412 Mr. Ivory on the astronomical refractions. 
occasionally observed in it. From these sources we learn 
that the air extends forty or fifty miles above the earth’s 
surface, and even at that altitude still continues to possess a 
density sufficient for refracting and reflecting the rays of 
light. 
The authors* who have written on the height and figure 
of the atmosphere have likewise assigned a boundary, beyond 
which it cannot reach. But in this they have rather fixed 
a limit to the domain peculiarly belonging to the earth, than 
reasoned upon any distinguishing properties of the atmo- 
sphere. If we conceive a body that circulates round the 
earth by the force of gravitation in the time of a diurnal re- 
volution, the path which it describes will mark the limit 
where the centrifugal force arising from the rotatory motion 
of the earth, will just balance the opposite centripetal force. 
Therefore any body that participates of the rotatory motion 
common to all, if placed beyond the boundary we have men- 
tioned, would continually recede from the earth, and would 
be lost in the immensity of space ; if placed within the same 
boundary, it would fall to the common centre. The radius 
of the orbit described by the revolving body is about 25,000 
miles, or something more than three diameters of the terres- 
trial globe. Now the air surrounding the earth cannot reach 
so far ; for if it did, it would be continually dissipated ; a 
supposition which is extremely improbable, since we are 
acquainted with no source from which a constant waste of so 
necessary a fluid might be supplied. 
But if we would acquire more correct notions as to the 
height of the atmosphere, we must consider more closely the 
* D’Alembert, Opus. Tom. 6. Laplace, Mec. Celeste, Liv. 3. Cap. 7. 
