1822.] On the Finite Extent of the Atmosphere, 251 
Articte III. 
On the Finite Extent of the Atmosphere. By William Hyde 
Wollaston, MD, VPRS.* 
THE passage of Venus very near the sun in superior conjunc- 
tion in the month of May last, having presented an opportunity 
of examining whether any appearance of a solar atmosphere 
could be discerned, I am in hopes that the result of my endea- 
vours, together with the views which induced me to undertake 
the inquiry, may be found deserving of a place in the Philoso-~ 
phical Transactions. 
If we attempt to estimate the probable height to which the 
eatth’s atmosphere extends, no phenomenon caused by its 
refractive power in directions at which we can view it, or by 
reflection from vapours that are suspended in it, will enable us 
to decide this question. 
From the law of its elasticity, which prevails within certain 
limits, we know the degrees of rarity corresponding to different 
elevations from the earth’s surface ; and if we admit that air has 
been rarefied so as to sustain only 1-100th of an inch barome- 
trical pressure, and that this measure has afforded a true estimate 
of its rarity, we should infer from the law, that it extends to the 
height of 40 miles, with properties yet unimpaired by extreme 
rarefaction. Beyond this limit we are left to conjectures founded 
on the supposed divisibility of matter: and if this be infinite, so 
also must be the extent of our atmosphere. For if the density 
be throughout as the compressing force, then must a stratum of 
given thickness at every height be compressed by a superincum- 
bent atmosphere, bearing a constant ratio to its own weight, 
whatever be its distance from the earth. But if air consist of 
any ultimate particles no longer divisible, then must expansion 
of the medium composed of them cease at that distance, where 
the force of gravity downwards upon a single particle is equal to 
the resistance arising from the repulsive force of the medium. 
On the latter supposition of limited divisibility, the atmosphere 
which surrounds us will be conceived to be a medium of finite 
extent, and may be peculiar to our planet, since its properties 
would afford no ground to presume that similar matter exists in 
any other planet. But if we adopt the hypothesis of unlimited 
expansion, we must conceive the same kind of matter to pervade 
all space, where it would not be in equilibrio, unless the sun, 
the moon, and all the planets, possess their respective shares of 
it condensed around them, in degrees dependent on the force of 
their respective attractions, excepting in those instances where 
* From the Philosophical Transactions, for 1822, Part I. 
