THE ROYAL ARTILLERY INSTITUTION. 
383 
point in this instance, both in accordance with the facts observed and 
with the laws of rotation already noticed. 
The concentric circles represent the base of the shot, and the dark 
spiral line the supposed deflection of the point. The eye is supposed 
to be looking always in a line tangential to the trajectory. The dotted 
line is the course the point would take were the trajectory to be pro¬ 
longed beyond the usual ranges, or which it would take were the shot 
not properly centred. The reason of this somewhat curious but very 
valuable motion of the point, appears to be due to the fact that the 
dip of the trajectory materially interferes with the tendency to gyrate. 
As the point is gyrating downward, the trajectory is also curving down¬ 
wards, and it is the comparative rates of these two motions that 
produce the effects observed. At first, the point will be dipping more 
rapidly than the trajectory; but as the resistance of the air diminishes 
and the point comes closer to the trajectory, by virtue of the forward 
motion of the shot in the direction of the axis and the inward motion 
of the point due to the resistance of the air acting above the head, the 
dip will be less, and so the point does not get round below the trajectory 
at all. It will be observed that the first gyration is small, and if we 
suppose it to occupy the time the shot would take to range 2200 yds., 
the effect will be that the axis of the shot will coincide pretty closely 
with the trajectory for that distance. The second gyration is shown 
larger, because the resistance of the air has then become less, and 
the gyration will be made under a greater exposure of surface. This 
second gyration is therefore what we should have as the first gyration 
if we were to increase the spin; for the greater stability the shot 
would have would cause it to wait until a greater amount of surface 
had been exposed before it would gyrate. It is important to notice 
