174 



KNOWLEDGE. 



May. mil. 



micrometric measures compared with l>urnham's 

 observations. At Heidelberg proper motions of faint 

 stars are discovered b_\' simph' looking in a stereoscope 

 at photographic plates of the same part of the sky 

 taken several years apart. Turner has discovered 

 large proper motions of stars as faint as the eleventh 

 magnitude marked on photographic plates, and 

 states that stars with proper motions greater than 

 15" per centuPi- are scattered with no sensible 

 reference to the galaxy. 



It may be asked what becomes of Herschel's 

 explanation of the apex caused by the Sun's proper 

 motion when there are not one but many apices. 

 It should be kept in mind that all nnjtion is relative, 

 and it is quite legitimate to attriiiute the apex of the 

 cluster that contains the largest number of stars to the 

 proper motion of the Sun. Boss has lateh' discussed 

 about six thousand proper motions, and he deter- 

 mines the apex without taking into account the 

 existence of different s\-stems, and also without 

 excluding an\- objects. 



It appears that all the la. Hants so far fdund lie 

 near the galaxy, and it is much easier to determine 

 additional radiants on tin- supposition tliat the\- all 

 lie near the galaxy, than if the\ were scattered o\er 

 the sky. If the pole of the galaw is projected on 

 Kohold's maps, and also the straight line (or rather 

 lines) corresponding to the great circle of the galax\-. 

 any straight line from the projected pole to an\- point 

 on the projected galaxy represents a great circle 

 perpendicular upon the galaw. and the piojected 

 poles of proper motions that lie near this line 

 indicate stars the radiant of whose proper iiiotions 

 is the pole of the galactic meridian in i|uestion. 1)\- 

 investigating the proper motions all round the sk\- in 

 this manner it should be easy to discover all the 

 radiants that lie near the galaxw 



Kapteyn, on the supposition that there are onl\- 

 two radiants, explains these as being caused bv stars 

 moving in two diametricall\- opposite directions, and 

 the fact that they do not appear to be diametrically 

 opjiosite he explains by combining the proper motion 



of the Sun with each of them. In this wa\' the 

 direction of the Sun"s proper motion is determined, 

 but it is seen that the Sun belongs to neither of the 

 clusters. However, there are probabh- more than 

 two star streams, and there is so far no reason to 

 suppose that thev mo\e in exactly opposite directions. 



It is conceivable that two globular clusters might 

 attract each other, approach, and pass through to 

 the opposite side. When the\' had reached the 

 same di.stance apart on the other side, thev would 

 stop and their motion would be re\-ersed. The 

 period of the swing of such a pendulum would 

 exceed millions of \'ears. But when the enormous 

 distances between the stars are taken into account 

 and their relativeh" insignificant sizes, the forces are 

 found too insignificant to account for existing proper 

 motions. The energy of runawa}' stars cannot be 

 derived from attractions between fixed stars or 

 clusters: it ma\- be due to explosions. 



As all observations of heavenb' bodies are made 

 hv comparison with [Positions of fixed stars, the 

 accurate determination of the proper motions of the 

 latter mav be said to be in some sense the most 

 important object of astronomical observation. 

 Several astronomers, especialh' Porter in Cincinnati, 

 have devoted their energies to the disco\-er\- and 

 determination ot proper motions. An niteniatinnal 

 coriperation between observatories in all parts ot the 

 World is being arranged to make a determination 

 with the utmost accuracy of the positions of a ver\' 

 great luimber of fixed stars evenly distributed over 

 the sk\' to ser\e as standards for determining the 

 positions of faint stars recorded on photographic 

 plates. But howe\er perfectb' the catalogue 

 embodxin:; the results of such cooperation ma\' 

 reflect the ai tual situation in the sk\' of the stars 

 it contains, it must in a tew \ears become ([uite 

 inaccurate as the stars change their places, and for 

 the determination of these changes the catalogue 

 must rely on observations made in the past, so that 

 the old observations, far from being superseded, gain 

 in imiMirtance : and the older the^• are. the more 

 imriortant it liecomes to take them into consideration. 



CORRESPONDENCE. 



THE FLIGHT OF BHiDS. 



To the Editors of " Knowledge." 



Sirs, — The representation by Gatke and others of the 

 prodigious rapidity of the flight of migrating birds does not 

 appear to rest on observations bringing conviction. I suggest 

 that some knowledge on the subject might be obtained. An 

 amazing number of birds are killed by dashing against 

 the lighted lanterns of lighthouses. Therefore, I should 

 suppose that, if on a " fly-line " near the shore there were 

 placed a dark screen, twenty feet square, and in the middle 

 thereof there were an aperture of, say, six feet diameter, 

 covered with such material as that which equestrians jump 

 through at a circus, and that it were very strongly lighted 

 from the land side, immigrant birds travelling in the dark 

 would make for it, and dash right through the flimsy 

 material. They should then come in sight of another similar 

 screen three hundred yards away, in the middle of which 

 should be a disc of strong glass very brightly hglited from 



behind. .At this the birds would dash as they do at the 

 lantern of the lighthouse, and would fall dead — to be 

 identified and registered by an attendant naturalist. The time 

 of the first screen being broken by a bird and its death at the 

 second should be taken by stop-watches. Between the screens 

 a man should have a camera with which to cinematograph the 

 approaching bird. There should be a contrivance for 

 immediatelv replacing the broken flimsy in the first screen. 



The birds on reaching the coast might, perhaps, have slowed 

 down through fatigue, but the bright lights might stimulate 

 them to renewed exertion. 



Years ago, crossing the Channel from France, I observed a 

 small bird accompanying the steamer for a mile or so. I 

 could not say pari passu, for it frequently spurted, going 

 with apparent ease ahead of us and then dropping behind. 

 The speed of that bird during that flight, till it disappeared, 

 could hardly have exceeded ten miles an hour. 



Some migrants fly high : but the light of a bright screen 

 might bring some of them down to its level. FRANCIS KAM, 



