260 Proceedings of the Royal Society of Edinburgh. [Sess. 
set with respect to the vertical ; and on a moving vehicle, subject to 
accelerations, any ordinary levelling device, such as a spirit-level or 
damped pendulum, is useless. Efforts have been made to use horizon 
glasses, or mirrors, for the purpose of setting the gyroscope. Such adjust- 
ments are troublesome to make, and are only possible when the horizon 
is visible. If the gyroscope is to be adjusted about both sets of pivots, 
the operation of setting the gyroscope becomes extremely difficult, if not 
impossible, to carry out. 
Apparatus, to be effective in war time, must be automatic in action as 
far as possible. Where practicable, all personal ” errors should he avoided. 
It is not fair to require an observer to set up a gyroscope by means of a 
horizon glass when the aeroplane, or other vehicle, is being shelled, nor is 
it to be expected that anything like accuracy can be obtained under such 
conditions. 
To endow the gyroscope device above described with a sense of the 
vertical, many investigators have advocated the use of a gyroscope, or 
system of gyroscopes, provided with gravity control. In fig. 3 is shown a 
gyroscope g mounted so as to possess gravity control, the arrangement 
being identical with that illustrated in fig. I, except that the centre of 
gravity of the pivoted system is brought below the plane of the pivots by 
the addition of the mass m, which is attached to the casing of the gyro- 
scope g by the rod r. An identical result would be obtained by mounting 
the gyroscope within the frame / so that its C.G. lies below the line of the 
