An AhJlraB of a Letter from Mr. J. Flamfteed, 
Math. Reg. isf F. of the R, S. giving an ac- 
count of the Eclipfes of iff SatellitSyanno 1686; 
and containing a Table of the Parallaxes ofv^' Orb^ 
and an EphemerisofV^^ Geocentric Places for the 
fame year. 
sir, " 
T Give you here the Eclipfes oi jupiters Satellite for 
X the next year 16% 6^ deduced from the fame num- 
bers by which I calculated thofeof the yeaj's 1^84, and 
i6%s^ printed in the Tranfailions. I ftiall not need to 
fhew here again, how to find within what limits on our 
Earth any of them , not vifible with us , will be ob« 
fervable i having done it already both in Englijh and 
Latin in thole Trails; I fliall therefore only tell you 
that all my obfervations of them fince made, (which 
have not been many by realon of our much cloudy 
weather and my frequent a vocations J have fully made 
good my affertions concerning them, and that I have 
not found my numbers at any time flnce to err above 
one minute in the Eclipfes of the firft or innermoft 
SateUit 5 fo that if a way were found of managing a 
Tube of 8, 10, or 12 foot long at Sea, and Obferv- 
ing the time as exaiSHy as we can at land, Obfervations 
of thefe Eclipfes would certainly give us the difference 
of Longitude, as exadly as the Latitudes are preten- 
ded to be Oblerved by our more knowing Sea-Men^ 
and certainly much better then the Obfervations of 
Lunar Eclipjes. How the differences of Longitude are 
gained by them, you will fee by the following Exam- 
ples, the latter of which I have purpofely produced to 
N n n fliew 
