728 
MECHANICS. 
To obtain'Tome fixed unalterable point by which a de¬ 
termined fcale might be difcovered, to which all thermo¬ 
meters might be accurately adjufted, was the BibjeCt 
which next drew the attention of philofophers. Mr. 
Boyle, who feems at an early period to have ftudied this 
fuhjefl with much anxiety, propofed the freezing of the 
efiential oil of annifeeds as a convenient point for gradu¬ 
ating thermometers; but this opinion he foon laid afide. 
Dr. Hallev next propofed that thermometers fhould be 
graduated in a deep pit under ground, where the tempe¬ 
rature both in winter and fummer is pretty uniform ; and 
that the point to which the fpirit of wine fhould rife in 
Rich a fubterraneous place Ihould be the point from which 
the fcale fhould commence. But this propofal was evi¬ 
dently attended with inch inconveniences, that it was 
foon abandoned. He made experiments on the boiling 
point of water, of mercury, and of fpirit of wine ; and 
he feems rather to give a preference to the fpirit of wine. 
He objected to the freezing- of water as a fixed point, be- 
caufe he thought that it admitted confiderable latitude. 
3. It Teems to have been referved to the all-conquering 
genius of fir Ifaac Newton to determine this important 
point, on which the accuracy and value of the thermo¬ 
meter depends. He chofe, as fixed, thofe points at which 
water freezes and boils; the very points which the expe¬ 
riments of fucceeding philofophers have determined to 
be the moll fixed and convenient. Seniible of the difad- 
vantages of fpirit of wine, he tried another liquor, which 
was homogeneous enough, capable of a confiderable rare¬ 
faction, about 15 times greater than fpirit of wine. This 
was linfeed oil. It has not been obferved to freeze even 
in very great colds, and it bears a heat about four times 
that of water before it boils. With thefe advantages it 
was made ufe of by fir Ifaac Newton, who difcovered by 
it the comparative degree of heat for boiling water, melt¬ 
ing wax, boiling fpirit of wine, and melting tin ; beyond 
which it does not appear that this thermometer was ap¬ 
plied. The method he ufed for adjufting the fcale of this 
nil-thermometer was as follows: Suppofing the bulb, when 
immerfed in thawing fnow, to contain 10,000 parts, he 
found the oil expand by the heat of the human body fo 
as to take up -g^th more fpace, or 10,156 fuch parts; and 
by the heat ot water boiling itrongly 10,725 ; and by 
the heat of melting tin 11,516. So that, reckoning the 
freezing-point as a common limit between heat and cold, 
he began his fcale there, marking it o, and the heat of 
the human body he made 12 0 ; and confequently, the de¬ 
grees of heat being proportional to the degrees of rare¬ 
faction, or 256 : 725 :: 12 : 34, this number 34 will 
exprefs tiie heat of boiling water; and, by the fame rule, 
72 that of melting tin. This thermometer was conftruCted 
in 1701. 
To the application of oil as a meafure of heat.and cold, 
there are infuperable objections. It is fo vifcid, that it 
adheres too ftrongly to the fides of the tube. On this 
account it afcends and defcends too fiov.ly in cafe of 
fudden heat or cold. In a Bidden cold, fo great a por¬ 
tion remains adhering to the fides of the tube after the 
reft has fubfided, that the furface appears lower than the 
correfponding temperature of the air requires. An oil- 
thermometer is therefore not a proper meafure of heat 
and cold. 
All the thermometers hitherto propofed were liable to 
many inconveniences, and could not be confidered as 
exaCt ftandards for pointing out the various degrees of 
temperature. This led Reaumur to attempt a new one, 
an account of which was publilhed in the year 1730 in 
the Memoirs of the Academy of Sciences. This ther¬ 
mometer was made with fpirit of wine. He took a large 
ball and tube, the dimenfions and capacities of which 
were known ; he then graduated the tube, fo that the 
fpace from one divifion to another might contain 1000th 
part of the liquor; the liquor containing xooo parts when 
it ftood at the freezing-point. He adjulled the thermo¬ 
meter to the freezing-point by an artificial congelation 
of water: then, putting the ball of his thermometer and 
part of the tube into boiling water, he obferved whether 
it rofe 80 divifions : if it exceeded thefe, he changed his 
liquor, and by adding water lowered it, till upon trial it 
fhould juft rife 80 divifions ; or if the liquor, being too 
low', fell fhort of 80 divifions, he raifed it by adding rec¬ 
tified fpirit to it. The liquor thus prepared fuited his 
purpofe, and ferved for making a thermometer of any 
lize, wlvofe fcale would agree with his ftandard. 
