( H 6 )

iäme Satellit made withm a Weck elfewhere, die Diftcrenee of Meridiant willbe had something better, than by compnrlng two Obsorvations of the samePhaßs of a Lunar Eclipse, made in Distant Place?, For whereas it is (bme*what difficult by reafon of the Penumlra to determine the True Time of theApplication of either of the Moons Limbs to the Shadow, the SatelliteUclipfcs , elpecially thole of the Fhß , are almost Momentany.

And whereas there can rarely happen 4 Eclipses of the Mcon Vifible thefame Year, thole of the Satellits happen so frequently, that there are moreof them Vifible in one year than we Count Days in it, though the PlanetJupiter lie hid under the Sun s Rays every Year a whole Month together.

I know our Navigators will Object against this Method, that it is difficultto Practice at Sca, becaule Long Telelcopes are required whicli the Motionof the Ship will not permit them to manage aboard. But if it be not Pra-cticable at S:a they cannot deny but that it is at Land j and that the TrueLongitude of Remote Coasts from us are the First: thing desired for the Cor-rection of their Charts: Let them attempt thde First, and I doubt not butthe fuccess will encourage them so much, that they will readily find meansto put it in Practice at Sca ; That the Frcncb have uled this Method fuc-celsfully both in Detonatis and in their Otvn Country, That a Telesoope of14 FootLongat most, or for need one 8 Foot, with broad Eye Glases, will besufficient for this purposo; That the difficulty cannot be known till it be tryed,and that Uso renders many things eaiie which our first thoughts conceivedUnpracticable.

If it be Required to know whether any of thosc Eclipses which are In-visible with us be Vifible in any other Given Place, Convcrt the Dijfcrenccof Meridians betwixt it and London into Time; and if the Place Lie to theEaß of London, Add it to, if to the IVcfl, Substract it from, the Time ofthe Appearance at London, the Sum or Difterence accordingly shall be theTrue Time of the Eclipse under that Meridian, at which it Jupiter be abovethe Horizon and the Sun beneath it,thc Eclipse is there Vifible, otherwaysnot.

. Or, By the help of the Ephemerides of the Planet's Places and a TerreßrialGide, the space on it in which any of these Eclipses will be Vifible may befound thus.

First soek the true Places of the Sun and Jupiter with his Latitude in theEphcm rides, whereby you may find their Declinations and Right Afcenfion?,either ^by the Vulgär Tables, or the Globe it seif, exactly enough for thisMethod.

Bring London on the Globe to the Meridian, and detaining it there notewhat Dc%. of the /Equator is cut by it. From this Substract the Time of theEclipse after Noon converted into deg. and min. the Remainder ffiews youthe Longitude of that Meridian on the Earth, where it is then Noon whenthe Satellit is Eclipscd', which I therefore call the Mcridional Longitude ofthe Eclipse. Bring this Mcridional Longitude under the Meridian, and Ele-vate the nearcr Pole to the Sun as much as is his Declination, keep theGide in this Position and is Jupiter be in Conscqucncc of the Sun, Draw a

Line

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