120 
Q and F will be equal to that of P, and if a horizontal circle 
be drawn through this point it will limit the part of the 
ball in which equilibrium is possible. 
For any deviation without this circle the equilibrium will 
be stable, i.e. if the centre of the ball gets so far from the 
jet that the ball is struck in some point without this circle, 
it will come back again. As to the nature of the equili- 
brium for any deviation within this circle, I cannot speak 
positively, but it is probably nearly neutral all over the 
enclosed area. 
This seems to agree very well with the appearances I 
have described, namely, that the ball appeared to be in equi- 
librium when struck at a point about 45° from its middle, 
about which point it oscillates. When the oscillations 
become so big that the ball leaves the jet, I have said that 
the ball instantly jumps back again. To account for this 
we have only to consider that the force P ceases as soon 
as the contact ceases, but not so with Q, for there will 
still be some water to be thrown off, so that perhaps for 
half of a revolution after the contact has ceased, the force 
Q will continue undiminished and so bring the ball back 
into the jet. 
