74 
one end, with the concave edge inward, and the plane side 
undermost, the plane making an angle of about forty degrees 
with the horizon; throwing as if to strike the ground at the 
distance of about thirty yards, and giving it, on leaving the 
hand, a rapid rotatory as well as progressive motion. In- 
stead of striking the ground at which it was aimed, its plane 
becomes horizontal at the distance of twenty-five yards, and 
so continues for about fifteen yards, when it commences 
rising in the air, and moving towards the left ; its plane then 
becomes inclined, and continues at an angle of from thirty to 
forty degrees, whilst it describes apparently a segment of a 
circle to the left. Having, at the distance of sixty or seventy 
yards, attained an altitude of from forty to sixty feet, the pro- 
jectile returns, descending to the point from which it was 
projected, when its plane becoming once more horizontal, it 
skims along within a few feet of the ground, and passes close 
by the right hand of the person who threw it. On passing, 
its plane becomes elevated once more, it rises a second time, 
and performs another smaller curve, (fifteen or twenty yards 
behind the projector,) in like manner as the first, with this 
singular exception, that the second curve is described from 
left to right, contrary to the course of its rotation and of the 
first curve, which is invariably from right to left.” 
In bringing the instrument under the notice of the Aca- 
demy, Mr. Mac Cullagh wished to draw attention to the 
theory of its motion. When a body of any form whatsoever 
is projected in vacuo, we know that its centre of gravity 
must describe a parabola in a vertical plane, while the body 
spins about an axis passing through that centre. In the pre- 
sent case, therefore, it is clear that the continued swerving from 
the vertical plane must be ascribed to the action of the air, 
But to compute accurately the mutual action of the air, and 
of a body endued, at the same time, with a progressive and 
a rotatory motion, is a problem far beyond the present powers 
of science. The problem can only be solved approximately ; 
