ME C H A N I G S. 
7~9 
of fpirit of wine, but it is great enough to anfwer tnoll 
of the purpofes for which a thermometer is wanted. 
The fixed points which are now univerfally chofen for 
adjufting thermometers to a fcale, and to one another, are 
the boiling and freezing water points. The boiling-water 
point, it is well known, is not an invariable point, but 
varies feme degrees according to the weight and tempera¬ 
ture of the atmofphere. In an exhaulted receiver, water 
will boil with a heat of 9S 0 or ioo°j whereas in Papin’s 
digefter it will require a heat of 412. Hence it appears 
that water will boil at a lower point, according to its 
height in the atmofphere, or to the weight of the column 
of air which prelles upon it. In order to enfure unifor¬ 
mity, therefore, in the conftruifion of thermometers, it is 
row agreed that the bulb of the tube be plunged in the 
water when it boils violently, the barometer {landing at 30 
Englifh inches (which is its mean height round London), 
and the temperature of the atmofphere. 55 0 . A thermo¬ 
meter made in this way, with its boiling point at 212°, is 
called by Dr. Horfley Bird's Fahrenheit , becaufe Mr, Bird 
was the firlt perfon who attended to the Hate of the baro¬ 
meter in conflru&ing thermometers. 
As artifts may be often obliged to adjuft thermometers 
under very different preffures of the atmofphere, philoso¬ 
phers have been at pains to difeover a general rule which 
might be applied on all occafions. M. de Luc, in his 
Reckerches far Us Mod. de l'Atmofphere, has given, from a 
l'eries of experiments, an equation for the allowance on 
account of this difference, in Paris meafure, which has 
been verified by fir George Shuckburgh; alfo Dr. Hor- 
iley. Dr. Mafkelyne, and fir George Shuckburgh, have 
adapted the equation and rules to Englifh meafures, and 
have reduced the allowances into Tables for the ufe of the 
artifl. Dr. Horfley’s rule, deduced from de Luc’s, is this: 
--- log. 2 — 92-804 — h ; where h denotes the height 
8990000 & t » o 
of a thermometer plunged In boiling water, above the 
point of melting ice, in degrees of Bird’s Fahrenheit, and 
2 the height of the barometer in ioths of an inch. From 
this rule he has computed the following Table, for find¬ 
ing the heights to which a good Bird’s Fahrenheit will 
rife when plunged into boiling water, in all ftates of the 
barometer, from 27 to 31 Englifh inches; which will ferve, 
among other ufes, to direct inllrnrnent makers in making 
a true allowance for the effe< 5 l of the variation of the ba¬ 
rometer, if they fliould be obliged to finifh a thermometer 
at a time when the barometer is above or below 30 inches; 
though it is belt to fix the boiling point when the baro¬ 
meter is at that height. 
Barometer. 
Equation. 
310 
+ l 'S 7 
3°'5 
+ °'79 
30*0 
o*oo 
20 ' C 
— o-So 
29'0 
— 162 
28-5 
— 2-45 
28-0 
— 37 1 
%T 5 
—4"i 6 
27*0 
—S'04 
Difference. 
078 
079 
o 80 
0’82 
o'8s 
0-85 
o'86 
o-88 
The numbers in the firfl column of this Table exprefs 
heights of the quickfilver in the barometer in Englifh 
inches and decimal parts : the fecond column fhows the 
equation to be applied, according to the fign prefixed, to 
212 0 of Bird’s Fahrenheit, to find the true boiling-point 
for every fuch Hate of the barometer. The boiling-point 
for all intermediate ftates of the barometer may be had 
with fufficient accuracy, by taking proportional parts, by 
means of the third column of differences of the equa¬ 
tions. See Philofophical Tranfaflions, vol. lxiv. Art. 30. 
alfo Dr. Mafkelyne’s Paper, vol. lxiv. Art. 20. 
