i4on 
fervations in order to the corredlipn of her Theory^ and 
as aground work for better T^^/^j- ; but the examinati- 
on will be a work of a long time, and if we iliould 
happily afterwards attain what we feek, yet the 
Calculation will be fo perplexed and tedious, that it will • 
be found much more inconvenient and difficult then that 
I propofe by obferving the Eclpjes oi Jupiters Satellits 
which however at prelent I muft prefer. 
Forlam perfuaded, th.2it thQ Eclipfes oi the firft will 
fcarcely be found above 4' Minutes of time different from 
my Calculation in the Catalogue^ nor thofe of the third 
above twice 'as much • Now an errour of 4 Minutes can- 
not caufe a fault of more then one degree in the Longitude 
coUeded by comparing an obferved Ingrefs of the firft 
Satellit into fhadow or emeriion from it, with the time 
givGiiiathc Catalogue i and I hope it wiilfcarce ever be 
found to err fo much. But if the fame Eclipfe may be ob- 
ferved in two diftant places at the fame time, or com- 
pared with an obfervation of the fame Jatellit made 
within a Week elfewhere, the difference of Meridians 
will be had fomething better then by comparing two ob- 
fervations of the lame phajis -of a Lunar Eclipje^ made 
in diftant places. 
For whereas it is fomewhat difficult by reafon of the 
Penumbra to determine the true time of the application of 
either of the Moons limbs to the fjiadow, the Satellits 
Eclipfes, efpecially thofe of the firft, are almoft momen- 
tany. 
And whereas there can rarely happen 4 Eclipfes of 
the vifible the fame year, thofeof tho, Satellits hap- 
pen fo frequently, that there are more of them vifible 
in one year then we count days in it, the the Planet 7^ lie 
hid under the Suns rales every year a whole moneth to- 
gether. 
I know our Navigators will objedt againft this Me- 
thod, that it is difficult to pradtice at Sea, becaufe long 
Telefcopes are required which the Motion of the Ship will 
not permit them to manage aboard, that it is hard to 
di- 
