( '® 9 <^ ) 
peculiar'fty either in the Quickfilver or the Glafs of 
which that Tube was made; or to an unperceived remnant 
of Air left in the Tube, from fome of which caufes that 
EfFedl and fome others of the fame kind were imagined 
to proceed. 
Cor. xd. In a Barometer made with a fmall Tube, 
the Mercury will rife and fall irregu'arly. For, as the 
height of the Mercury depends partly upon the Dia- 
meter of that part of the Tube that touches the up- 
per Surface of the Mercury, it is plain, that the unavoi- 
dable inequalities in the Diameter of the lube will be 
more conflderable, in refpedi to the whole Diameter ; 
and confequently will afTc<^ the height of the Mercury 
more in a fmall Tube than in a wider. And this I 
take to be the reafon, why it is fo very difficult, noc 
to fay impoffible, to make two barometers, which ffiall 
exaiSfly agree in the height of the Quickfilver in all 
conftirutions of the Air, efpecially if the Tubes be ve: 
ry narrow. This irregularity is flill more conflderable 
in the Pendent Barometer in which the Quickfilver 
moves through a large fpace, in order to make a fmall 
alteration in the length of the Column fufpendedJ The 
fame confideration is eafily extended to ihofe Levels, 
that depend upon the rifing of Mercury to the fame 
height in the oppofite Legs of a bent Tube ; an loftru- 
ment of which kind has been lately offer’d for the 
fervice of the Publick. And as the effedi is juft con- 
trary in Levels made with Water or Spirit of Wine, 
due regard ought to be had to this Property in the 
conftrudlion of thofe Inftruments, by making the Tubes 
fufficiently wide, in order to diminilh the Error as much 
as poffible. 
III. Fart 
