248 DESCRIPTION of an 
yet nearly fill the caliber. It is to be allowed to move down- 
wards till it be fully immerfed in the fpirit. After this has 
once been effeéted, it will be found, that it is not difpofed to 
part again from the fpirit ; but if the thermometer be held ver- 
tically, with the bulb uppermoft, it will immediately defcend 
to the extremity of the column. but no farther: There it refts ; 
and if by a diminution of heat the fpirit contra@, it is drawn 
upwards at the extremity of the column, as this recedes towards 
the bulb. Now, let the thermometer be fo difpofed, that its 
ftem, inftead of being vertical fhall be horizontal, (and fuch is 
the ordinary pofition of this thermometer), it may readily 
be imagined, that, in this cafe, the conical piece fhall equally, 
as before, follow any retraction of the column; but fhould this 
lengthen again, in confequence of an increafe of temperature, 
the conical piece does not advance with it, but remaining fixed 
at the loweft point to which the column had defcended, it allows 
the liquor to pafs freely beyond it, as that again expands. Hence 
the point of the fcale at which the conical piece is found to reft, 
denotes the loweft degree to which the liquor of the thermo- 
meter has funk in the interval of the obfervations. To reétify 
the inftrument for a frefh obfervation, nothing farther is requi- 
fite than to elevate the bulb of the thermometer, in order that 
the conical piece may fink, by its proper gravity, to the extre- 
mity of the column. ~ 
2. Ir it be required, that the thermometer fhould mark the. 
higheft point to which the fluid has afcended in any given 
time, then a mercurial thermometer is to be employed, fuch as 
is reprefented by DE; into the tube of which is introduced a 
conical piece of ivory F, with its bafe turned towards the bulb. 
When fuch a thermometer is placed in a vertical fituation, 
the bit of ivory will fall down, and reft upon the furface of — 
the mercury in the tube ; it will rife as the column is 
lengthened, and defcend as this contracts. But if the item be 
placed 
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