258 Proceedings of the Royal Society of Edinburgh. [Sess. 
the ground it is necessary that at the instant at which the bomb is dropped 
the aeroplane should be travelling in a line whose trace on the ground 
passes through the target, and this condition can only be fulfilled with 
certainty by stabilising the sight employed. 
The bombsight used by the R.N.A.S. in 1917 was made up of systems 
of “ line,” and ‘‘ range ” or “ transit,” wires or bars mounted on a frame- 
work. The “ line ” bars consisted of two horizontal wires or bars, one 
above the other, placed fore and aft with respect to the aeroplane. 
When bombing “ up wind ” or “ down wind ” the aeroplane was steered so 
that the target was kept as nearly as possible in the plane of these 
bars. The “ range ” or “ transit ” bars consisted of horizontal wires or bars, 
and by observation of the transit of the target across these the proper 
instant at which to drop the bomb was determined. To be accurate 
such a contrivance must not partake of the pitching and rolling of the 
aeroplane on which it is mounted ; the frame with its attachment is said 
to be stabilised when it is maintained flat (so that the line bars lie in a 
true vertical plane, a true vertical plane being one containing the thread 
of a simple pendulum set up on a fixed support, a support which is 
moving in a straight line at constant speed) no matter how the aeroplane 
may pitch or roll. If the line bars can be maintained in a truly vertical 
plane, and the target is on the plane of the bars at the instant of dropping 
the bomb, the first condition required for accuracy is fulfilled. Obviously 
it is of equal importance that the ‘‘range” bars, or their equivalent, should 
be stabilised. Hence it is necessary that the sight should be stabilised 
not only laterally, but longitudinally ; errors due to rolling and pitching 
of the aeroplane must be entirely eliminated if the bombing is to be 
accurate. 
Many inventors have planned apparatus for keeping a platform, 
mounted on a vehicle horizontal — making use of a gyroscope, or system 
of gyroscopes, as the means of defining the vertical or horizontal. It is 
widely supposed that a gyroscope mounted on gimbals so as to be free 
to process in all directions will of itself assume the desired position, 
namely, that in which the axis of the gyroscope is vertical, or horizontal, 
as the case may be. To be successful a stabiliser must, so to speak, have 
a sense of the true vertical, and it must not be appreciably disturbed by the 
forces which it encounters. 
Figs. I and 2 show, in elevation and plan respectively, a gyroscope 
mounted on trunnions with its axis of spin vertical. The casing of the 
gyroscope g is attached by means of pivots to a gimbal frame / 
which surrounds the gyroscope casing, with enough clearance to permit 
