184 



KNOWLEDGE. 



[August 2, 1897. 



at last, wishes to join his comrades who are drying their 

 wings ou a rock not far off. The wind is blowing towards 

 the rock in question ; accordingly he begins by flying in 

 the opposite direction, so that the wind may assist him as 

 he struggles to rise from the water. This feat accomplished, 

 he turns round and makes all sail for his destination. The 

 lark and the cormorant are quite right in thinking that 

 the wind will help to lift them, but it does so only because 

 it increases in velocity as they rise. Near the surface of the 

 land or the sea it is retarded by friction. An anemometer 

 reveals a very marked increase in velocity at successive 

 elevations not far from the ground. By means of kites 

 experiments have been made up to a height of a thousand 

 feet, and even there the increase continues, though it is 

 much more gradual. Were it not for this increase of 

 velocity with altitude, the lark, after his first jump from 

 the ground, would, but for his own efl'orts, move back- 

 ward with the wind. It would not Uft him any more 

 than it will Uft a baUoon from which the gas has 

 escaped. But perpetually passing from a slower to a 

 more rapid current (in theory it is convenient to divide the 

 air into definite strata), he has always an inertia which 

 enables the wind to lift him. He is still, so to speak, part 

 of the slowly moving stratum below when he emerges into 

 the rapidly moving stratum above. In fact, he illustrates 

 the principle of the kite — inertia, which is equivalent to 

 momentum, taking the place of the string. As long as 

 the kite is held by the string, it will rise ; cut the string, 

 and [the wind will at once resign it to the mercies of 

 gravitation. We see, then, that if a lark wishes the wind to 

 help him to rise, he must face it — incline his body forward, 

 as a kite is inclined ; and, further, that the inequality of 

 the wind must give him inertia or momentum, or else his 

 wings will have to raise him without assistance. When 

 gulls advance at right angles to the wind without any 

 beating of their wings, the principle applied is the same. 

 From some slight elevation they glide downwards till they 

 are near the ground or the surface of the sea, then 

 (possibly by a twist at the waist) they turn suddenly and 

 face the wind, which lifts them to their former elevation ; 

 after which the process begins again. 



It may be that when birds soar, i.e., rise in spirals 

 without moving their wings, they are availing themselves 

 at each upward step of the increasing velocity of the wind. 

 But they sometimes attain an elevation of a mUe or more, 

 and there the increase must be a negligeable quantity, if 

 it has not at length reached the vanishing point ; and, 

 therefore, we have to look elsewhere for an explanation of 

 soaring. 



This brings us to the subject of up-currents, obviously 

 much to be desired if wing labour is to be saved ; and birds, 

 whenever such assistance is to be had, are always ready to 

 profit by it. When the wind blows against a clifl', it is, of 

 course, deflected upward, and gulls may sometimes be seen 

 enjoying the fun of being lifted like a foam bubble by the 

 up-cun-ent, then descending to be lifted again. Oftener 

 they may be seen soaring over cliffs or hills near the shore, 

 ascending in noble spirals without a motion of their wings. 

 The secret lies in the upward slant of the wind. But 

 soaring may be seen under quite different circumstances. 

 The adjutant bird, one of the noblest of soarers, rises till 

 he looks a mere speck or vanishes altogether, over the 

 plains of Burmah. Can there be up-currents there ? It is 

 possible that, owing to the ground being unequally heated 

 by the sun, there may be such currents here and there. 

 But in Egypt I have seen kites soaring over fields where 

 the young corn was growing thick and green, and where, 

 consequently, the earth's surface was not much heated. 

 Probably the adjutant depends on the ii-regularity of the 



wind. Even what we call a steady wind, we know by 

 Prof. Langley's experiments, is very unsteady, and varies 

 enormously in velocity from one moment to another. This 

 fact the adjutant may make use of, deriving from it the 

 indispensable inertia or momentum. As long as he can 

 offer resistance to the breeze, not being carried by it like a 

 balloon, but feeling it blowing in his face, so long the 

 breeze will continue to lift him. Whenever the wind 

 fi-eshens he will turn and face it, and wiU rise like a kite. 

 When it slackens he will sail with it, descending slightly. 

 Obviously there will be a good deal of leeway, a phenome- 

 non vouched for by good observers ; but there will be a gain 

 in altitude. And thus the grand bird, making even the 

 wind, that " chartered libertine," his slave, rises higher 

 and higher — often, no doubt, watching as he soars for 

 carrion on the earth below, but often, probably, delighting 

 only in the cool upper air, and in the exercise of his own 

 skill. 



To descend now to a humbler but very wonderful per- 

 formance. Gulls may often be seen following in the wake 

 of a steamer without a motion of their wings, though the 

 wind is blowing strong in their faces. They are taking 

 advantage of an up-current. As the steamer advances, 

 the air at the stern rushes down to prevent a vacuum 

 being left, and this down-draught rebounds off the surface 

 of the sea and a strong up-draught is formed. The gull puts 

 himself in this up-draught, which lifts him like a feather- 

 weight, then he glides downward and onward at a great 

 pace. Meanwhile, the point at which the up-draught is 

 formed advances. He soon finds himself in it again, is 

 raised to his former elevation, and the process is repeated. 



But often a gull will hover over the stern, apparently 

 almost motionless, in realiiy steadily advancing with the 

 steamer. Here, too, there must be an up-current, the 

 wLud blowing at a slight angle to the steamer being 

 deflected upward when it strikes her side. This buoys the 

 gull up. In rising he loses way ; when he advances, he 

 descends. But he does not descend or rise rapidly, as in 

 the case last described. Like a consummate artist, the 

 gull conceals the art which he employs. 



For the solution of this and other flight problems more 

 observations are needed. The interest of the subject will 

 amply repay anyone who will devote time and energy to 

 the study of it. 



♦ 



ENGLISH MEDALS.-II. 



By G. F. Hill, M.A. 



OUR last article* brought us up to the Stuart 

 Restoration in the history of EngUsh medals. 

 That event naturally excited great interest, and is 

 illustrated by a number of medals which accord to 

 the King the usual measure of fulsome praise. The 

 most important medallists of this time were the Eoettiers, 

 a family of Antwerpian origin, of whom John, the eldest, 

 had been brought to the notice of Charles whUe residing in 

 Holland. Thomas Simon.who had worked for the Common- 

 wealth, did not retire from his post of Engraver to the Mint 

 until 1664. He died of the plague in 1665, and the fine 

 medal (No. 1) relating to the "Dominion of the Sea," 

 struck in 1665, is supposed to be his last work. The King 

 rides (in the character of Neptune) in triumph over the 

 waves, and the design commemorates the naval victory 

 obtained by the English over the Dutch ou the 3rd of June, 

 ofi" Lowestoft. 



The work of John Roettier may be illustrated by a medal 

 (No. 2) commemorating the Restoration, although apparently 



* See Knowledge for April, 1897. 



