730 M E C H A 
The correction in this Table is exprefled in ioooth parts 
of the diftance between the freezing-point and the furface 
of the ice: e. gr. If the freezing-point ttands feven inches 
above the furface of the ice, and the heat of the room is 
62, the point of 32 0 fliould be placed 7X'00261, or *018 of 
an inch lower than the obferved point. A diagonal fcale 
will facilitate this correction. 
The committee obferved, that, in trying the heat of 
liquors, care fhould be taken that the quickfilver in the 
tube of the thermometer be heated to the fame degree as 
that in the ball; or, if this cannot be done conveniently, 
the obferved heat fliould be corrected on that account; 
for the manner of doing which, and a table calculated for 
that purpofe, we muft refer to their excellent report in the 
Phil. Tranf. vol. lxvii. or Abr. vol. xiv. p. 269. 
With regard to the choice of tubes, they ought to he 
exactly cylindrical. But, though the diameter fliould vary 
a little, it is eafy to manage that matter in the manner 
propofed by the abbe Nollet, by making a fmall portion 
of the quickfilver, e. gr. as much as fills up an inch or 
half an inch, Aide backward and forward in the tube; 
and thus to find the proportions of all its inequalities, 
and from thence to adjufi the divifions to a fcale of the 
mofl perfect equality. Capillary tubes are preferable to 
others, becaufe they require fmaller bulbs; and they are 
alfo more fenfible, and lefs brittle. The mofl convenient 
fize for common experiments has the internal diameter 
about the 40th or 50th of an inch, about nine inches 
long, and made of thin glafs, that the rife and fall of the 
mercury may be better leen. 
It is commonly obferved of thermometers, that upon 
equal augmentations and diminutions of heat they feldom 
vary equally, though they are filled with the fame liquor. 
To account for this circumftance, it fhould be recollected, 
that the variation of a thermometer is direCtly as the ca¬ 
pacity of the ball, and inverfely as the bafe of the Item. 
Thus, if there be two mercurial thermometers, for in- 
ftance, and we call the capacities of the balls C and c, and 
the bafes of the ftems B and b, the variations will be as 
C c 
C to c direftly, and as B to b inverfely, or as — to 
eonfequently the variations will not be equal in thofe 
thermometers unlefs ~—=z T ; and this cannot be the cafe 
B 0 
unlefs C : c :: B : 6; therefore, to render the varia¬ 
tions in the two thermometers equal, the capacities of 
their balls muft be to each other as the bafes of their 
cylindrical ftems. 
The next thing to be confidered, is of what number of 
degrees or divifions the fcale ought to confift, and from 
what point it ought to commence. As the number 
of the divifions of the fcale is an arbitrary matter, the 
fcales which have been employed differ much from one 
another in this circumftance. Fahrenheit has made 180 
degrees between the freezing and boiling water point; 
Amontons made 73 ; and fir lfaac Newton only 34. There 
is, however, one general maxim, which ought to be ob¬ 
ferved : That fuch an arithmetical number Jhould. be cko/en as 
can eafily be divided and fubdivided ; and that the number of 
divifions- jhould be [0 great, that there fiall Jeldom be occafon 
for jraElions. The number 80 chofen by Reaumur anfwers 
extremely well in this refpeCt, becaufe it can be divided 
by fcveral figures without leaving a remainder; but it is 
too fmall a number: the cottfequence of which is, that 
the degrees are placed at too great a diftance from one 
another, and fractions muft therefore be often employed. 
We think, therefore, that 160 would have been a more 
convenient number. Fahrenheit’s number 180 is large 
enough ; but, when divided, its quotient foon becomes an 
odd number. 
