July 26, 191 7] 



NATURE 



425 



with an aeroplane that it must in some way or 

 other come to the earth as soon as its stock of 

 petrol is exhausted. It cannot, like a ship out- 

 side a port with a dangerous bar, wait until con- 

 ditions are more favourable ; it must come down, 

 whatever the risk. Once in the air, a steady wind 

 has no effect upon the flying of an aeroplane, 

 although it has a great effect upon the direction of 

 the course. So much misapprehension exists on 

 this point that it cannot be put too clearly. First, 

 however, it must be stated frankly that a per- 

 fectly steady wind does not exist in practice, but 

 the ordinary wind becomes more and more steady 

 as the height increases, and in so far as the drift 

 of an aeroplane is concerned it has the same effect 

 as a steady wind of the same mean velocity. 



The pilot, therefore, if the earth is hidden from 

 him by a sheet of clouds, is absolutely and entirely 

 ignorant of the strength and direction of the wind 

 in which he is flying ; it is just the same to him 

 if it be a dead calm or if it be blowing at the rate 

 of a hundred miles an hour from the east or from 

 the west; he is, indeed, as unconscious of the 

 motion which he is sharing with the air as he 

 is of his daily revolution at a rate in these latitudes 

 of some 600 miles an hour round the axis of the 

 earth. But the effect upon the drift of his machine 

 may be very considerable, and as he does not know 

 what it is he cannot allow for it. The sailor 

 is also concerned with the drift of his vessel, but 

 he has in general a fairly good knowledge of how 

 much it is ; the currents due to the tide can be 

 predicted, and the leeway due to wind can be esti- 

 mated, but it is not so with the airman. More- 

 over, the rate of drift of a vessel is mostly small 

 in comparison with her motion through the water, 

 but in exceptional instances the velocity of the 

 wind may equal the velocity of the aeroplane. 



Thus Glasgow lies very close to a point 400 

 miles due north of Plymouth, and an aero- 

 plane leaving Plymouth and flying due north at 

 eighty miles an hour would find itself close to Glas- 

 gow in five hours' time. Should, however, a 

 strong west wind be blowing of which the pilot 

 did not know, and also clouds so that he could not 

 see the earth, he would, if steering by compass, 

 find himself in five hours' time over the North 

 Sea, and quite possibly much nearer to the Danish 

 than to the English coast. In the present state 

 I of our knowledge he could obtain information 

 I at starting of the general direction and strength 

 i of the wind, but not in such detail that he could 

 j hit off Glasgow within 100 or 200 miles. If he 

 j could see the ground he could ascertain that he 

 was not travelling in the way his machine was 

 1 pointing, and would thus become aware of his 

 drift, but if he could see the ground he could steer 

 by the known landmarks. There would be few 

 [landmarks over the sea, but the appearance of the 

 'surface should give him information as to the 

 strength of the wind, and also of its direction. 



Hence it seems likely that in countries like Eng- 

 land, where clouds prevail, long-distance flight, 

 if It is to be carried on at regular times day after 

 day, will have to be at low elevations. About 

 3000 ft. is the usual height of the winter cloud 



sheet, but it may on occasion easily descend to 

 2000 ft. 



Wind, therefore, though when it is steady and 

 in a favourable direction it may be of assistance 

 for a journey in its own direction, will in general 

 be a hindrance to aerial navigation, and when 

 combined with low clouds may become an insuper- 

 able hindrance. In cases where its velocity and 

 direction can be accurately foretold, the difficulty 

 about allowing for the drift can be overcome, but 

 such precise forecast is not yet practicable. 



A gusty wind introduces difficulties of its own ; 

 the so-called holes in the air, of which one heard 

 so much in the early days of aviation, were due 

 to gustiness, but greater stability and speed in the 

 machine are eliniinating these difficulties. 



Clouds introduce a difficulty of their own, apart 

 from the point that has been already considered. 

 It would seem at first sight as though a man would 

 retain his sense of the vertical direction in any 

 circumstances, but this is not so. Were a man 

 placed inside a hollow vessel that was falling 

 freely without air resistance, he would be entirely 

 without sense of weight or direction, and the 

 pilot of an aeroplane in an extensive mass of cloud 

 is in much the same position. He cannot see any 

 definite object, and apart from sight his sense of 

 direction depends upon the reaction between him 

 and the seat he is sitting on. So long as the 

 motion is uniform this reaction is vertical, but any 

 acceleration of the machine alters the direction and 

 intensity of the reaction, and so confuses the sense 

 of level. The same effect is produced upon a 

 spirit-level or similar instrument, and so confusing 

 is the effect that it is said the machine may 

 almost be upside down without the pilot knowing 

 it. It W'Ould seem as though a gyroscope might 

 to some extent meet the difficulty. One result of 

 this uncertainty of level is that astronomical obser- 

 vations for the determination of latitude and longi- 

 tude are not possible unless the horizon can be 

 seen, and thus the amount of the error produced 

 by want of knowledge of the drift cannot be 

 known. 



Fog is to all intents and purp>oses simply a cloud 

 touching the earth. Landing places for aviators 

 have naturally been put in low, sheltered positions, 

 partly because a shelter from wind is required, 

 but probably chiefly because more or less of a dead 

 level is necessarj', and such flat places are more 

 likely to be found at low altitudes. Such positions 

 are especially liable to fog. The danger of a fog 

 lies in its concealing the landing place and hiding 

 from the pilot until the last moment his distance 

 from the ground. 



Thus it appears that the demand of the airman 

 on the meteorologist is that he shall be able to 

 forecast wind and fog, and, to a less extent, 

 clouds, on tHe route the airman is proposing to 

 follow. It has long been the business of the 

 Meteorological Office to forecast wind, and a cer- 

 tain amount of precision has been attained. Dur- 

 ing last winter Major Taylor investigated 

 the possibility of forecasting fog, and gave the 

 results in lectures to the Roval Meteorological 

 and .Aeronautical Societies. His work constitutes 



