MECHANICS. 
by 180; dividing therefore the intermediate fpace into 
180 equal parts, will give the fcale of the thermometer, 
which may afterwards be continued upwards and down¬ 
wards at pleafure. Other thermometers of a fimilar con- 
ftru&ion have been accommodated to common ufe, having 
but a portion of the above fcale. They have been made 
of a fmall fize and portable form, and adapted with ap¬ 
pendages to particular purpofes; and the tube with its 
annexed fcale has often been enclofed in another thicker 
glafs tube, alfo hermetically lealed, to preferve the ther¬ 
mometer from injury. And all thefe are called Fahren¬ 
heit's thermometers. 
a. In 1733, M. Delijle, of Peterfburgh, conftruffed a 
mercurial thermometer on the principles of Reaumur’s 
fpirit-thermometer. In his thermometer, the whole bulk 
of quickfilver, when imrnerfed in boiling water, is con¬ 
ceived to be divided into 100,000 parts 5 and from this 
one fixed point the various degrees of heat, either above 
or below it, are marked in thefe parts on the tube or 
fcale, by the various expanfion or contraflion of the 
quickfilver, in all imaginable varieties of heat. Dr. Mar- 
tine apprehended it would have been better if Delifle had 
made the integer 100,000 parts, or fixed points, at freezing 
water, and from thence computed the dilatations or con- 
denfations of the quickfilver in thofe parts; as all the 
common obfervations of the weather. See. would have 
been expreffed by numbers increafmg as the heat increafed, 
inftead of decreafing, or counting the contrary way. How¬ 
ever, in practice it will not be very eafy to determine ex¬ 
actly all the diviiions from the alteration of the bulk of 
the contained fluid. And befides, as glafs itfelf is dilated 
by heat, though in a lefs proportion than quickfilver, it 
is only the excefs of the dilatation of the contained fluid 
above that of the glafs that is obferved ; and therefore, 
if different kinds of glafs be differently affeCted by a given 
degree of heat, this will make a feeming difference in the 
dilatations of the quickfilver in the thermometers con- 
ftrufted in the Newtonian method, either by Reaumur’s 
rule or Delifle’s. Accordingly it has been found, that 
the quickfilver in Delrfle’s thermometers has flood at dif¬ 
ferent degrees of the fcale when imrnerfed in thawing 
■fnow : having flood in fome at 154 0 , while in others it 
has been at 156°, or even 158®. 
3. The thermometer at prefent ufed in France is called 
Reaumur's-, but it is very different from the one originally 
invented by Reaumur in 1730, and deferibed in the Me¬ 
moirs of the Academy of Sciences. The one invented 
by Reaumur was filled with fpirit of wine; and, though 
its fcale Was divided by the author into 80 parts, of which 
o was the freezing-point, and 80 the boiling-point, 
yet in faiit 80 was only the boiling-point of the fpirit of 
wine that he employed, which, as Dr. Martine computes, 
correfponded with about j8o of Fahrenheit. But the 
thermometer now in ufe in France is filled with mercury ; 
and the boiling-water point, which is at 80, correfponds 
with the 212th degree of Fahrenheit. The fcale indeed 
commences at the freezing-point, as the old one did. The 
new thermometer ought more properly to be called De 
Luc's thermometer, for it was firrt made by De Luc ; and is 
in fadt as different from Reaumur’s as it is from fir Ifaac 
Newton’s. When De Luc had fixed the fcale, and finifhed 
an account of it, he fliowed the manufeript to M. de la 
Condamine. Condamine advifed him to change the num¬ 
ber 80; remarking, that fuch was the inattention of phi- 
lofophers, that they would probably confound it with 
Reaumur’s. De Luc’s modefty, as well as a predilection 
for the number 80, founded, as he thought, on philofo- 
phical reafons, made him decline following this advice. 
But he found by experience that the prediction of Con¬ 
damine was too well founded. 
4. The thermometer of Celfius, which is ufed in Sweden, 
has a fcale of 100 degrees from the freezing to the boil¬ 
ing water point. 
