( 206 ) 
I fuppofe to have occafion’d the Miflake in the Mea- 
fares. 
Monf. CaJJini , as well as the Engli/h Aflronomers, 
believes that the Earth makes one Revolution about its 
Axis, once in 23 Hours 5 6', becaufe in that Time, the 
Plane of any Meridian returns to the fame fix’d Star 
from which it had departed. 
Let * H be taken in any Parallel of Latitude, as for 
Example, in the Latitude of 51 0 46', a Plumb Line, 
-L H, will be perpendicular to the Curve BH, at H, and 
produced pafs thro* the Zenith of the Point H, if the 
Earth had no Diurnal Rotation j but fince the Earth 
moves round its Axis, all Bodies upon its Surface, en- 
deavour to fly from the Axis of their Motion with a 
Force proportionable to their Diftance from it in a Di- 
rection along the Plane of that Parallel, in which they 
are. Let that Force (explain’d by Monf. Huygens , and 
called a Centrifugal Force) be reprefented by the Line 
H /, or its Equal and Parallel L h • now a Plummet 
placed at L, if the Earth flood (till, would defcend in 
the Line LH, but as it is at the fame Time aCted upon 
by the Force H / in the Direction L h , it will move 
in the Direction L / Diagonal of the Parallelogram H /, 
according to the known Laws of Mechanicks ^ and the 
Plumb Line L H, inftead of being perpendicular to the 
Curve at H, will in the Latitude 51 0 4 6' make an An- 
gle of 5 1 with H L. This Angle will be lefs towards 
the Poles, till at the very Pole it quite vanifhes, as it 
alfo does at the Equator. The Demonf ration of the 
Quantity of this Angle , I Jhall give before I end this 
Differtation. Now fince there is no iuch Angle ob- 
ferv’d, but in all Water Levels we find the Plumb 
Line always perpendicular to the Line of Level, the 
Surface 
* . 11. 
