box 
'| Description of the Rotascope. 267 
vary the relative position of its poles; but if the globe receive a 
rapid rotary motion, previously to the commencement of its oscilla- 
tions, there will be a vigorous effort visible at every oscillation, tend- 
ing to alter the direction of the axis of the globe. 
Alittle observation will serve to ascertain, that the position which 
the axis affects, is such as to bring the rotary motion referred to the 
centre of the sphere into the same plane and the same direction as 
that of the oscillation, referring the latter to the centre or point of 
suspension of the pendulum. Hence, if we watch the motion of the 
exterior of the globe, we shall perceive that the rising side in the 
rotation, is the anterior side in the oscillation, and that the inverting 
lakes place, the instant the pendulum begins to descend in the arc of 
oscillation. 
8. There is in use a small apparatus for striking fire, composed of 
asemi-cylindrical box of tinned iron a few inches in length, at one 
end of which is a small cavity for receiving the tinder, and above it 
iS mounted on an axis a disk of steel to strike, when in rapid motion, 
upon a flint, held just above the tinder. The steel disk is put in mo- 
tion by the-friction of a string, drawn briskly over a small pulley on 
the axis. If, when the wheel, which is about two inches in diame- 
ter, is revolving vertically, we hold the whole loosely in the hand ex- 
tended, and carry the latter alternately right and left before the body, 
80.88 to cause the wheel and appurtenances to describe a horizontal 
“uve, to which the direction of its axis at the commencement of the 
Motion is a tangent, we shall perceive a strong tendency in the wheel 
to leave the vertical and assume the horizontal position.* & 
4. To illustrate the effect of removing atmospheric resistance and 
obviating friction, I have constructed a delicate metallic wheel about 
ree inches in diameter, through the centre of which passes a steel 
axis about one tenth of an inch in diameter and three inches long. 
Near each end of this axis a hole is drilled to receive a delicate 
thread of loosely twisted silk, by which the two ends are supported 
and the axis kept ina perfectly horizontal position ; the other ends 
of the silk threads are fastened to two hooks on the bottom of a cyl- 
mass of lead, which is in turn supported by a single hook = 
the centre of its upper base. ‘The wheel is made to revolve on its 
* This neat little experiment had been made, and was first communicated to me 
by Mr. William Mason, who, to render it the more striking, had mounted the whole 
‘0 a pointed axis passing longitudinally through the centre of gravity. 
