168 GYROSTATIC BALANCE. 
instrument stands on this knife-edge on a plate of glass 
or other hard and smooth plane. 
It is evident that the sole characteristic difference be- 
tween this and the gyroscope, is that in the former the 
point of support is under the centre of the wheel and in 
the equatorial plane, while in the latter, it is in the line 
of the axis produced. The so-called gyrostats in Sir W. 
Thomson’s balance are supported at points in the axis, 
and therefore are gyroscopes. ‘To make them in the form 
of gyrostats, 7. e., with jackets about them, would in- 
volve very considerable additional expense with no cor- 
responding advantages. Tor this reason I have used in 
my apparatus the ordinary form, 7. é., a wheel supported 
in a ring. 
As you see, the wheels in Sir W. Thomson’s diagram 
(Fig. 1) all rotate in the same direction, 7. e., if a watch 
be placed at right angles to each axis, and at the end 
farthest from the upper hook, and with its back towards 
it, the rotation of each wheel is opposite to that of the 
hands. 
This being so, Sir W. Thomson declares ; 
1st. ‘‘That the system as a whole will have no moment 
of momentum ; in other words there will be no rotation 
around a vertical axis.” 
2d. ‘‘ That in this condition of no rotation around a 
vertical axis, the index will, at first, oscillate up and 
down, until stopped by friction, and, but for the friction, 
it would, he says, vibrate forever.’’ 
3d. ‘‘ If we check the vibration by hand, the weight 
will hang down at rest, the pin drawn out to a certain 
degree ; and the distance drawn out will be simply pro- 
portional to the weight pune on the lower hook, as in 
ordinary spring balances.’’ | 
Now I propose to show you by actual trial that the 
system, as a whole, will have moment of momentum, 
and that there will be rotation around a vertical axis; 
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