the Royal Society’s Injlruments . 383 
The firft column is the infide diameter of the tube, 
exprefled in decimals of an inch; the fecond is the weight 
of a quantity of quicklilver fufficient to fill one inch in 
length of it ; and the third is the correfponding depref- 
lion of the convex furface of the quicklilver in a ciftern 
barometer, whofe tube is of that lize. The reafon of 
giving the fecond column is, becaufe the ealieft way of 
afcertaining the infide diameter of the tube is, by finding 
the quantity of quicklilver fufficient to fill a given length 
of it. It is needlefs faying, that the part of the tube, 
whofe diameter is to be meafured, is that anfwerinp; to 
the upper part of the column of quicklilver; and that 
the table can be of no ufe but to thole only who obferve 
by the convex furface. 
In this barometer, the infide diameter of the tube is 
about , 2 5 of an inch, and confequently the deprelfion is 
,05 ; the area of the ciftern is near 120 times as great as- 
that of the bore of the tube; fo that as the quantity of 
quicklilver was adj ufted when the barometer flood at 
29I, the error arifing from the alteration of the height 
of the quicklilver in the ciftern can fcarce ever amount, 
to fo much as T ^th of an inch. As the tube appeared to 
be well filled, it was thought unneceffary to have the 
quicklilver boiled in it; but that is certainly the fur eft 
way of filling a barometer well. 
The principal reafon of fetting down the mean heat 
of the thermometer within doors, during each month, 
in the journal of the weather, is this: fuppofe that any 
one defires to find the mean height of the. barometer in 
any 
