( iH ) 
of to obferve fmall Rifes or Falls, having two fine and 
well made Indices to the Tube. 
Dr. Edmund Halley, our Royal Aftronomer, has, 
fome Years ago, given us, in the Thilofopbical 
Tran faff ions, the Falls of Mercury in the Barometer, 
corresponding with the Heights to. which the Barometer 
inuft be carried to produce thofe Falls. The firfi tenth 
Part of an Inch in the Fall of the Mercury , he makes 
to anfwer to an Height of 90 Feet j the next tenth, to 
an Height Something greater, and So in Proportion, as 
the Air diminifhes in Denfity, according as we rife in 
Height. The Proportion of the firfi Tenth of the Mer- 
aJrys Fall, he has built upon the Comparison of the 
different Specifick Gravities of Air and Mercury ; and 
taking Mercury to be 137 Times heavier than Water, 
and Water (in cold Weather) to be 800 Times heavier 
than Air-, it follows, that 13, 5x800, will give 10800; 
which Number, if it be taken in Feet, and divided by 
I2 ,o (the Number of the 10th of an Inch in a Foot; 
we (hall have 90 Feet anlwerable to the 10th of an 
Inch, and 75 Feet to a Line or the ixth Part of an Inch. 
As the Dodor’s Tables may be of ufe to the Curious, 
.1 have inferted them here. 
Nor 
