( 54 * ) 
M. Mariotte , partly from the fpecifick Weight of Air 
and Mercury, which were found by Experiments to be 
as i to 10,800 •, Air being to Water as 1 to 800, and 
Water to Mercury as 1 to 13 or very near it. If 
fo, as the Column of Mercury in the Barometer is 
counterpoifed by a Column of Air of equal Weight, a 
Cylinder of Air of 10,800 Inches, or 900 Feet will be 
equal to one Inch of Mercury, and 90 Feet to * 0 of an 
Inch, or 75* to Part of it. The Height of the Air, 
as it anfwers to one Inch of Mercury, being thus de- 
termined, and the Expanlions of the Air being recipro- 
cally as the Heights of Mercury, Dr. Halley , by the 
Help of the Hyperbola and its Afymptotes, calculated 
two Tables, one (hewing the Altitude to given Heights 
. of Mercury, the other the Heights of Mercury at given 
Altitudes. Thefe Tables, the firft that ever were cal- 
culated, together with the Do&or’s whole Method of 
proceeding, and an ingenious Attempt of his to difcover 
the true Reafon of the Rife and Fall of Mercury upon 
Change of Weather, were printed in the Thilofofhical 
Tranfattions *, and the Tables themfelves were very 
lately re-printed, with fome Obfervations upon them, 
by Dr. c Defaguliers f . 
In the Year 1703, when the Meridian Line, firft 
begun by M. Ticard in 1669, afterwards continued in 
1683, was farther purfued, feveral Obfervations of this 
Kind were made, and the Heights of feveral confidera- 
ble Mountains, particularly in the Southern Parts of 
France , determined as well by Trigonometrical as Ba- 
rometrical Obfervations. Monlieur CaJJini the Younger 
took that Opportunity to compare thefe Obfervations 
J N° 181 . pag. 106; f phi1, Tranfaft. li® 386. 
with 
