I ^53 3 
A Ktmar\ concerning the Longitude of the Cape of 
Good Hope. 
IN the lecond Book of this Voiagc are related two Obfer- 
vations of the Satellites of Jufiter^ capable, if well made,, 
to aicertain the Longitude of the Cafe of Good Hope. The 
firft was there made 3^»;;^ ad.Jt. n. 1685, when at iih. ^^m. 
20/; the firft or innermoft Satellite touched the WefierHQ^gt of 
Jupiter and at i ih, ^om. 5c/ it appeared no more ; this Ob- 
iervation is laid to be made with an excellent Telefcope of- 
twelve Foot: The other was on June ^h, following/. 
when the Emerfwn oi the fame Satellite was obferved at 9^. 
gyw. 40/. from which latter is concluded, that the Longitude 
of the Cape is iS^.to the Eaft of Paris^ for that the faid Emer- 
■^fim^ according to the 0/^«//^ of Slff", Caffim, in the Meridi- 
an of P^r^ ought no have happened at 8/?. 26m. This fame 
Emerfton is computed by M.v.Flamfteed at i^m. at London^ 
that is, 3 later th^n by Sign ; and confidering that 
neither is verified by Obfer vation in Europe, the Longitude 
hence deduced is doubtful at leaft 3 minutes j if this had been 
the onlyObfervation : But the former being confidercd will 
yet fhew that there is a much greater doubt fiill remain- 
ing ; For from certain Jfironomical principles the parallax of 
the Ork or difference between the place of Jupiter feen from 
the Sm and Earth was,at the time of the firft Obfer vat ion,9^n 
19/». which Arch that Satellite moves in ih. 6m, and the ut- 
moft duration of an Eclipfe thereof in this pofition of Jupiter 
being fcarce 2h. 20m. ( as appears by the accurate Obferva- 
tions of Mr. C^^^iand Mr. Flamfteed) it will follow, that 
from the Immerfion behind J up iters Wcftern Edg, to the 
Emerfion out of the niaddow,there could not be full 3 26m, 
wherefore the Emerfion out of the fhaddow^on June 7d, 
ought, according to the time of the Immerfion, to be at 14/;. 
^6m. atlateft, at the O/x? ; which by Mr. Flamfieeds Calcw 
Im was at London i^h. ^im. or according to Sigr.C^/^iw/ at 
I i 13 
