in the Bay of
Fundy. Here the Atlantic passes into a long channel whose sides
gradually converge. When the great pulse of the tide rushes up this
channel, it is gradually accumulated into a mighty volume at the
upper end, the ebb and flow of which at spring tides extends through
the astonishing range of not less than fifty feet.

These discrepancies between the tides at different places are chiefly
due to the local formations of the coasts and the sea-beds. Indeed, it
seems that if the whole earth were covered with an uniform and deep
ocean of water, the tides would be excessively feeble. On no other
supposition can we reasonably account for the fact that our barometric
records fail to afford us any very distinct evidence as to the
existence of tides in the atmosphere. For you will, of course,
remember that our atmosphere may be regarded as a deep and vast ocean
of air, which embraces the whole earth, extending far above the
loftiest summits of the mountains.

It is one of the profoundest of nature's laws that wherever friction
takes place, energy has to be consumed. Perhaps I ought rather to say
transformed, for of course it is now well known that consumption of
energy in the sense of absolute loss is impossible. Thus, when energy
is expended in moving a body in opposition to the force of friction,
or in agitating a liquid, the energy which disappears in its
mechanical form reappears in the form of heat. The agitation of water
by paddles moving through it warms the water, and the accession of
heat thus acquired measures the energy which has been expended in
making the paddles rotate. The motion of a liquid of which the
particles move among each other with friction, can only be sustained
by the incessant degradation of energy from the mechanical form into
the lower form of diffused heat. Thus the very fact that the tides are
ebbing and flowing, and that there is consequently incessant friction
going on among all the particles of water in the ocean, shows us that
there must be some great store of energy constantly available to
supply the incessant draughts made upon it by the daily oscillation of
the tides. In addition to the mere friction between the particles of
water, there are also many other ways in which the tides proclaim to
us that there is some great hoard of energy which is continually
accessible to their wants. Stand on the bank of an estuary or river up
and down which a great tidal current ebbs and flows;