In the following Table we have the refult of fifteen dif- 
Vot.. XIV. No. too?. 
ferent obfervations made by fir George Shuckburgh, com¬ 
pared with the refult of M. de Luc’s rules. 
Height of the 
Barometer re¬ 
duced to the 
fame Tempe¬ 
rature of 50°. 
Mean 
Boiling- 
Point by 
Obfer- 
vation. 
Boiling- 
Point by 
De Luc’s 
Rules. 
Height 
of Baro¬ 
meter. 
Boiling- 
Point b\ 
Obfer-' 
vation. 
I 
Boiling- 
Point by 
De Luc’s 
Rules. 
Inches. 
0 
O 
Inches 
O 
O 
25-498 
207*07 
208*54 
30-008 
213*22 
213-47 
27-241 
208-64 
208-84 
30*207 
213-58 
213-79 
2 7 * 9 54 - 
209-87 
210*03 
30-489 
214*15 
2I4*23 
28-377 
210*50 
210 ’8 i 
30-763 
214-37 
214-66 
28-699 
211-27 
211-34 
30-847 
214-83 
214-79 
28-898 
211-50 
211 -67 
3°’957 
214-96 
214-96 
28-999 
21160 
211-85 
2 9'447 
2!2 '55 
212-74 
29-805 
212-95 
213-15 
Sir George Shuckburgh has alfo fubjoined the follow¬ 
ing General Table for the ufe of artifis in conftru£!ing 
the thermometer, both according to his own obfervations 
and tliofe of M. de Luc. 
Height of the 
Barometer. 
Correction 
of the 
Boiling-Point. 
Differ¬ 
ence. 
Correction 
according to 
de Luc. 
Differ¬ 
ence. 
26-0 
26- 5 
27- 0 
2 7'5 
*8-o 
28- 5 
29- 0 
29- 5 
30- 0 
3°‘5 
; 31-0 
— 7 '° 9 ° 
— 6-18 
— 5-27 
— 4-37 
— 3-48 
— 2 ‘59 
•— 1-72 
— 0-85 
o*oo 
+ 0-85 
-j- 1-69 
•91 
•91 
•90 
•89 
•89 
•87 
•87 
•85 
•85 
•84 
— 6-83° 
“ S '93 
S '°4 
■— 4-16 
— 3 ' 3 * 
— 2-45 
— 1-62 
— o-8o 
0 00 
+ °'79 
+ 1 '57 
•90 
•89 
•88 
■87 
•86 
•83 
•82 
•80 
79 
•78 
The Royal Society, fully apprifed of the importance of 
adjufting the fixed points of thermometers, appointed a 
committee of feven gentlemen to confider of the beft me¬ 
thod for this purpofe; and their report is publifhed in the 
Phil. Tranf. vol. lxvii. part ii. art. 37. They obferved, 
that, though the boiling-point be placed fo much higher 
on fome of the thermometers now made than on others, 
yet this does not produce any confiderable error in the 
obfervations of the weather, at leaft in this climate y for 
an error of a degree and a half in the pofition of the 
boiling-point, will make an error only of half a degree in 
the pofition of 92 0 , and of not more than a quarter of a 
degree in the point of 6 z°. It is only in nice experi¬ 
ments, or in trying the heat of hot liquors, that this error 
in the boiling-point can be of much importance. 
In adjufting the freezing as well as the boiling point, 
the quickfilver in the tube ought to be kept of the fame 
heat as that in the ball. When the freezing-point is 
placed at a confiderable diftance from the ball, the pounded 
ice fliould be piled to fuch a height above the ball, that 
the error which can arife from the quickfilver in the 
remaining part of the tube not being heated equally 
with that in the ball, fliall be very fmall; or the obferved 
point muft be corrected on that account according ts 
the following Table: 
Height of the Air. 
Correftion. 
4 2 ° 
•00087 
5 2 
•00174 
62 
"00261 
72 
•00348 
82 
•00435 
8 S3 Tks 