This thermometer was far from being perfeCt. As the 
bulbs were three or four inches in diameter, the furround¬ 
ing ice would be melted before its temperature could he 
propagated to the whole fpirits in the bulb, and confe¬ 
quently the freezing-point would be marked higher than 
it Ihould be. Dr. Martine accordingly found, that, in- 
ftead of coinciding with the 32d degree of Fahrenheit, it 
correfponded with the 34th, or appoint a little above it. 
Reaumur committed a miltake allb refneCting the boiling- 
point ; for he thought that the fpirit of wine, whether 
weak or ftrong, when immerfed in boiling water, received 
the fame degree of heat with the boiling water. But it 
is well known that highly-rectified fpirit of wine cannot 
be heated much beyond the 1751!'. degree of Fahrenheit, 
while boiling water raifes the quickfilver 37 degrees 
higher. 
4. At length a different fluid was propofed, by which 
thermometers could be made free from moft of the defeats 
hitherto mentioned. This fluid was mercury , and feems 
fir ft to have occurred to Dr. Halley in the laft century 5 
but was not adopted by him on account of its having a 
fmaller degree of expanfibility than the other fluids ufed 
at that time. Boerhaave fays, that the mercurial thermo¬ 
meter was firft conftmCted by Olaus Roemer; but the 
honour of this invention is generally given to Fahrenheit, 
of Amfterdam, who prefented an account of it to the 
Royal Society of London in 1723. 
That we may judge the more accurately of the propriety 
of employing mercury, we will compare its qualities with 
thofe of the fluids already mentioned ; air, alcohol, and 
oil. Air is the moft expanfible fluid, but it does lot re¬ 
ceive nor part with its heat fo quickly as mercury. Al¬ 
cohol does not expand much by heat. In its ordinary 
ftate it does not bear a much greater heat than 175 0 of 
Fahrenheit; but when highly rectified it can bear a greater 
degree of cold than any other liquor hitherto employed 
as a meafure of temperature. At Hudfon’s Bay, Mr. 
Macnab, by a mixture of vitriolic acid and fnow, made it 
defeend to 69 below o of Fahrenheit. There is an incon¬ 
venience, however, attending the ufe of this liquor ; it is 
not pofiible to get it al ways of the fame degree of ftrength. 
As to oil, its expanfion is about 15 times greater than 
that of alcohol; it fultains a heat of 6oo°, and its freez¬ 
ing-point is fo low that it has not been determined ; bat 
its vifeolity renders it ufelefs. Mercury is fuperior to al¬ 
cohol 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 conftruCtion 
of thermometers, that which rneafures moft exactly equal 
differences of heat by equal differences of its bulk : its 
dilatations are in faCt very nearly proportional to the aug¬ 
mentations of heat applied to it. 2. Of all liquids, it is 
the moft eafily freed from air. 3. It is fitted to meafure 
high degrees of heat and cold. It fuftains a heat ol 6oo° 
of Fahrenheit’s fcale, and does not congeal till it falls 39 
or 40 degrees below o. 4. It is the moft fenlible of any 
fluid to heat and cold, even air not excepted. Count 
Rumford found that mercury was heated from the freezing 
to the boiling point in 58 feconds, while water took 2 
minutes 13 feconds, and common air 10 minutes and 17 
feconds. 5. Mercury is a homogeneous fluid, and every 
portion of it is equally dilated or contracted by equal 
variations of heat. Any one thermometer made of pure 
mercury is-, aeteris paribus , pofleffed of the fame properties 
with every other thermometer made of pure mercury. Its 
power of expanfion is indeed about fix times lefs than that 
