222 CAUSES OF DRIFT. 
Fig. 3. 
_ shot, which is therefore small. The former, O, acting as normal to the 
tangent, and representing the resisting force of the air, which causes 
the movement of the head of the shot. Draw qg from. G, perpendicular 
to O; then Og is the moment tending to turn the point up, and will 
continue to act until O passes through 6 at right angles to cb. 
If the pressure #, acting upon the flat head below a, be resolved, the 
component O pressing perpendicular to the end Cb of the shot, and 
therefore parallel to its long axis, will pass below the centre of gravity. 
The component P parallel to Cb represents merely the friction 
between the air and the surface of the end, and is therefore very small 
indeed compared to O, the resistance opposed to the motion of translation. 
Fig. 4. 
These effects may be demonstrated practically by means of a gyroscope, 
Fig. 5, provided with a small elongated shot, instead of the disc used 
for ordinary experiments. The projectile must be 
made with the greatest care, so that its centre of 
gravity coincides exactly with that of the two rings 
within which it is placed: the rings are so arranged 
that one can turn round a vertical axis, and the other 
round a horizontal axis, the projectile being there- 
fore free to turn in any direction. A cylindrical 
portion of metal extends beyond the base of the shot, 
in prolongation of its longer axis, round which the 
string is wound to give the required rotatory 
motion to the projectile.* 
As the shot in the gyroscope has no motion of 
translation, a strong current of air must be directed 
upon it, so as to represent the resistance of the atmos- 
phere to a projectile moving with a high velocity. 
The diameter of the nozzle of the blower should 
be equal, or perhaps rather larger than that of the shot, and the centre 
of the blast should be directed below the point of the shot, in the posi- 
tion indicated by A in Figs. 1 and 2. 
Fig. 5. 
*T had such a gyroscope made by one of the best instrument makers in London, I 
daresay itis at the R.M. Academy still ; also a blower, from which a strong blast of air 
could be directed on the shot, 
