98 Dr. Wollaston on the finite extent of the atmosphere. 
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 yiy of a second, a quantity not to be regarded in an 
estimate, 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 
possible effects of heat which cannot be appreciated, it is evi- 
dent 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 pre- 
valence of such a medium cannot be maintained; while, on 
the contrary, all the phenomena accord entirely with the sup- 
position 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. 
