undamental law governing tides.

There are many other conditions that modify tidal effects. Neither the
sun nor the moon is always at the same distance from the earth. So that
there will be a variation at times in high and low tides. For instance,
it will happen sometimes that when both the sun and moon are acting
conjointly they will both be at their nearest point to the earth, and
when this is the case the spring tide will be much higher than usual.

For many years the writer has observed that artesian wells, made by deep
borings of small diameter into the earth to a water supply, have a daily
period of ebb and flow, as well as a neap and spring tide, the same as
the tides of the ocean, except that the process is reversed. The time of
greatest flow of an artesian well will occur at low tide in the ocean.
This might be accounted for from the fact that when the tide is at its
height the moon is also pulling upon the crust of the earth, which would
tend to take the pressure off the sand rock which lies one or two
thousand feet below the surface and through which the flow of water
comes, and thus slacken the flow. When the moon is in position for low
tide, the crust of the earth would settle back and thus produce a
greater pressure upon the water-bearing rock. This is the only theory
that has suggested itself to the writer that would seem to account for
these phenomena.

Looked at from one standpoint, it is easy to account for tidal action.
But when we attempt to make up a table giving the hour and minute as
well as the height of the tide at that particular time we find that we
have a very complicated mathematical problem. However, tables are made
out so that we know at just what time in the day a tide will occur every
day in the year.

CHAPTER XXI.

WHAT IS A SPONGE?

Before entering upon the great subject of water and ice--two of the most
tremendous factors in world-building--let us consider a small matter, so
far as its permanent effects are concerned, yet one which enters largely
into the comfort and health of mankind, and which, though an animal, may
be discussed where it belongs--under "Water."

There are few things more familiar about the ordinary household than a
piece of sponge, and yet, perhaps, there are but few things about which
there is so little known. The sponge had been in use many, many years
before it was given a place in either the animal or vegetable kingdom.
The casual observer, becau