An instrument to indicate whether, under all conditions, the path of the aeroplane is a straight line or whether it is turning to the right or left, is much required. It is well known that when flying in a cloud or at night, and no fixed object is visible, even the most experienced pilot may, without realising it, be flying on a sharp curve. If, however, the angle of banking is great the muscular reaction of his body will have to be increased to counteract the increased force acting on it. This will tell him that his course is not straight, but there is nothing to show whether he is on a right-hand or left-hand turn. If he tries to straighten his course he is just as likely to increase the curvature as to straighten out.