4 Lieut. Col. Miller’s description of a percussion shell, 
through a roll of paper, or a. gun barrel, merely from its 
own weight pressing against the air, so as. to be distinctly 
visible to the eye. If the bottom of the tube is closed to pre¬ 
vent the air from escaping, it will be easily ascertained when 
the ball possesses the spiral motion, and when it does not do 
so, from the peculiar sound it makes against the sides of the 
tube. 
5. If it is asserted, that the spiral motion in a rifle ball is 
sustained solely by the impulse it receives from the barrel, I 
am aware of no proof that can be produced in support of that 
opinion, either from analogy or experiment. If, on the other 
hand, we suppose it to be a motion, both produced and sustained 
by the action of the air, it will be in strict accordance with 
the effect the air is known to produce in all similar cases. 
Among other familiar instances, may be given that-of the 
arrow, the windmill, the shuttlecock, the smoke-jack, a win¬ 
dow ventilator, and a grooved paper cylinder dravm through 
the air. And it may be observed of the arrow,; shuttlecock, 
and cylinder, that each revolves in equal distances of air, 
whatever their velocity may be, and the spiral motion-is 
maintained undiminished to the end of the flight. 
Having been led to infer from the above-mentioned reasons, 
that the spiral motion is in all cases, both produced and sus¬ 
tained by the action of the air ;. and believing it to be impos¬ 
sible to reconcile theory and experiment upon any other 
principle, the next idea that suggested itself, was the possibi¬ 
lity of giving the. spiral motion to grooved shot, when fired 
from a plain barrel. The same-thought appears to have 
occurred to Mr. Robins sometime before his death, about the 
year 1745 ; for, in a memorandum found among his papers^ 
