
side, beginning with 0 at the bottom, ascends. 
The divisions are ascertained, by placing this 
thermometer with a good standard mercurial 
one in water, gradually heating or cooling, 
and marking the divisions of the new scale at 
every 5\ The method of shewing how high 
the mercury had risen in the observer’s ab- 
sence, is effected in the following manner : 
Within the small tube of tire thermometer, 
above the surface of the mercury on either 
side, immersed in the spirit of wine, is placed 
a small index, so fitted as to pass up and down 
as occasion may require; that surface of the 
mercury which rises, carries up the index 
with it, which index does not return with tiie 
mercury when it descends; but, by remain- 
ing fixed, shews distinctly, and very accu- 
rately, how' high the mercury had risen, and 
consequently what degree of heat or cold had 
happened. Fig. 240 represents these in- 
dexes drawn larger than the real ones, to 
render it more distinct ; a is a small glass 
tube of an inch long, hermetically sealed at 
each end, inclosing a piece of ;leel wire, 
nearly of the same length ; at each end c d, 
is fixed a short piece of a tube of black glass, 
of such a diameter as to pass freely up and 
down within the small tube of the thermo- 
meter. The lower end, floating on the sur- 
face of the mercury, is carried up with it 
when it rises ; while the piece at the upper 
end, being of the same diameter, keeps the 
body of the index parallel to the sides of the 
thermometrica! tube. From the upper end 
of the body of the index at 0, is drawn a spring 
of glass to the fineness of a hair, about §■ of an 
inch in length, which being set a little oblique, 
presses lightly against the inner surface of 
tiie tube, and prevents the index from follow- 
ing the mercury when it descends, or being 
moved by tiie spirit passing up or down, or 
by any sudden motion given to the instru- 
ment by the hand or otherwise ; but at the 
same time the pressure is so adjusted, as to 
permit this index to be readily carried up by 
the surface of tire rising mercury, and down- 
wards whenever the instrument is to be recti- 
fied for observation. To prevent tiie spirit 
from evaporating, the tube at the end i is 
closely sealed. 
This instrument in its frame must be secur- 
ed against the wall out of doors, to prevent 
its being shaken by violent winds. “ Towards 
evening,” saysMr. Six, r ‘l usually visit my ther- 
mometer, and-see at one view', by the index on 
the left side, the cold, of the preceding night; 
and by that on tire right, the heat of the day. 
These I minute down, and then apply a small 
magnet to that part of the tube against which 
the indexes rest, and move each of them 
down to the surface of the mercury; thus, 
without heating, cooling, separating, or at all 
disturbing the mercury, or moving the in- 
strument, may this thermometer, without a 
touch, be immediately rectified for another 
observation.” 
Tne common contrivance for a self-register- 
ing thermometer, now sold in most of tiie 
London shops, consists simply of two ther- 
mometers, one mercurial and the other of ak 
cohol (fig. 243), having their stems horizontal!; 
the former has for its index a small bit of magj- 
netical steel wire, and the latter a minute 
thread ot glass, having its two ends ion * cl 
into small knobs by fusion in the flame of a 
candle. 
The magnetical bit of wire lies in the va- 
cant space of the mercurial thermometer, 
and is pushed forward by tiie mercury when- 
ever tiie temperature rises, and pushes that 
fluid against it ; but when the temperature 
falls and the fluid retires, this index is left 
behind, and consequently shews the maxi- 
mum. j iie other index, or bit of glass, lies 
in the lube of the spirit thermometer im- 
mersed in the alcohol : and when the spirit j 
retires by depression of temperature, the iu- j 
dex is carried along with if in apparent con- | 
tact with its interior surface ; but* on increase 
of temperature the spirit goes forward and 
leaves the index, which therefore shews tiie 
minimum of temperature since it was set. 
As these indexes merely lie in the tubes] 
their resistance to motion is altogether incon] 
siderab*e. Tiie steel index is brought to the 
mercury by applying a magnet on the out- 
side of the tube, and the other is duly placed 
at the end ot the column of alcohol by inclin-j 
ing the whole instrument. 
I he operation of this instrument has beeui 
thus explained: When the surface of tiie] 
column of spirit is viewed by a magnifier, J 
it is seen to have the form of a concave he-fj 
misphere, which shews that the liquid is at-1 
traded by the glass. The glass in that place] 
is consequently attracted in the opposite di-| 
rection, by a force equal to that which is sol 
employed in maintaining that concave figure 
and if it was at liberty to move, it would bei 
drawn back till the flat surface was . restored J 
Let us suppose a small stick or piece of glassl 
to be loose within the tube, and to protrude] 
into the vacant space beyond the surface of] 
the alcohol. r l he fluid will be attracted also] 
by the glass, and form a concave between its! 
surface, and that of the bore of the tube. 
But the small interior piece being quite at 
liberty to move, will be drawn towards tiie 
spirit so long as the attractive force possesses 
any activity ; that is, so long as any addition- 
al fluid hangs round the glass; or, in other 
words, until the end of the stick of glass is 
even with the surface. Whence it is seen 
that the small piece of glass will be resisted, 
in any action that may tend to protrude it: 
beyond the surface of the fluid ; and if this 
resistance is greater than the force required 
t° slide it along in the tube (as in fact it is), 
the piece must be slided along as the alcohol 
contracts ; so as always to keep the piece 
within the fluid. And this fact is accordingly 
observed to take place. 
It might at first sight be imagined that equal 
increments of heat would cause fluids to ex- 
pand equably ; viz. that if the heat is increas- 
ed gradually by one degree, two degrees, 
three degrees, & c. the fluid thus heated 
would expand its bulk by a certain quantity, 
then by twice that quantity, three times that 
quantity, and so on. But "this is not the case, 
and every fluid seems to follow a particular 
law ef expansion. 
