♦ KNO^VLEDGE ♦ 



^L ¥^ AN ILLUSTRATED %^^ 



^^ MAGAZINE OF SCIENCE ^f 



PLAINIY\yQRDED-£XACTLYD£SCRIB£D| 



LONDON: FlilDAY, SEPTEMBER 25, 1«85. 



Contents of No. 201. 



SWIFTLY-MOVING METEORS.* 



rpil]'] earth is now traversing that part of her annual 

 JL Cdurse in which her northern hemisphere is most 

 (lircclly cxiHisrd tn meteoric downpour. The northern 

 lu;ini>liliri-f limy be compared at this season to an 

 umbrLlLi earriud somewhat in front by one who travels 

 through a rainstorm drifted by a wind which he is facing, 

 whereas in the spring months the northern hemisphere 

 is like an umbrella carried over the shoulders for pro- 

 tection against rain blown after the traveller. When we 

 face the drifting rain it comes down more sharply on us 

 than when we are retreating from it. So is it with the 

 earth ; and thus we have at this season a greater number 

 I if meteors falling on our northern hemisphere, and those 

 >vhich reach us fall on our air (the eartli's umbrella, one 

 might almost say, seeing that it in reality protects her 

 from the meteoric shower) more sharply than during the 

 months of spring. 



At this season, then, the observers of meteors and 

 falling stars have more favourable opportunities than 

 during any other part of the year. And meteoric 

 observations are now much needed, for a discovery has 

 l)ccn announced respecting meteors which, if real, is one 

 of the most interesting ever made. But it needs very 

 eareful testing. 



Every one knows, or should know, that meteors are 

 bodies which enter the earth's atmosphere from outside. 

 They had been travelling on orbits of planetary, perhaps 

 more than planetary, extent, with velocities even ex- 

 ceeding the swift rush of our earth on her annual course. 

 I'Jucountering her upon their way, they rushed into the 

 air which shields her from actual mischief, were caused 

 to glow by the friction arising as they swept through 

 this air at the rate of tliirty or forty miles per second, 

 "-nd, being vaporised by the intense heat, their careers 

 as separate bodies came to an end. 



So swift have been the movements of all meteors yet 

 observed, that the motion of the observer as he is carried 

 I'ound the earth's axis (at the rate of ten or twelve 



* I'roin the Timet of Sept. 10. 



!■" 1 rjf appreciibly 



. ...lUvh. Thib 



II iM 1 1 ,x display 

 I, I- I isted five or 

 ' f the meteors has 

 Their " radiant 

 of their parallel 

 -J ill I re. has remained 

 ^t 1 ill the time, 

 111 V liM li the earth's 

 _n,l till ugh a right 

 xpicted, becOT 



apj)reciabl_\ iim li n .i 

 point," or the '' v.uiinIiihj- jhhii 

 tracks, as seen jirojeeti.il on tin ~i n 

 in tlio same position amonic tin 

 although in six hours the dirn 1 1 n 

 rotation carries the observer l^ > li.i 

 angle. This, of course, was to hi 



velocity of ten or twelve miles per minute is a3 nothing 

 compared with a velocity of thirty or forty miles per 

 second. 



But it is quite different with the earth's motion of 

 revolution. This carries the observer along at the rate of 

 18^ miles per second. If the direction of this motion 

 changed while a shower of meteors lasted, then assuredly 

 we might expect that the apparent directions of the 

 meteors' courses would change too. We might, indeed, 

 be sure that they would change in a marked degree even 

 though the meteors were travelling along with the 

 greatest velocities meteors can have when crossing the 

 earth's path on any closed orbit — viz., twenty-six miles 

 per second. 



Bat then the direction of the earth's motion around 

 the sun does not appreciably change during the usual 

 continuance of a display of falling stars. In a whole day 

 it only changes one degree, which is as nothing. It 

 takes three months to change through a right angle, 

 instead of six hours, the time during which the direction 

 of her motion of rotation changes by that amount. So 

 that it might have been supposed that we need never 

 look for any change in the apparent directic n of meteoric 

 motion on account of a change in the direction of the 

 earth's motion in her course around the sun. 



Now, however, comes the announcement — or, rather, 

 the announcement has been in the air for full five years — 

 that there are meteors which pour in upon the earth in 

 the same unchanged direction for four or five months at 

 a stretch, some of these belonging to well-known systems, 

 or, rather, to systems supposed to be well known till this 

 strange discovery was made respecting them. Mr. 

 Denning, of Bristol, who has long been known as a 

 careful observer of meteors, stated in 1880 that he had 

 observed meteors coming night after night from the same 

 radiant point among the stars for several months in suc- 

 cession. It was pointed out immediately that if such 

 were the case, and such meteors travelled only with 

 ordinary meteoric velocities, the apparent persistence 

 of the radiant point proved incontcstably that the 

 meteors belonged to different ,-_\ -i iiu-. iml i.> :, .is Mv. 

 Denning supposed, to one ami i' The 



reasoning on which this deiun I . Imt 



demonstrative. It may be very i-' ! > _ "se 



of tweuty-MX miles an hoiu- in :iu :iii\ ;i ry ^i? ■ ,ll.v, I, n, 



say from due north. Then it is rni hm i\ i:. I;i]i 



changes her course through a iijlii r i , \si wilt 

 round, witli unchanging velnei 



•rle, th 



n.l > 



If. ( 



the 



the ship uiis :it llr-i stiMmiui: 'lue in rtli, meeting the 

 wind, shn was ixfu'snl to an aiipifeiit north wind of 

 forty-four and a-lialf miles an hour. If she turned round 

 in a large are towards the east the wind would seem to 



