[ 12(^2 J 
An AhfiraSi of a Letter from Mr J. Flamfteed* Math. 
Kcg.isr¥.of the R.S. giving the defer iptionis ufes of 
an Infirumentfor finding the diftances ofl^' SateU 
lits from hk Axis^ with the help of the Table ofVz^ 
rallaxes and Catalogue of Eclipfes ; printed in the 
preceding Tranfaftions. See Tab. 2, Fig. 2- 
THE little Circle in the middle reprefents the P/«- 
net Jupiter, the four concentrick Circles the pro- 
per Orbits of his four SatellitSy duly proportioned to 
the breadth of his body; the diftances betwixt the 
parallel lines inter(ed:iag them, being each equall to ^ 
one of his Semidiameters. 
The 4 divided Circles next without thefe, arc diftin- 
guiflied into fo many parts as there are days and hours 
m each Satellits revolution; the Innermoft of them ferv- 
ing for the firft, or innermoft Satellic s that next it, 
for the 2d, that next without this for the 3d, and the 
outermoft for the 4th; above which is a fmall divid- 
ed Arch of 1 1' degrees. 
By this with the aforementioned tables to find the di- 
ftances of the Satellits ixomi^^ Jxis to 2l propofed time. 
1. In the Table of Parallaxes of the Orb, find the 
the Parallax to the time propofed, and note whether 
it be to be Added or Subjiracled. 
2. Extend the thrid from the center of the Inftru- 
ment over the Parallax numbred in the fmall Arch : 
it cuts off in the 4 divided Circles , fo many hours as 
each Satellit fpends in palling from the Axis of the 
ftiadow to the Axis of viewed from our Earth i thefe 
I call the Simple FaraUa^iic\ Intervalls, which if the 
Parallax was to be added, are alfo additionally if to be 
Subftrad:ed, SubduUive. 
3. To thefe FarallaBick Intervalls add the times of 
half the duration of the Eclips of each Satellite 
which for the I'^may be alTunied i^.io'jfor the 30". 
greater 
