OBSERVATIONS OF LUMINOUS METEORS. 73 



relatively to the earth is fully 30 miles per second, it follows that in this 

 case, where the meteor was evidently overtaking the earth, moving nearly in 

 the same direction with it, its real velocity in space must have exceeded that 

 of the earth's motion in its orbit by not much more than 7 miles per second. 

 The excess of the velocity of a meteor overtaking the earth directly in a para- 

 bolic orbit, above that of the earth's mean motion in its own nearly circular 

 orbit, is found by Dr. Weiss to be about 9| miles per second*. 



The following letter in ' Nature,' May 16, 1872, from Mr. G. C. Thomson, 

 at Cardiff, affords another instance of bright meteors noted during the past 

 year, the real course of which appears to have not differed greatly in their 

 direction from that of the earth's motion in its orbit, at the time of their 

 appearance: — "I observed a meteor at about half-past eleven on the night 

 of the 8th inst., in the constellation Scorpio, which passed very close to the 

 star Antares, travelling from right to left. It appears to me worth remark- 

 ing, from the fact of its course lying very near and roughly parallel to that 

 part of the ecliptic which corresponded to the earth's position in her orbit. 

 It traversed some 8° or 10° of arc, and was visible for three or four seconds, 

 gradually increasing in brightness until it was nearly on a par with Antares, 

 which star it also resembled in colour. Its slow apparent motion imme- 

 diately suggested the idea that it was moving in the same plane and direction 

 as the earth, in fact that it was overtaking us in an orbit just outside our 

 own. The course of another meteor seen about half an hour earlier from a 

 westerly window, and described to me as not inferior to Jupiter in brightness, 

 appears also to have lain in the direction of the ecliptic, but from left to 

 right, in the neighbourhood of the constellations Gemini, Cancer, or Leo. 

 It is rash to generalize from insufficient data ; but I conceive these meteors 

 may both have belonged to a system whose orbit lies nearly in the j^lane of 

 the earth's orbit, the apparent retrograde motion of the last named being 

 caused by the direction of its path crossing our orbit at a point behind the 

 earth's then place, instead of in advance of it." The two meteors here noticed 

 appear to have belonged to the meteor-system denoted by the radiant-point 

 Y, presenting itself during the first half of May, near the centre of the con- 

 stellation Leo, and scarcely more than 20° distant from the point in the 

 ecliptic from which the earth's motion is directed during the early portion of 

 that month. The apparent motion of the two meteors in opposite directions 

 (in the former case moving eastwards towards Scorpius, and in the latter 

 case westwards towards the constellations Gemini and Cancer) is most 

 readily explained by the effect of perspective upon their, probably, not far 

 from really parallel courses, joined with the circumstance that in their 

 appearance above the observer's horizon, at Cardiff, the meteors successively 

 presented themselves upon opposite sides of their common radiant-point. 

 In relation to the probable positions of their apparent radiant-centres, both 



* As a convenient means of exactly estimating the vei-y short intervals of time occupied 

 by meteors in their flight, it may be suggested to observers to repeat the English alpha- 

 bet (or as many letters of it as are required, rapidly and distinctly) immediately after the 

 meteor's appearance. With ordinary fluency of pronunciation one alphabet occupies 

 about four seconds, and fifteen alphabets can usually be repeated in one minute, the time 

 occupied by a single syllable, or by one letter of the alphabet, wlien thus repeated, being 

 about one-sixth part of a second. By beginning the repetition during or immediately after 

 the meteor's passage, and continuing it during an equal period of time to that in which it 

 appeared to move, a pretty exact estimate of the interval may thus be obtained from 

 memory. In ordinary oases (where the time of the meteor's passage does not allow more 

 than five or six letters of the alphabet to be repeated) the observation may be repeated 

 once or twice, and by counting the number of letters, in each case, a more exact average 

 determination, amounting generally to a very close approximation, may be obtained. 



1872. ' ' G 



