a
composition of terrestrial with meteoric motion. Hence, unless that of
the earth in its orbit be by comparison insignificant, the visual line
of encounter must shift, if not perceptibly from day to day, at any
rate conspicuously from month to month. The fixity, then, of many
systems observed by Mr. Denning seems to demand the admission that
their members travel so fast as to throw the earth's movement
completely out of the account. The required velocity would be, by Mr.
Ranyard's calculation, at least 880 miles a second.[1248] But the
aspect of the meteors justifies no such extravagant assumption. Their
seeming swiftness is very various, and--what is highly significant--it
is notably less when they pursue than when they meet the earth. Yet
the "incredible and unaccountable"[1249] fact of the existence of
these "long radiants," although doubted by Tisserand[1250] because of
its theoretical refractoriness, must apparently be admitted. The first
plausible explanation of them was offered by Professor Turner in
1899.[1251] They represent, in his view, the cumulative effects of the
earth's attraction. The validity of his reasoning is, however, denied
by M. Bredikhine,[1252] who prefers to regard them as a congeries of
separate streams. The enigma they present has evidently not yet
received its definitive solution.

The Perseids afford, on the contrary, a remarkable instance of a
"shifting radiant." Mr. Denning's observations of these yellowish,
leisurely meteors extend over nearly six weeks, from July 8 to August
16; the point of radiation meantime progressing no less than 57 deg.
in right ascension. Doubts as to their common origin were hence freely
expressed, especially by Mr. Monck of Dublin.[1253] But the late Dr.
Kleiber[1254] showed, by strict geometrical reasoning, that the
forty-nine radiants successively determined for the shower were all, in
fact, comprised within one narrowly limited region of space. In other
words, the application of the proper correction for the terrestrial
movement, and the effects of attraction by which each individual
shooting-star is compelled to describe a hyperbola round the earth's
centre, reduces the extended line of radiants to a compact group, with
the cometary radiant for its central point; the cometary radiant being
the spot in the sky met by a tangent to the orbit of the Perseid comet
of 1862 at its intersection with the orbit of the earth. The reality of
the connection between th