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in the Time of the ancient Chaldeans , who have 
left us fome Obfervations of Eclipfes. But as we 
meafurc the Length of Years by the Number of 
Days, and Parts of a Day, which are contained in 
each of them j it is a new Queftion, Whether the 
Days, or the Revolutions of the Earth round its Axis, 
have always been of the fame Length. This is una- 
nimoully fuppofed, without our being able to pro- 
duce the leaft Proof of it: Nor indeed do 1 lee, how 
it could be poflible to perceive fuch an Inequality, 
in cafe it had really exifted. At prefent we meafurc 
the Duration of a Day by the Number of Olcillations, 
which a Pendulum of a given Length makes in this 
Space of Time: But the Ancients were not ac- 
quainted with thefe Experiments, whereby we might 
have been informed, whether a Pendulum of the 
fame Length made as many Vibrations in a Day 
formerly as now. But even tho’ the Ancients had 
a&ually made fuch Experiments, we could draw no 
Inferences from them, without fuppofing, that Gra- 
vity, on which the Time of an Olcillation depends, 
has always been of the fame Force : But who will 
ever be in a Condition to prove this Invariability in 
Gravity? Thus, even fuppofing that the Days had 
fuffered conftderable Changes ; and that Gravity had 
been altered fuitably thereto, fo that the fame Pen- 
dulum had always completed the fame Number of 
Vibrations in a Day; it would ncvcrthelefs be Bill 
impoffible for us to perceive this Inequality, were it 
ever fo great. And yet I have fome Reafons, deduced 
from Jupiter ' s Adlion on the Earth, to think, that 
the Earth's Revolution round its Axis continually 
becomes more and more rapid. For the Force of 
Jupiter 