As to the point at which the fcale ought to commence, 
various opinions have been entertained. If we knew the 
beginning or loweft degree of heat, all philofophers would 
agree that the loweft point of the thermometer ought to 
NICS, 
be fixed there; but we know neither the loweft nor the 
higheft degrees of heat; we obferve only the intermediate 
parts. All that we can do, then, is to begin at fome 
invariable point, to which ^thermometers made in different 
places may eafily be adjufted. If poflible, too, it ought 
to be a point at which a natural well-known body re¬ 
ceives fome remarkable change from the effefts of heat 
or cold. Fahrenheit began his fcale at the point at which 
fnow and fait congeal; Kirwan prepofes the freezing- 
point of mercury; fir lfaac Newton, Hales, and Reaumur, 
adopted the freezing-point of water. The objection to 
Fahrenheit’s loweft point is, that it commences at an ar¬ 
tificial cold never known in nature, and to which we can¬ 
not refer our feelings ; for it is what few can ever expe¬ 
rience. There would be feveral great advantages gained, 
we allow, by adopting the freezing-point of mercury. It 
is the loweft degree of cold to which mercury can be ap¬ 
plied as a meafure; and it would render unneceffary the ufe 
of the figns plus and minus, and the extenfion of the fcale 
below o. But we objeft to it, that it is not a point well 
known; for few, comparatively fpeaking, who ufe ther¬ 
mometers, can have an opportunity of feeing mercury 
congealed. As to the other advantage to be gained by 
adopting the freezing-point of mercury, namely the abo¬ 
lition of negative numbers, we do not think it would 
counterbalance the advantage to be enjoyed by ufing a 
well-known point. Heat and cold we can only judge of 
by our feelings: the point then at which the fcale fliould 
commence, ought to be a point which can form to us a 
ftandard of heat and cold ; a point familiar to ns from 
being one of the moft remarkable that occurs in nature, 
and therefore a point to which we can with moft clearnefs 
and precilion refer in our minds on all occafions. This 
is the freezing-point of water, chofen by fir lfaac Newton, 
which of all the general changes produced in nature by 
cold is the moft remarkable. It is therefore the moft: 
convenient point for the thermometers to be ufed in the 
temperate and frigid zones, or we may fay over the globe ; 
for even in the hotteft countries of the torrid zone many 
of the mountains are perpetually covered with fnow. 
Having now explained the principles of the thermo¬ 
meter as fully as appears necelfary in order to make it 
properly underftood, we will here lubjoin an account of 
thofe thermometers which are at prefent in moft general 
ufe. Thefe are Fahrenheit’s, Delifte’s, Reaumur’s, and 
Celfius’s. Fahrenheit’s is ufed in Great Britain, Delifle’s 
in Ruflia, Reaumur’s in France, and Celfius’s in Sweden. 
They are all mercurial thermometers. 
1. Fahrenheit 's thermometer confilts of a (lender cylin¬ 
drical tube and a fmall longitudinal bulb. To the fide 
of the tube is annexed a fcale which Fahrenheit divided 
into 600 parts, beginning with that of the fevere cold 
which he had obferved in Iceland in 1709, or that pro¬ 
duced by furrounding the bulb of the thermometer with 
a mixture of fnow or beaten ice and fal ammoniac or fea- 
falt. This he apprehended to be the greateft degree of 
cold, and accordingly he marked it, as the beginning of 
his fcale, with o 5 the point at which mercury begins to 
boil he conceived to (how the greateft degree ot heat, 
and this he made the limit of his fcale. The diftance 
between thefe two points he divided into 6 00 equal parts 
or degrees; and by trials, he found that the mercury 
flood at 32 of thefe divifions, when water juft begins to 
freeze, or fnow or ice juft begins to thaw; it was there¬ 
fore called the degree of the freezing-point. When the 
tube was immerfed in boiling water, the mercury rofe 
to 212, which therefore is the boiling-point, and is juft 
180 degrees above the former or freezing-point. But 
the prefent method of making the fcale ot thefe thermo¬ 
meters, which is the fort in moft common ufe, is firft 
to immerfe the bulb of the thermometer in ice or fnow 
juft beginning to thaw, and mark the place where the 
mercury ftands with 32 ; then immerfe it in boiling wa¬ 
ter, and again mark the place where the mercury ltands 
in the tube, with the number 212, exceeding the former 
