[ 302 •] 
the common degrees of heat pointed out by the top 
of the mercury, as much as the column of fpirit of 
wine expands, and therefore are greateft when that 
column is fo ; that is to fay, when the greateft heat 
to which the inftrument has been expofed is leaft. 
A difference of 30 degrees of Fahrenheit’s fcale, in 
the greateft rife of the thermometer, would require 
the fcale to be altered one fixtieth part : and the 
error arifing from making ufe of the fame fcale will 
be about one fixth of a degree, if the thermometer 
is obferved when it has fallen ten degrees. 
In the inftrument here defcribed, the bore of the 
tube is about 0.027 inches 5 and one inch of it con- 
tains two grains of mercury, and anfwers to about 
ten degrees, the cylinder containing about 2280 
grains. If a much fhorter tube was made ufe of, 
a confiderable error might arife from too great a 
quantity of fpirits adhering to the fides of the tube, 
in that part, which is filled with mercury ; efpecially 
when the thermometer riles faft. This makes it ne- 
ceflary to employ a cylinder of a confiderable big- 
nefs, if it is defired to have the fcale of degrees 
pretty large. 
If the weight of the mercury is thought inconve- 
nient, it may be avoided by the conftrudtion de- 
fcribed in fig. 2. where the bottom of the tube is 
bent fo as to point upwards, and is joined to a ball A, 
which communicates with a cylider placed above it. 
In all other refpedts it is the fame as the inftrument 
before defcribed. 
It is filled with fpirit of wine and mercury ; the 
quantity of the latter being fufticient to fill the 
whole tube and the ball A. 
No 
