290 Proceedings of the Koyal Society of Edinburgh. [Sess. 
In fig. 19 is shown the instrument fitted up for rolling trials. Attached 
to the frame / of the instrument, with its length perpendicular to the 
pivots P 2 P 2 ’ is ^ rigid bar of wood. Carried at one end of this bar 
is a white screen, as shown, composed of millimetre paper, and to the 
other, with its length horizontal and parallel to the screen, is a knife-edge. 
The apparatus was mounted on the aeroplane with the pivots P 2 P 2 iji^g 
fore and aft. By flying with the knife-edge and screen in line with the 
Fig. 19. 
sun, a clearly defined shadow of the sharp edge was obtained on the screen, 
and hence the performance of the instrument could be ascertained. The 
effects of manoeuvres on the instrument were investigated by performing 
the manoeuvre so that at its conclusion the bar was in line with the sun. 
A very extensive series of trials was carried out during the spring 
and summer months of 1917 by officers of the B.N.A.S., and these finished 
up with the sun-shadow tests above described. It was concluded that 
the Gray stabiliser furnished an accuracy, for bombing purposes, of from 
ith to xVfh of a degree, or 30 feet on the ground from a height of 
15,000 feet. The instrument was adopted by the Naval Air Service, and 
a great effort was made to construct a very large number of the instru- 
ments for use in 1918 over the German lines. 
