t 5 6 3 ] 
pendicular to the line of their motion ; tho’ die dia- 
meters of the planets, in other directions, is very much 
wanted j it being highly probable, from the laws of 
motion, and what we fee in Jupiter, that fuch planets, 
as revolve round their axes, have their polar diameters 
fhorter than their equatorial ones. 
The diftances of Jupiter’s fatellites from one an- 
other, or from Jupiter’s body, cannot be meafured, 
with any certainty, in the common way, as their po- 
rtion is always very far from being at right angles with 
the line of their motion : Neither can the moon’s 
diameter, which muft be taken from horn to horn, 
fcarce ever be obtained that way, becaufe it very 
rarely happens, that the diameter, to be meafured, lies 
at right angles to the line of her motion. The fame 
may be faid of the diftance between two ftars. But 
this micrometer gives angles, in every direction, with 
equal eafe and certainty ; the obfervation being alfo 
finifhed in an inftant, without any trouble or fatigue 
to the obferver. For as there are no wires made ufc 
of, this way, in the held of the telefcope ; fo the ob- 
ferver has no concern about any illumination. The 
largenefs of the fcale deferves alfo to be taken notice 
of, as it may, in this micrometer, be increas’d almoft 
at pleafure, according as the fmalnefs of the objeCt 
requires. Another inconvenience attending the com- 
mon micrometer is, the variation of the fcale, ac- 
cording to the diftance of the objeCt. As the telefcope 
muft be lengthen’d, or drawn out farther, for fhort 
diftances ; the fcale, which depends upon that length, 
is thereby increas’d ; which renders the meafure of 
the angle very uncertain: Whereas, in this micro- 
meter, the fcale is the fame at all diftances ; fo that 
the angle may be meafured with the utmoft cer- 
4 B 2 tainty, 
