PRESIDENT’S ADDRESS. 59 
sway, we may expect that the wave produced will be a forced 
wave, travelling round the earth with the same speed as the 
controlling forces. If we have an ocean in which such a 
forced wave occurs. and connected to it are other seas in 
which no forced wave occurs, it is clear that when the forced 
wave passes the entrance of a connecting sea it wiil cause a 
free wave to be propagated along that sea, with a velocity 
depending on the depth of the water. Now the only place 
on the earth’s surface where the tidal forces have uninter- 
rupted sway is in the Southern and Southern Pacific Oceans. 
There only is it possible for a wave to go right round the 
earth without being interrupted by land. Such a wave, 
when it came to the Southern Atlantic, would tend to spread 
itself, and to travel up the Atlantic as a free wave. But 
if the Atlantic were land-locked, we should have tidal oscil- 
lations generated in it. Thus it comes about that the actual 
tidal effect in the North Atlantic is a combination of the 
true forced Atlantic tide with a free tidal wave which travels 
up from the Southern Ocean. In smaller seas the free wave 
which has travelled from the great connecting oceans consti- 
tutes the principal effect observed. Thus it comes about 
that we have tidal waves proceeding in every direction. 
They may run N.toS.orS.toN., E.to W.,as when approach- 
ing the eastern coast of Australia, or W. to E., as along a 
great portion of the northern coast. In the course of their 
travelling these free tidal waves, especially when they travel 
through shallow channels, undergo remarkable changes, of 
the exact nature of which we know very little. The loss of 
energy owing to friction will, however, be greater in those 
components in which the particles move most. rapidly.. We 
may therefore expect the semi-diurnal waves to be more 
diminished than the diurnal and long-period, and the semi- 
diurnal waves of large amplitude to be reduced more in pro- 
portion than semi-diurnal wavesof smallamplitude. ‘Thus the 
tendeney is, when the tidal wave has travelled freely for 
great distances, over comparatively shallow water, for the , 
semi-diurna]l components to become small and more and 
more nearly equa! co one another, while the diurnal waves, 
not losing by friction to the same extent, Cee relatively 
large. 
We might be inelined to expect that the relative sizes of 
the principal sotar and lunar semi-diurnal waves would 
always be pretty weil the same, or, at any rate, that the 
lunar tide would always be greater than the solar. Analysis 
shows, however, that this is by no means the case. Thus at 
‘Pert Adelaide and Fremantle the semi-diurnal waves pro 
