"Stuart" wrote in message
k...
When did everybody start calling a tidal wave a tsunami and why?

When they learnt the difference. A tidal wave is something
completely
different to a tsunami, but a lot of people don't seem to realise
this.


If you were to crack a book on hydrodynamics you will find that there
are essentially four different types of wave in an ideal non-viscous
liquid

Capillary waves
Surface waves
Deep water waves
Solitons

(actually, there are also linear combinations of the first two)

Capillary waves are what you get if you induce a wave by vibrating a
tuning fork witih one tine just touching the liquid
Surface waves are what you normally see on the surface of the ocean
before they break near the shore. That includes the so-called "Giant
waves". These are simply distinguished by having a very large
amplitude of oscillation. In both those types of wave any "piece" of
water moves vertically up and down.
In the case of a deep water wave, any *piece" of water executes a
circular motion, with the diameter of the circle equal to the depth of
the water.
Both tidal waves and tsunamis are synonyms for deep water waves.
In the open ocean with a constant depth, a deep water wave is
essentially harmless and may pass practically unnoticed. It only
becomes dangerous when it meets a sloping ocean floor, so that the
nornal circular motion can no longer be accomodated. It is at that
stage that it begins to "suck" in water ahead of it and begins to pile
up in height.

Solitons are peculiar beasts in which certain specifically-shaped
transient surface disturbances can move unchanged in shape.

Franz