Thefe are the principal thermometers now ufed in Eu¬ 
rope; and the temperatures indicated by any of them may 
bs reduced into the correfponding degrees on any of the 
others by means of the following Ample theorems; in 
which R fignifies the degrees on the fcale of Reaumur, F 
thofe of Fahrenheit, and S thofe of the Swedilh thermo¬ 
meter. 1. To convert the degrees of Reaumur into thofe 
of Fahrenheit; —— 5 -j-32 = F. 2. To convert the de- 
4 
( F V a 
grees of Fahrenheit into thofe of Reaumur; v --- 
= R. 
. S X 9 
Fahrenheit;-(-32 =F 
into Swedilh; 
To convert the Swedilh degrees into thofe of 
4. To convert Fahrenheit’s 
- ( F 3 2 ) X S ._1 
:S. 5. To convert Swedilh 
degrees into thofe of Reaumur ; 
S x 4 
= R. 6. To con- 
R X 3 
vert Reaumur’s degrees into Swedilh ; -- = S. 
. 4 * 
To fuch readers as are unacquainted with the algebraic 
expreflion of arithmetical formula;, it will be fufheient to 
exprefs one or two of thefe in words to explain their ufe. 
1. Multiply the degree of Reaumur by 9, divide the pro¬ 
duct by 4, and to the quotient add 32; the fum exprefles 
the degree on the fcale of Fahrenheit. 2. From the de¬ 
gree of Fahrenheit fubtraCt 32, multiply the remainder 
by 4, and divide the product by 9, the quotient is the de¬ 
gree according to the fcale of Reaumur ; &c. 
As in meteorological obfervations it is neceflary to at¬ 
tend to the greatefl rife and fall of the thermometer, it 
was definable to have a thermometer which might regifier 
the greatefl degree of heat, or greatefl degree of cold, 
which took place during the abl'ence of the obferver. 
Accordingly in 1757, lord Charles Cavendilh prefented 
to the Royal Society of London a thermometer in two 
different forms; the one contrived to mark the greatefl 
degree of heat, and the other the greatefl degree of cold. 
The firft confifts of a glafs tube A B, fig. 36. with a 
cylindrical bulb B at the lower end, and capillary at the 
top, over which there is fixed a glafs ball C. The bulb 
and part of the tube are filled with mercury, the top of 
which fliows the degrees of heat as ufual. The upper 
part of the tube above the mercury is filled with fpirit of 
wine; the ball C is alfo filled with the fame liquor almoft 
to the top of the capillary tube. When the mercury rifes 
the fpirit of wine is alio raifed, and falls into the ball C, 
which is fo made that the liquor cannot return into the 
tube when the mercury finks; confequently the height 
of the fpirit of wine in the ball, added to that in the 
tube, will give the greatefl degree of heat to which the 
thermometer has pointed fince the lafl obfervation. Whei\ 
a new obfervation is to be made, the inftrument mull be 
inclined till the liquor in the ball covers the end of the 
capillary tube.- In this thermometer it is evident that 
the mercury mult be affeCted by the weight and elaflicity 
of the fpirit of wine, and therefore it will not correfpond 
to any of the common mercurial thermometers. 
The thermometer for (bowing the greatefl degree of cold 
is represented at fig. 37. by the crooked tube ABCD. 
This inftrument is filled with fpirit of wine, with.the ad¬ 
dition of as much mercury as is fufheient to fill both legs 
of the fiphon, and about a fourth or fifth part of a hol¬ 
low ball C. We are not told what the proportion of 
mercury was to that of fpirit of wine. The degrees of 
heat are Shown by the rife or fall of the mercury in the 
longer leg A B. The thermometer marks the greatefl 
fall by means of the hollow ball C. When the mercury 
in the longer leg finks by cold, that in the Shorter will 
rife and run over into the ball C, from which it cannot 
return when the mercury fubfides in the Ihorter and rifes 
in the longer leg. The upper part of the Ihorter leg will 
therefore be filled with a column of fpirits of a length 
proportional to the increafe of heat; the bottom or lower 
furface of which, by means of a proper fcale, will lhow 
hov? 
