THE DRIFT OF SERVICE PROJECTILES. 
4(59 
if you play a billiard ball putting on, say left side, and strike bard, there is a 
negligible deflection from the line of aim of the ball, but that if you play very 
slowly, so as to give a good spin to the ball with a very small velocity of trans¬ 
lation, it is well known that the ball will drift to one side or other of the 
line of aim. As showing how difficult these questions are to consider, I 
should like to observe about a billiard ball, that if you play with the left side 
and with the nap of the table, even with a well-worn table (it is well known), 
the drift of the ball is to the left, but if you play against the nap it is 
also fairly well known that the drift of the ball is not to the left, but 
to the right, so that whether you play with the nap or against the nap 
makes all the difference as to whether the ball drifts to the right or to the left 
when you put on left side. This fact is mentioned by Major Broadfoot 
in the Badminton book on billiards and I have seen it abundantly verified 
on billiard tables, on some of which there was little nap. There is also 
the question of how high velocity comes in with regard to the game of golf. I 
have not played golf to any large extent, but I have noticed that the golf ball 
only begins to show the effect of spin upon it, making it either rise or go to one 
side or the other when its velocity has diminished to a considerable extent, that 
is to say, that the drift of the ball, out of what would be the normal path for an 
unspun ball, is scarcely observable until a certain point is reached when the 
velocity has considerably fallen and then the effect of the spin of the shot, in 
causing drift, appears. 
An interesting matter that Captain Shortt has brought forward is the periodicity 
in sound that is heard when a projectile is fired \ it is quite new to me and I look 
upon it as evidence very strongly in favour of the views that he has advanced. 
Personally, I feel very much obliged to him for opening this discussion and 
giving us the result of what one can see is well thought-out work in regard to 
the matter {applause). 
Professor Greenhill, F.R.S.—Captain Shortt has shown considerable 
intellectual courage m bringing forward his arguments on this very difficult and 
baffling subject of drift. I have studied carefully his remarks and I consider that 
his arguments are quite sound and clear ; and that he has provided for us a popular 
explanation, devoid of mathematical technicalities, of the reason why a projectile 
deviates from a vertical plain of fire. We who see the thorny analytical difficulties 
which obstruct the path are more timid in coming forward with an explanation ; 
besides, we are like the daughters of the horse-leech as mentioned by Tennyson— 
our province consists in “ setting the how much before the how; ” it is not for 
us to explain why a projectile deviates to the right; we are called upon to say 
how much the drift should be by calculation beforehand. 
I have made a collection in a volume of all the foreign pamphlets I could lay 
my hands upon bearing upon this subject; they are not very light reading, and 
sometimes unconvincing, especially when the author is compelled to adopt some 
approximations in order to avoid an analytical cul-de-sac —another man’s approxi¬ 
mations always are unconvincing. 
We have ventured, sir, to send down again the apparatus that we had before at 
General Owen’s lecture. The curved track that you see before you, on which a 
gyrostat mounted on wheels is made to run; this time we have borrowed a 
gyrostat belonging to the Boyal Military Academy to exhibit these phenomena 
on a larger scale and I hope Captain Cooper will show the gyrostat in action at 
the end of the proceedings. The idea of exhibiting the swerve of the gyrostat on 
an inclined curve is due to Mr. Smith ; rve are indebted to him for that idea 
{applause). The apparatus is also useful in other ways, for illustrating, for 
instance, the struggles which are observed in the bearings of the dynamo on a 
ship, due to pitching or rolling; and also in showing how it is possible in a 
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