256 On the Finite Extent of the Atmosphere. [Ocr. 
sary to those who have considered the common phenomena 
observable in the occultations of Jupiter’s satellites by the body 
of the planet. Their approach, instead of being retarded by 
refraction, is regular, till they appear in actual contact ; showing 
that there is not that extent of atmosphere which Jupiter should 
attract to himself from an infinitely divisible medium filling 
space. 
Sides the mass of Jupiter is full 309 times that of the earth, 
the distance at which his attraction is equal to gravity must be 
as 4/ 309, or about 17-6 times the earth’s radius. And since his 
diameter is nearly eleven times greater than that of the earth, 
176 
ai 
centre, at which an atmosphere equal to our own should occasion 
a refraction exceeding one degree. To the fourth satellite this 
distance would subtend an angle of about 3° 37’, so that an 
increase of density to 31 times our common atmosphere would 
be more than sufficient to render the fourth satellite visible to us 
when behind the centre of the planet, and consequently to make 
it appear on both (or all) sides at the same time. — 
The space of about six miles in depth, within which this 
increase of density would take place, according to known laws 
of barometric pressure, would not subtend to our eye so much 
as 1-300th ofa second, a quantity not to be regarded in an esti- 
mate, where so much latitude has been allowed for all imaginable 
sources of error. 
Now though, with reference to the solar atmosphere, some 
degree of doubt may be entertained in consequence of the pos- 
sible effects of heat which cannot be appreciated, it is evident 
that no error from this source can be apprehended in regard to 
Jupiter ; and as this planet certainly has not its due share of an 
infinitely divisible atmosphere, the universal prevalence of such 
a medium cannot be maintained; while, on the contrary, all the 
phenomena accord entirely with the supposition that the earth’s 
atmosphere is of finite extent, limited by the weight of ultimate 
atoms of definite magnitude no longer divisible by repulsion of 
their parts. 
= 1°6 times his own radius will be the distance from his 
Articie IV. 
Analysis of Mica with 4 One Avis of Double Refraction. 
By M. Rose, of Berlin.* 
Amone a great number of different kinds of mica which Dr. 
Seebeck had examined with respect to their action on light, only a 
From Gilbert’s Annals, yoh. y. 1822, 
