29 
It is difficult to explain in a few words what is meant by the 
dynamical theory, but speaking generally the position of the tidal 
cone according to this theory depends mainly upon the rotation of 
the earth upon its axis, and the cone of water would be in great part 
due to a heaping up of the water owing to centrifugal force and 
would of necessity travel along lines parallel to the equator. In 
contradistinction to this the equilibrium theory, as already men- 
tioned, demands that the cone of water is drawn up by the gravita- 
tional attraction of the moon and sun. 
This main distinction between the two theories must be carefully 
borne in mind, especially in the case of the Port Hedland tides, so 
that the arguments in favour of the equilibrium theory may be 
appreciated when these tides are examined in detail. 
A glance at Plate VIII., Fig. 1, entitled “Diagram explaining 
Diurnal Irregularity/’ will make one of the arguments in favour of 
this theory quite clear. The central circle represents the earth and 
the shaded area an imaginary shell of water — enormously exagger- 
ated in proportion to the size of the earth. The moon is supposed to 
be at M, at its furthest distance from the equator and the maximum 
bulge of water is occurring under and opposite to M. Now it can 
be readily seen that a port at H where a high tide is taking place, 
will be carried by the earth’s rotation on its axis, SN, through a low 
water at L to another high water at H 1 and here, owing to its being- 
nearer to the major axis of the ellipsoidal shell of water, the tide 
is higher than at H. As the moon approaches the equator, it carries 
the tide-bulge with it and the difference in height between the H 
and H 1 becomes less and less, until at the equator it vanishes. It 
should be further noticed that as the moon travels to the other side 
of the equator, the H tide becomes higher than the H 1 tide. When 
the tides of Port Hedland are examined it will be found that their 
behaviour can be explained by reference to this diagram, and to a 
lesser extent the action of the Fremantle tides. So much by way of 
introduction. 
Before, however, discussing in detail the Fremantle and Port 
Hedland tides, a brief reference to the history of tidal observations 
in Western Australia will not be out of place. 
Tidal Observations in Western Australia. 
Probably on no coast in the world is there less exact knowledge 
of the tides than on the coast -line extending from Wyndham in the 
far north to Eyre in the extreme south-east, and therefore it will 
not be surprising to learn that satisfactory records have been taken 
at only one port, namely Fremantle. 
/ As far back as 1873 the Admiralty recognised the importance 
of a knowledge of the tides on the Western Australian coast and 
caused a series of observations to be carried out at Fremantle. The 
work was. performed chiefly by «Staff-Commander Archdeacon, R.N., 
the officer in charge of the Admiralty Survey of the coast and per- 
