THERMOMETER. 
the air-thermometer, but was itself not free 
from objections. The liquor could not 
easily be obtained of the same strength, 
and hence different tubes filled witli it, wlien 
exposed to the same degree of heat, would 
not correspond. Another defect was the 
want of some fixed guide as a standard to 
commence the graduation. Philosophers 
soon saw that some fixed and unalterable 
point must be found, by which all thermo- 
iheters might be accurately adjusted. Dr. 
Halley proposed that thermometers should 
be graduated in a deep pit, where the tem- 
perature in all seasons was nearly the same. 
This however could not generally be prac- 
tised. He thought of the boiling point of 
water, of mercury, and of spirit of wine, 
preferring the latter, on account of the 
freezing of water, not knowing that this was 
fixed and uniform. At length Sir Isaac 
Newton determined this important point, 
on which the accuracy and value of the 
thermometer depends. He chose, as fixed, 
those points at which water freezes and 
boils ; the very points which the experi- 
ments of succeeding philosophers have de- 
termined to be the most fixed and conveni- 
ent. Sensible of the disadvantages of spirit 
of wine, he tried another liquor which was 
homogeneous enough, and capable of a con- 
siderable rarefaction, several times greater 
than sphit of wine. This was linseed 
oil. It has not been observed to freeze 
even in very great colds, and it bears a heat 
very much greater than water before it 
boils. Wkh these advantages it was made 
use of by Sir Isaac Newton, who disco- 
vered by it the comparative degree of heat 
for boiling water, melting wax, boiling spi- 
rit of wine, and melting tin ; beyond which, 
it does not appear that this thermometer 
was applied. The method he used for ad- 
justing the scale of this oil-thermometer was 
as follows : supposing the bulb, when im- 
merged in thawing snow, to contain 10,000 
parts, he found the oil expand by the heat 
of the human body so as to take up one 
thirty-ninth more space, or 10,256 such 
parts ; and by the heat of water boiling 
strongly 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 scale there, marking it 0, 
and the heat of the human body he made 
12“ ; and consequently, the degrees of heat 
being proportional to the degrees of rare- 
faction, or 256 : 725 : : 12 : 34, this number 
34 will express the heat of boiling water ; 
and by the same rule, 72 that of melting tin. 
This thermometer was constructed in 1701, 
To the application of oil as a measure of 
heat and cold, there are insuperable objec- 
tions, It is so viscid, that it adheres too 
strongly to the sides of the tube. On this 
account it ascends and descends too slowly 
in case of a sudden heat or cold. In a sud- 
den cold, so great a portion remains ad- 
hering to the sides of the tube after the rest 
has subsided, that the surface appears 
lower than the corresponding temperature 
of the air requires. An oil thermometer is 
therefore not a proper measure of heat and 
cold. All the thermorueters hitherto pro- 
posed were liable to many inconveniences, 
and could not be considered as exact 
standards for pointing out the various de- 
grees of temperature. This led Reaumur 
to attempt a new one, an account of which 
was published in the year 1730 in the Me- 
moirs of the Academy of Sciences. This 
thermometer was made with spirit of wine. 
He took a large ball and tube, the dimen- 
sions and capacities of which were known ; 
he then graduated the tube, so that the 
space from one division to another might 
contain l,000tb part of the liquor; the li- 
quor containing 1,000 parts when it stoodat 
the freezing point. He adjusted the ther- 
mometer to the freezing point by an artifi- 
cial congelation of water; then putting the 
ball of his thermometer and part of the tube 
into boiling water, he observed whether it 
rose 80 divisions j if it exceeded tliese, he 
changed h's liquor, and by adding water 
lowered it, till upon trial it should just rise 
80 divisions ; or if the liquor, being too low, 
fell short of 80 divisions, he raised it by 
adding rectified spirit to it. The liquor 
thus prepared suited his purpose, and served 
for making a thermometer of any size, whose 
scale would agree with his standard. At 
length a different fluid was proposed, by 
which thermometers could be made free 
from most of the defects hitherto men- 
tioned. This fluid was mercury, and seems 
first to have occurred to Dr. Halley, but 
was not adopted by him on account of its 
having a smaller degree of expansibility 
than the other fluids used at that time. 
The honour of this invention is generally 
given to Fahrenheit of Amsterdam, who pre- 
sen ted an account of it to the Royal Society of 
London in 1724. That we mayjudge the more 
accurately of the propriety of employing 
mercury, we will compare its qualities with 
those of the fluids already mentioned, air, 
alcohol, and oil. Air is the most expansible 
fluid, but it does not receive nor part witli 
