405 
other words, it is held, that “ when a body revolves in a cir- 
cular orbit, by means of a force directed to the centre of the 
circle, the centripetal and centrifugal forces will be equal.” * 
Now, if you will only keep in mind the fact, that when a stone 
is made to revolve at the end of a string, the string does 
nothing but hold or restrain the stone, and is not “ a centri- 
petal force ” that “ draws back,” according to the Princijoia , 
you will reject this popular notion, or at least see that it 
requires some better proof before acceptance. This imagined 
“ balance '' between a force of projection and gravity, reminds 
me of the mythical balance of Mahomet's coffin between 
heaven and earth, — only it is less rational. Granting that it 
might be just possible to balance an iron box half-way between 
a magnet and the earth, (which I do not, however, admit,) still 
the balance would necessarily be so fine, that the slightest 
breath, or anything producing the slightest vibration, would 
destroy the balance, and down the box would fall. Mahomet's 
body would then certainly “ go to the mountain '' of the earth, 
and that with a vengeance ! But however conceivable such a 
“ balance '' might be as a statical problem, or (perhaps I should 
rather say) puzzle, I venture to add that as a dynamical con- 
ception, when carefully considered, it is well-nigh foolish. A 
constant force like gravitation must needs always overcome 
any single impulse once given to a gravitating body, however 
great the impulse might be. And Sir Isaac Newton's other 
illustration, with his reasoning upon it, is extremely weak and 
faulty. For small distances on the surface of the earth, such 
as for two or even twenty miles, when the earth may be 
regarded as a level plain, and gravity as acting in parallel 
directions downwards, what he says as regards the distance a 
ball might be projected is approximately true. The parabolic 
theory of projectiles is, in fact, based upon these two assump- 
tions. But when he goes on to apply the same reasoning to 
a projectile supposed to be made to “ fall at the distance of 
10, 30, or 90 degrees,'' the curvature of the earth's surface, and 
the converging of the lines of direction in which gravitation 
really acts, ought not to have been disregarded. I know not 
whether to consider it as amusing or sad, to find such an 
instance as this of “ absence of mind'' on the part of the great 
Newton. His whole reasoning, to prove that gravitating 
bodies might be projected so as to go forward and revolve in 
the celestial spheres, is really based upon results previously 
arrived at upon the supposition that the earth is a level plain, 
and that gravity acts in parallel lines, and not in lines con- 
* Grant, Hist, of Pliys. Astr.. p. 23, note. 
