266 On the Eclipses of Jupiter’s Satellites. 
an emersion*, It denotes the distance of the satellite from Ju- 
piter’s limb, at the moment of its re-appearance; the diameter 
of Jupiter being taken for unity. This distance is to be mea- 
sured either in a line with Jupiter’s equator (or longer axis), or 
in a line parallel thereto. Or, which is the same thing, in a 
line with the belts: for the satellites generally appear a little 
above or below the centre. 
Before I dismiss this subject of the eclipses of Jupiter’s satel- 
lites, I would call the attention of the practical astronomer to 
that of the shadows of the satellites passing over the face of 
Jupiter. On the importance of such observations M. Laplace 
has the following remark: ‘ Les observations de |’entrée et de 
la sortie de leurs ombres sur le disque de Jupiter, répandraient 
beaucoup de lumiére sur plusieurs élémens de cette théorie. 
Ce genre d’observations, jusqu’ict trop négligé par les astro- 
nomes, me parait devoir fixer leur attention, car il me semble 
que les contacts intérieurs des ombres doivent déterminer )’in- 
stant de la conjonction, avec plus d’exactitude encore que les 
éclipses. La théorie des satellites est maintenant assez avancée 
pour que ce qui lui manque ne puisse étre déterminé que par 
des observations trés-précises. I] devient, donc, nécessaire 
d’essayer de nouveaux moyens d’obser vation, ou du moins, de 
s’assurer que ceux dont on fait usage, méritent la préférence t.” 
I am not aware of any recorded observations of this nature: and 
a new and interesting field is thus opened to those practical 
astronomers who are fortunately possessed of powerful tele- 
scopes. 
* « The telescopes, proper for observing the eclipses of Jupiter's satel- 
lites, are common refracting telescopes from fifteen to twenty feet.” So 
says the Nautical Almanac: but I much doubt whether any one of the Com- 
missioners of the Board of Longitude ever saw a telescope of this kind ; nor 
do I think there is such a thing in existence. How absurd then it appears 
to recommend the use of them; and thus mislead (as I know it has done) 
those entering the career of science! A great part of the utility and im- 
portance of observations of these eclipses arises from the use of telescopes 
of nearly the same form and power: by which means the times of the phe- 
nomena are more readily compared. Telescopes with three object glasses 
are now rarely made: and those with two object glasses, of 46 inch focal 
length and 32 inches aperture, will perhaps, in the present state of the art, 
be found the most proper for observations of this kind. If the observation 
of occultations of the fixed stars by the moon should be introduced into the 
Navy, a much smaller oat TE a will answer for such purposes. 
t Systéme du monde, page 252, 4th edition. ' 
1822. 
