drocarbon gas, and iron vapour, and
it is for this reason that Bredichin suggested the above-mentioned
composition of the various types of tail. Spectroscopic evidence of the
presence of hydrogen is yet wanting.

[34] This illustration, as well as the figure of the path of the
meteors, has been derived from Dr. G.J. Stoney's interesting lecture on
"The Story of the November Meteors," at the Royal Institution, in 1879.

[35] On the 27th November, 1885, a piece of meteoric iron fell at
Mazapil, in Mexico, during the shower of Andromedes, but whether it
formed part of the swarm is not known. It is, however, to be noticed
that meteorites are said to have fallen on several other occasions at
the end of November.

[36] Hooke had noticed, in 1664, that the star Gamma Arietis was double.

[37] Perhaps if we could view the stars without the intervention of the
atmosphere, blue stars would be more common. The absorption of the
atmosphere specially affects the greenish and bluish colours. Professor
Langley gives us good reason for believing that the sun itself would be
blue if it were not for the effect of the air.

[38] The declination of a star is the arc drawn from the star to the
equator at right angles to the latter.

[39] The distance of 61 Cygni has, however, again been investigated by
Professor Asaph Hall, of Washington, who has obtained a result
considerably less than had been previously supposed; on the other hand,
Professor Pritchard's photographic researches are in confirmation of
Struve's and those obtained at Dunsink.

[40] I am indebted for this drawing to the kindness of Messrs. De la
Rue.

[41] _See_ Chapter XIX., on the mass of Sirius and his satellite.

[42] As the earth carries on the telescope at the rate of 18 miles a
second, and as light moves with the velocity of 180,000 miles a second
very nearly, it follows that the velocity of the telescope is about one
ten-thousandth part of that of light. While the light moves down the
tube 20 feet long, the telescope will therefore have moved the
ten-thousandth part of 20 feet--_i.e._, the fortieth of an inch.

[43] _See_ Newcomb's "Popular Astronomy," p. 508, where the discovery of
this law is attributed to Mr. J. Homer Lane, of Washington. The
contraction theory is due to Helmholtz.

[44] The theory of Tidal Evolution sketched in this chapter is mainly
due to the researches of Professor G.H. Darwin, F.R.S.

[45] The hour varies with the locality: it wo