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the jet is supposed to strike the ball underneath, and form 
a cup in which the ball rests. 
A few seconds’ observation, however, will show that the 
above is not satisfactory : before any explanation can be 
complete, it must show how it is that the ball will remain in 
equilibrium on one side of the jet ; nay, that it will fly back 
into the jet, when driven out of it. I will first point out the 
nature of the forces which act on the ball. Its weight acts 
at its centre in a vertical line, and is the only force which 
is not due to the water. If the water strikes the ball 
directly underneath its centre, it will produce a force acting 
upwards in a vertical line, the magnitude of which will 
depend on the height, and may therefore be made to 
balance the weight of the ball. In this position the ball 
would by the action of those two forces be in equilibrium 
in the same way as if it were balanced on a point ; the 
slightest deviation would upset it, and then the jet would 
strike it on one side. If so struck there would be two forces 
at the point of contact, the one normal or through the centre 
of the ball, due to the impulsive action of the water, which 
I shall call P ; and another in the tangent due to the 
friction of the water, which I call R 
If W be the weight of the ball, then P, R and W are the 
only forces which at first sight appear to exist, and the 
question is, can P, It and W be in equilibrium ? This is 
easily answered ; for, these three forces are necessarily in the 
same plane, but they do not all pass through the same point 
and therefore they are not in equilibrium. Hence there 
must be some other force acting on the ball which does not 
pass through its centre, or the point in which it is struck. 
Since this force cannot arise from the action of the water 
as it strikes the ball, or the weight of the ball, it must be 
due to the action of the water as it leaves, and since that 
water which is splashed off or deflected at the point of 
contact does not touch the ball again, the force must be due 
