elliptical as shown
in Fig. 107. It was no longer free to escape and go away into the depths
of space: it was enchained and made a member of the solar system. It
also ceased to be a comet; it was degraded into a shoal of meteors.

This is believed to be the past history of this splendid swarm. Since
its introduction to the solar system it has made 52 revolutions: its
next return is due in November, 1899, and I hope that it may occur in
the English dusk, and (see Fig. 97) in a cloudless after-midnight sky,
as it did in 1866.

NOTES FOR LECTURE XVII

The tide-generating force of one body on another is directly as the mass
of the one body and inversely as the cube of the distance between them.
Hence the moon is more effective in producing terrestrial tides than the
sun.

The tidal wave directly produced by the moon in the open ocean is about
5 feet high, that produced by the sun is about 2 feet. Hence the average
spring tide is to the average neap as about 7 to 3. The lunar tide
varies between apogee and perigee from 4.3 to 5.9.

The solar tide varies between aphelion and perihelion from 1.9 to 2.1.
Hence the highest spring tide is to the lowest neap as 5.9 + 2.1 is to
4.3 -2.1, or as 8 to 2.2.

The semi-synchronous oscillation of the Southern Ocean raises the
magnitude of oceanic tides somewhat above these directly generated
values.

Oceanic tides are true waves, not currents. Coast tides are currents.
The momentum of the water, when the tidal wave breaks upon a continent
and rushes up channels, raises coast tides to a much greater height--in
some places up to 50 or 60 feet, or even more.

Early observed connections between moon and tides would be these:--

1st. Spring tides at new and full moon.

2nd. Average interval between tide and tide is half a lunar, not a
solar, day--a lunar day being the interval between two successive
returns of the moon to the meridian: 24 hours and 50 minutes.

3rd. The tides of a given place at new and full moon occur always
at the same time of day whatever the season of the year.

LECTURE XVII

THE TIDES

Persons accustomed to make use of the Mersey landing-stages can hardly
fail to have been struck with two obvious phenomena. One is that the
gangways thereto are sometimes almost level, and at other times very
steep; another is that the water often rushes past the stage rather
violently, sometimes south towards Garston, sometim