for adjujling ’Thermometers. 847 - 
■fee apt to make it ftick, in which cafe the included va- 
pour might be fo much compreffed as to caule an error. 
We would alfo by no means advife lining the tin plate 
with leather, or any other foft fubftance, for the fake of 
making it fhut clofer, as that alfo might be apt to make 
it ftick. The chimney alfo ought not to be made lefs 
than half a fquare inch in area: for though a fmaller 
chimney would be fufficient to carry off the fteam, un- 
lefs the veffel is much larger than what we ufed; yet the 
adhefion which is apt to take place between it and the 
tin plate when wet, might perhaps bear too great a pro- 
portion to the power which the included fteam has to lift 
it off, if it was- made much lefs. It is convenient that 
the chimney be not lefs than two or three inches 
long, as thereby the obferver will be lefs incommoded 
by the fteam ; but it would be improper to make it much, 
longer, for the longer the chimney is, the greater difpo- 
fition has the air to enter into the pot between it and the 
cover.. 
It is moft convenient not to make the cover fit on 
tight, but to take on and off eafily ; and to wrap fome 
fpun cotton round that part of the cover which enters 
into the pot, in order to make it fhut clofer ; or, what 
feems to anfwer rather better, a ring of woollen cloth 
maybe placed under the cover, fo as to lie between the top 
o£ 
