its heat so quickly as mercury. Alcohol 
docs not expand much by heat. In its or- 
dinary state it does not bear a much greater 
heat than 175“ of Fahrenheit ; but when 
highly rectified it can bear a greater de- 
gree of cold than any other liquor hither- 
to employed as a measure of temperature. 
At Hudson’s Bay, Mr. Macnab, by a mix- 
ture of vitriolic acid and snow, made it to 
descend to 69“ below 0 of Fahrenheit. 
There is an inconvenience, however, at- 
tending the use of this liquor ; it is not pos- 
sible to get it always of the same degree of 
strength. As to oil, its expansion is about 
15 times greater than that of alcohol ; it 
sustains a heat of 600°, and its freezing 
point is so low that it has not been deter- 
mined ; but its viscosity renders it useless. 
Mercury is far superior to alcohol and oil, 
and is much more manageable than air. 
1. As far as the experiments already made 
can determine, it is of all the fluids hitherto 
employed in the construction of thermome- 
ters, that which measures most exactly 
equal differences of heat by equal differ- 
ences of its bulk : its dilatations are, in fact, 
very nearly proportional to the augmenta- 
tions of heat applied to it. 2. Of all liquids 
it is the most easily freed from air. 3. It 
THERMOMETER. 
pin’s digester, it will acquire a heat of 412“. 
Hence it appears, that water will boil at a 
lower point, according to its height in the 
atmosphere, or to the weight of the column 
of air which presses upon it. In order to 
ensure uniformity, therefore, in the con- 
struction of tliermometers, it is now agreed, 
that the bulb of the tube be plunged in the 
water when it boils violently, the barome- 
ter standing at 30 English inches, and the 
temperature of the atmosphere 55“. A 
thermometer made in this way, with its 
boiling point at 212“, is called, by Dr. Hors- 
ley, “ Bird’s Fahrenheit,” because Mr. Bird 
was the first person who attended to the 
state of the barometer in constructing ther- 
mometers. 
As artists may be often obliged to adjust 
thermometers under very different pres- 
sures of the atmosphere, philosophers have 
been at pains to discover a general rule 
which might be applied on all occasions. 
M. de Luc, from a series of experiments, 
has given an equation for the allowance on 
account of this difference, in Paris measure, 
which has been verified by Sir George 
Sehuckbnrg ; also Dr. Horsley, Dr. Maske- 
lyne, and Sir George Schuckburgh, have 
adapted the equation and rules to English 
fitted to measure high degrees of heat^ measures, and have reduced the allowances 
and cold. It sustains a heat of 600“ of Fah- 
renheit’s scale, and does not congeal till it 
fall 39 or 40 degrees below 0. 4. It is 
the most sensible of any fluid to heat and 
cold, even air not excepted. Count Rum- 
ford found, that mercury was heated from 
the freezing to the boiling point in 58 se- 
conds, while water took 2 minutes 13 se- 
conds, and common air 10 minutes and 17 
seconds. 5. Mercury is a homogeneous 
fluid, and every portion of it is equally di- 
lated or contracted by equal variations of 
heat. Any one thermometer, made of pure 
mercury, is cceteris paribus, possessed of the 
same properties with every other thermo- 
meter made of pure mercury. Its power 
of expansion is indeed about six times less 
than that of spirit of wine, but it is great 
enough to answer most of the purposes for 
which a thermometer is wanted. The fix- 
ed points, which are now universally chosen 
for adjusting thermometers to a scale, and 
to one anotlier, are the boiling and freezing 
water points. The boiling water point, it 
is well known, is not an invariable point, 
but varies some degrees according to the 
weight and temperature of the atmosphere. 
In an exhausted receiver, water will boil 
with a heat of 98“ or 100“ j whereas, in Pa- 
into tables, for the use of the artist. Dr. 
Horsley’s rule, deduced from De Luc’s, is 
this: 
99 
log. z — 92.804 = h. 
Where h denotes the height of a thermo- 
meter plunged in boiling water above the 
point of melting ice, in degrees of Bird’s 
Fahrenheit, and z the height of the barome- 
ter in lOths of an inch. From this rule he 
has computed the following table, for find- 
ing tlie heights to which a good Bird’s Fah- 
renheit will rise, when plunged irr boiling 
water, in all states of the barometer, from 
27 to 31 English inches ; which will serve, 
among other uses, to direct instrument- 
makers in making a true allowance for the 
effect of the variation of the barometer, if 
they should be obliged to finish a thermo- 
meter at a time when the barometer is 
above or below 30 inches ; though it is best 
to fix the boiling point when the barometer 
is at that height. 
