Over established air routes quite elaborate measures are now adopted to keep pilots informed of the direction and speed of the wind at different levels, so that they can make due allowance for this factor in shaping their course. In clear weather this information is easily obtained by sighting the drift of a pilot balloon with a theodolite, or by observing in a specially designed graduated mirror or pair of mirrors the drift of the smoke cloud from a shell fired by an anti-aircraft gun and timed to burst at any desired altitude. In cloudy weather the smoke trails can often be successfully observed through small breaks in the clouds. When the sky is completely overcast, a succession of shells is fired at definite short intervals of time and the distances apart of the puffs of smoke and the direction of the line in which they lie are determined from an aeroplane flying above the spot. Another method, which was devised by the French military meteorological service during the war, is to send up small balloons loaded with bombs which burst after a certain time, the position of each burst being determined by sound-ranging from the ground. These methods of providing information concerning the winds at flying levels have, however, their serious limitations, and aeronauts now look hopefully to the perfection of the existing systems of “directional wireless,” whereby the pilot will receive whenever desired, or at regular intervals, a wireless signal from the terminus of his route or some other known point, the direction from which the signal comes being indicated by suitable apparatus on the aeroplane. Thus aided, he should never deviate far from his course, unless he chooses to.