: 
} 
OBSERVATIONS OF LUMINOUS METEORS. 277 
periodic star-showers by moving in hyperbolas instead of in parabolas or 
long ellipses, so as to have motions of their own beyond the sphere of the 
sun’s attraction, carrying them apparently from star-system to star-system, 
instead of in constant revolutions round a single solar centre. Observations 
of the duration, length of path, and points of first appearance of meteors of 
the August and November star-showers Dr. Galle suggests will be most valu- 
able to show if any shooting-stars of those well-known streams present 
speeds that cannot belong to other than hyperbolic orbits, as in those cases it 
must be assumed that the excessive velocities observed have their expla- 
nation in some physical cause, to which it will then be very desirable to 
direct special and the most accurate possible investigations. 
The combination of at least two good observations needful for determining 
a shooting-star’s real speed of flight is the difficulty that will present itself to 
carry out Dr. Galle’s experiments on the Perseids and other annual meteoric 
showers. This objection, however, does not apply to the apparent speed, if 
even a single observer records that speed without very serious errors; but 
eyen such a record is not often reached. Observers’ estimates vary chiefly 
as to the apparent lengths of meteors’ courses and their time of flight. An 
incomplete view of the course at the beginning, and sometimes also (from 
dimness of the meteor owing to distance) at the end, is often the cause of 
this, unsuspected by the observer. The time of flight and length of path 
recorded should, however, always correspond together, a short observed time 
of flight for a partially observed path being never coupled in a record with an 
ideal length of course supposed to make up the whole length of a meteor’s 
line of flight judged by such indications of it as the meteor may have left. It 
may also be forgotten to record the times of flight of some shooting-stars at 
all—a very unfortunate omission, because the value contributed by such an 
observation to a simultaneous observation of the same meteor made at a 
distant station is enhanced immensely by a statement of this astronomically 
important datum. Much is due here to hurry in the rapid succession of 
meteors in periodic star-showers, and comparatively little to inability to note 
and appreciate small intervals of time. The best time for noting the duration 
(as well as the magnitude and colour) is while fixing with the eyes the posi- 
tion of the path just seen, often marked for some time after the meteor’s dis- 
appearance by the persistent streak among the stars; and it can then easily 
be borne in mind, and presently afterwards recorded. Stop-watches, how- 
ever, or chronographs of the best description, must be resorted to if results of 
the most reliable character only are desired to be obtained. 
It may be added that if the visual radiant-point and the real height in 
miles at disappearance (/ miles) are determined, and the following particulars 
of a meteor’s apparent course are taken from a single observer’s description 
of its apparent path, viz. the distance from the radiant-point in degrees (d) 
of the point of commencement, the altitude in degrees from the horizon (a) 
of the point of extinction, the length (7) in degrees of the apparent path, as 
well as the time of flight in seconds (s), then the real length of path (L) in 
miles is L=A i and the real velocity in miles: per second (V) is 
- _ sin a. sin ad 
Vv = = For many meteors of a shower, like the Perseids, from a 
s sina.sind 
single radiant-point, an average value of h, about 52 miles, may be assumed ; 
‘and an average real velocity of the meteors of the shower may then be 
obtained by the last of these expressions from careful observations, by a 
single observer only, of their times of flight or durations and apparent paths. 
