[ 3°4 ] 
The common degrees of heat are ftiown by the 
top of the mercury in the longeft leg, or by the top 
of the fpirit, in cafe any of it is left above the mer- 
cury. 
When the mercury in the longeft leg finks by 
cold, that in the ftiorter leg will rife, and will run 
over into the ball A ; from whence it cannot return 
back when the thermometer rifes again, as the fur- 
face of the mercury in the ball is below the orifice 
of the tube Therefore the upper part of the 
ihorter leg will be filled with a column of fpirits of 
a length proportional to the increafe of heat ; the 
bottom of which, by means of a proper fcale, will 
fhow how much the thermometer has been lower 
than it then is ; which being fubftraCted from the 
prefent height, will give the loweft point that it has 
been at. 
If no further contrivance was ufed, the mercury 
would fall into the ball A in large drops; which 
would make the inftrument lefs accurate. For the 
thermometer’s beginning to rife immediately after a 
drop is fallen, or juft as it is going to fall (in which 
cafe it will return back into the tube), will make a 
difference of fuch part of a degree nearly as that drop 
anfwers to. To prevent this inconvenience, the top 
of the ftiorter leg, clofe to the ball, is contracted, by 
being held in the flame of a lamp ; and the paffage 
is further ftreightened by a folid thread of glafs placed 
within the tube, and extending from the bottom of 
the ftiorter leg to the part near the ball A, where it 
is moft contracted. By this means, as loon as any 
fmall portion of mercury is got beyond the end of 
the thread of glafs, it breaks off, and falls into the 
ball 
3 
