CAUSES OF DRIFT. 241 
than it is on the right, and therefore the lateral pressure of the air on 
the right should be greater and the flame ought to deflect to the left, as 
I think you will see that it does. (Applause.) That is just a little 
illustration of Professor Magnus’ point. How does that apply to a pro- 
jectile descending from a great height with a very low velocity of 
translation ? This is a most important point, as the lecturer has told us 
with high angle fire recently an ultimate J/e/¢ drift of the projectile was 
noticed. 
Fig. 2. 
SS aed 
<a 
Now consider the projectile in the descending branch. Let Fig. 2 
represent a section of it as seen from the gun, the resistance of the air 
acting upwards. On the right side the current of rotation is against the 
resistance of the air, while on the left it is with the resistance of the 
air. Consequently the resultant velocity of the air is lower on the right 
hand side and therefore the lateral pressure is greater, and therefore the 
tendency of the shot is to slip to the J/e/¢ in the descending branch when 
the velocity of translation is low. Now that is all theory, and as Major 
MacMahon has said it is desirable that we should have practical results. 
I will therefore take the lecturer’s own table of the 8-in. Howitzer. He 
points to that and says that with a time of flight of 2°7 seconds there is 
a drift to the right of two yards, and witha time of flight of 19:2 seconds 
there is a drift of 85 yards, which is very large in proportion, and he 
takes a simple multiple for the comparison. But there is nothing to 
show that we should take a simple multiple in the connection between 
the time of flight and the drift. I have looked into the subject roughly 
and I have found that with all high-velocity guns of the present day 
the relation between time of flight and drift is expressed in a very 
simple form with every gun that I have been able to get out. When 
the shot is projected at a high velocity and low angle of elevation it is 
a matter of agreement, neither rolling effect nor slip affect the question 
of drift. In fact the gyroscopic effect determines drift. Under these 
conditions I find the relation between time of flight and drift is ex- 
pressed by the equation— 
As 1S DY 
where T is time of flight in secs. and D is drift in yards, K and n being 
constants for the particular gun. Now the points given by the above 
equation, if plotted on a logarithmic chart, lie on a straight line, and 
can be easily studied when thus graphically expressed. Taking the 
Range table for the 8-in. Howitzer, firing 10 lbs. R.L. gunpowder, I find 
that up to an elevation of 5° 20’ the points so plotted lie absolutely on 
2 
