( »93 ) 
^Demonpathneoncernhgthe Mothn cf Ught, cmmmiutei 
from Paris, in Journal des Scavans^ mA here made E^gliJIx 
PHilofophers have been labouring for many years to decide 
by fome Experience, whether the aSionof Light be con- 
veyed in an inftance to diftant places^ or whether it requiretb 
time. hl Romer of ihc R, Jcademy of the Sciences hath devi- 
fed a way, taken from the Obfervations of the firft Satellit of 
Jupiter, by whichhedemonftrates , that forthe diftance of a- 
bout 3000 leagues, fuch as is very near the bignefs of the Dia- 
mererof the Earth, Light needs not one fecond of time* 
Let (in Fig, 1 1.) A be the Sun, B Jupiter, G the firft Satelik 
of Jupiter y which enters into thefhadowof Jupiter to come 
out of ic at D • and let EFGHKL be the Emh placed at divers 
dillancesfrotn Jupiter, 
Now, fuppofe the Earth , being in L towards the fecond 
Quadrature of Jupiter, hath feen the firft Satellit at the time 
of its emerfion or iffaing out of the fhadow in D | and that 
about 42f hoursafcer, ('z^i^» after one revolution of this Satel- 
lit J the Earth being in K, do fee it returned in D ; it is mani- 
feft, thatif the Light require time to traverfe the interval LK^, 
the Satellit will be feen returned later in D5 than it would have 
been if the Earth had remained in L, fo that the revolution of 
this Satellit being thus obferved by the Emerfion?, will be re- 
tarded by fo much time, as the Light fhali have taken in pafling 
from L to K, and that, on the contrary ,in the other Quadrature 
FG, where theEarth by approaching goes to meet the Light,the 
revolutions of the Immerfions will appear to be fhortned by 
fo much, as thofe of the Emerfions had appeared to be length- 
ned* And becaufe in 42^hours ^ which this Satellit very near 
takes to make one revolution, the diftance between the JE^r^^ ' 
and Jupiter in both the Qaadratures varies at leaft 210 Diame* 
£ers of the £^y^^f, it follows, that if for the account of every 
Diameter of the E^r/I^ there were required a fecond of time^ 
theLighc would take 3 £^ minutes for eachof the intervals GF, 
KL ; which would eaufe near half a quarter of an hour be- 
tween two revolutions of the firft Satellit , one obferved in 
FG, and the other in KL, whereas there is not obferved any 
fenfible difFerence» 
