96 



KNOWLEDGE 



[May 1, 1891, 



that their velocity is comparable to that of the earth, they 

 always (so far as I have traced) appear to move faster or 

 slower according as the earth is receding from or approach- 

 ing the radiant. The explanation of the phenomenon 

 must therefore be sought elsewhere, but there is as yet no 

 satisfactory theory on the subject. 



It seems clear, however, either that the meteors belong- 

 ing to the same long-enduring shower (though, no doubt, 

 physically connected with each other in some way) are not 

 originally moving in parallel lines, or else that their 

 original parallehsm is by some means destroyed before they 

 are sufficiently heated in the atmosphere to become 

 luminous. And this departure from parallelism (whether 

 original or acquired) must follow a -peculiar law, which 

 almost compensates the displacement of the apparent path 

 caused by the earth's motion. Totally unconnected 

 meteors could not be collected into groups by terrestrial 

 aberration in the manner that we find them collected 

 when we analyse any catalogue ; just as a number of 

 stones thrown at random into a current will not be 

 collected into a heap by the action of the water. What 

 we experience is plainly the result not of chance but of 

 law ; but of a law which remains to be discovered. 

 Yours faithfully, 



W. H. S. MoxcK. 



[The degree of certainty with which meteor radiants 

 have been determined has, I think, been generally much 

 over-rated. Observations made during rich showers prove 

 that the meteor-tracks di'a'mi backwards amongst the stars 

 do not all meet in a point, but they appear to radiate from 

 an area of as much as five or six degrees in diameter ; the 

 reason for this divergence no doubt being that meteors 

 are irregularly shaped bodies, which, on plunging into the 

 air, are deflected from their original course in a manner 

 similar to that in which an irregularly shaped body, such as 

 a shell, is turned out of its original course when thrown 

 into water. Occasionally meteors have such projections 

 or irregularities that their course through the air appears 

 sensibly curved. The degi'ee of certainty with which a 

 radiant can be determined depends on the magnitude of 

 the radiant area, as well as on the number of meteors 

 observed. If there are several radiants supposed to be 

 active on any night, an observer has to exercise his judg- 

 ment in determining which among the many intersections 

 with other meteor-paths he will select for the radiant. No 

 doubt the length of the meteor-path gives some indication 

 as to whether the \'isible part of the track is near or far 

 from the radiant ; but the length of the path is also 

 affected by the magnitude of the meteor and its velocity, 

 and there must fi-equently be very considerable uncertainty 

 in selecting from amongst many intersections. In sucla 

 cases the bias of the observer will no doubt produce its 

 effect. We cannot judge as to the probability of the actual 

 existence of any radiant without knowing the number of 

 intersecting meteor-tracks from which it has been deter- 

 mined. Radiants determined from only two or three 

 Intersections of meteor-paths not all observed on the same 

 night ought, I think, to have very little weight attached 

 to them. 



I cannot accept Mr. Proctor's suggestion that such 

 stationary radiants correspond to a rain of sporadic 

 meteors coming from certain directions out of space ; 

 for if the earth's motion in its orbit causes no shift of 

 the radiant, we should have to assume such a velocity for 

 these sporadic meteors that they would at once be detected 

 by the difl'erent character of the streak left. According to 

 Mr. Denning, some of the meteors from these stationary 

 radiants are quite slow-moving, 



No one has at present, as far as I am aware, plotted 

 down all the observed radiants on a globe ; marking those 

 which fall near together, but do not occur in the same 

 month ; and taken a photograph of the globe to exhibit 

 the grouping. Such a graphic method would no doubt 

 enable the eye to judge better of the grouping, and the 

 relation of the groups to the places of bright stars, than 

 any inspection of tables. I have plotted down the radiants 

 referred to in Mr. Monck's letter. They fall into three or 

 four well-marked groups ; but the dates of the adjacent 

 radiants are not uniformly distributed round the year, 

 as one would expect on Mr. Monck's hypothesis. — 

 A. C. Ranyakd.] 



THE FACE OF THE SKY FOR MAY. 



By Hebbert Sadler. F.R.A.S. 



BOTH spots and faculae continue to appear on the 

 solar surface. The following are the times of 

 minima of some of the Algol type variables ('/. 

 Knowledge, March and April, 1891), which may 

 be conveniently observed at Greenwich. 

 U Cephei.— May 4th, Oh. 31m. a.m. ; May 9th, Oh. 11m. 

 A.M.; May 13th, llh. 51m. p.m.; May 18th, llh. 31m. 

 P.M.; May 23rd, llh. 11m. p.m.; May 28tb, lOh. 50m. 



P.M. 



S Cancri.— May 7th, llh. 51ui. p.m.; May 2Cth, llh. 

 7m. P.M. 



S Librse.— May 7th, 6h. 44m. p.m. 



U Coronffi.— May 14th, Oh. 27m. a.m. ; May 20th, lOh. 

 9m. P.M. ; May 27th, 7h. 51m. p.m. 



Owing to his proximity to the sim. Mercury is not well 

 situated for observation in May. On the 1st he sets at 

 8h. 39m. P.M., or Ih. 18m. after the sun, with a northern 

 declination of 20° 27', and an apparent diameter of 10|". 

 He is then inferior to a oth-magnitude star in brightness, 

 about YHo o^ the disc being illuminated. He is in inferior 

 conjunction with the sun at 3h. a.m. on the 10th, at a 

 distance of about olj millions of miles from the earth, 

 being then in transit over the solar disc ; a phenomenon 

 which may be well observed in the western part of the 

 United States and of South America. At Greenwich the 

 sun rises at 4h. 18m. a.m. on that day, and external 

 contact at egress takes place at 4li. 50m. 25s. a.m., at an 

 angle of 168° fi-om the north pole towards the west 

 (counting for direct image). At egress the sun will be 

 only about 4° above the horizon at Gre. nwich. After 

 this Mercury becomes a morning star, but is too near 

 the sun to be observed, as on the 31st he only rises 

 30m. before that luminary. 



Venus is a morning star, but is too near the sun to be 

 very conveniently observed. On the 1st she rises at 

 3h. 37m. a.m., or 57m. before the sun, with a northern 

 declination of 0"^ 49', and an apparent diameter of 13^' , 

 eight-tenths of her disc being then illuminated. On the 

 31st she rises at 2h. 4(im. a.m., or Ih. 5m. before the sim, 

 with a northern declination of 13^ 45', and an apparent 

 diameter of Hi". She has then only a little more than 

 a quarter of the brightness she possessed at the beginning 

 of .lannary, about f'Jj of her disc being illummated. 

 During the month she passes through Pisces into Aries, 

 but without approaching any conspicuous star very 

 closely. 



Mars and .Jupiter may be considered to be, for the pm-- 

 poses of the amateur, invisible. Saturn is an evening 

 star, rising on the 1st at Ih. 20m. p.m., with a northern 

 dechuation of 9^ 37', and an apparent equatorial diametef 



