September 1, 1897.] 



KNOWLEDGE. 



211 



wings will fall, no matter how unequal in velocity the 

 horizontal wind may be." Mr. Headley supposes that the bird, 

 at the moment of his ascending from the slower moving 

 stratum of air into the swifcer moving one, " is part of the 

 slowly moving stratum below " ; but surely, if he is part of 

 the slowly moving stratum, it is because he is beiwj carried 

 uith the stratum, and not because he has risen through it. 

 Again, when he has passed through the second stratum 

 and is emerging into a third still more swiftly moving one, 

 if he " is a part of the second stratum it is because he 

 has been carried with the stratum, and so its force has 

 been expended in carrying the bird along ; bat Mr. 

 Headley's theory is that the force of the current has gone 

 to lift the bird. On which horse will Mr. Headley ride ? 

 It seems clear that the bird cannot be " part of the 

 current" at the same time that the current has expended 

 its force in lifting him. The lark's motion upward is 

 surely the resultant of the forces which are acting upon 

 him. One of these forces is that of the wind he breasts ; 

 but to say that a steady wind would have no eflect and an 

 unequal wind a great eflect is to say a thing which is hard 

 of belief. 



I do not quite follow Mr. Headley's meaning when he 

 speaks of birds "advancing at right angles to the wind." 

 Perhaps he means " in the teeth" of the wind ; but that 

 is surely not at right angles to it. There is something 

 also of a portentous wildfowl which can, by a "twist at 

 the waist," turn suddenly; but this must be a "bird of 

 paradise," for no bird of this world could perform such a 

 feat. A very puzzling passage, too, is that in which we 

 are told of birds that rise " in spirals, without a motion 

 of their wings." To do this they must face every point of 

 the compass in succession, and cannot therefore breast the 

 wind, which it seemed part of Mr. Headley's theory they 

 should do. The subject is truly interesting, and has long 

 been so. The " way of an eagle in the air '" was a puzzle 

 to Solomon. It is to be hoped Mr. Headley will not be 

 deterred by criticism, but be spurred to find the true 

 solution. J. H. S. Moxley. 



Koyal Hospital, Chelsea, S.W. 



[Mr. Moxley's objections have been many times stated 

 and met. 



Lord Rayleigh and other high authorities have shown 

 that a horizontal wind of tmequal velocity can help a bird 

 to rise, whereas a tmiform horizontal wind is of no 

 assistance at all. A few years back there appeared in 

 Nature a number of letters dealing with this subject. In 

 these letters, writers who thoroughly understood dynamics 

 made clear what is very easy to grasp — that a bird may 

 be lifted by a horizontal wind that increases in velocity 

 with altitude, if only he has inertia. Inertia he must 

 have at the outset, and as he rises into each successive 

 stratum of air he will still be, relatively to the breeze in 

 which he is, an inert body, since he has just emerged from 

 the slower current below. 



It is imimportant to distinguish inertia and momentum in 

 this connection. When a boy, holding the string of his 

 kite, stands still, the kite has inertia and rises ; if he runs 

 with the string in his hand, the kite has momentum and 

 rises. 



With regard to mere irregularities — and they are great 

 even in what we call a steady wind — Prof. Langley has 

 made out a strong case for his view that a soaring bird 

 may make use of these no less than of the more dependable 

 increase of velocity with altitude. As to the power of 

 some birds to rise in spirals without a motion of their 

 wings, they do achieve this feat, and I can only recommend 

 Mr. Moxley to go and see them. The gain in elevation 

 takes place when they are facing the wind — whenever 



through their momentum or inertia they can feel the wind 

 blowing in their faces. 



Gulls do advance at right angles to the wind. Having 

 risen to some not great height above the grotmd or sea, 

 they descend, making at the same time progress onward; 

 then they turn and face the wind, which, since its forca 

 increases with altitude, lifts them.— F. W. He.^dley.] 



THE BERLIX URAXIA STEEXWARTE. 

 To the Editors of Knowledge. 



Sirs, — The Urania Sternwarte having been mentioned 

 in your last issue as an example of a public institution 

 devoted to the interests of popular astronomy, may I be 

 permitted to add a few words concerning this excellent 

 enterprise, as well as concerning the facilities generally 

 which are oflered to the amateur in Berlin ? The Urania 

 Institution was founded in 1888, and has been since then 

 considerably enlarged — a new branch having been opened 

 only quite recently. It now comprises various depart- 

 ments for the illustration of physics, optics, chemistry, 

 electricity, etc., besides the astronomical observatory, 

 which, however, alone concerns us at present. Among the 

 five large telescopes which are here placed at the disposal 

 of the public every fine evening until eleven o'clock, there 

 is a fine twelve-inch refractor mounted in a dome with rising 

 floor, as also a reflector of about eight inches aperture. 

 In the daytime the solar spots and faculfe are shown by 

 projection, and the various details of the observatory are 

 explained by the attendants. There are also exhibited 

 various examples of astronomical photography, as well as 

 models of telescopes and plaster casts representing lunar 

 craters, while a reading room is provided where current 

 astronomical literature may be consulted. Attached to the 

 institute are two theatres — one for the delivery of lectures 

 illustrated by the usual lantern shdes, while the other 

 is reserved for more spectacular efi'ects, and is therefore 

 fitted with the usual accessories of an ordinary theatre, 

 the lecturer occupying a small balcony at the side of the 

 stage. The lectures given in this latter theatre are, how- 

 ever, chiefly noted for their scenic effects, and are not 

 always astronomical. Nevertheless, some tableaux recently 

 produced here, representing a series of lunar landscapes, 

 were admirable, both in artistic eflect and selenographic 

 accuracy. Though this may perhaps border on the 

 "playgrotmd of science" rather too closely, there is still 

 sufficient solid matter to suit the tastes of the more 

 sober minded, and this (exclusive of the lectures) may 

 be obtained for an entrance fee equivalent to sixpence. 

 Mention must also be made of the well-known monthly 

 publication Himmel und Erde, which is issued by the 

 Urania Society. 



Should the amateur's interest impel him to seek still 

 further details, he may visit the Koyal Observatory, where 

 all the instruments will be fully explained to him — the 

 Observatory being open to the public free on two mornings 

 in the week ; or he may visit the Royal Astrophysical 

 Observatory at Potsdam, which is open free to visitors 

 every Friday afternoon. Here he will also receive fuU 

 particulars of the principal instruments forming the equip- 

 ment of this splendid observatory, soon to be further 

 enriched by the addition of a thirty-two inch photographic 

 refractor. 



Adjoining Herr Archenhold's telescope at the Berlin 

 Industrial Exhibition last year there was an astronomical 

 museum on a small scale. It consisted mainly of exhibits 

 which had been lent for the occasion, e.g., maps, docu- 

 ments, and instruments of historical interest, astronomical 

 photographs (the most popular amongst which were 

 Dr. Weinek's beautifi;! lunar enlargements) telescope 



