35 
masses of the two bodies. Full information on this point and many 
others dealing with the tides can be obtained from almost any text- 
book on the tides, and perhaps for preference The Tides, by G. 
SH. Darwin. Without, however, going into these points it will be 
-seen from the curves that at new and full moon when the sun and 
moon are in line, the maximum tides occur and are known as 
[“Springs”; while at first and last quarter the tidal range is only 
•about one-third that of the springs, and these are called “Neaps.” 
Under ideal conditions this would probably apply but in practice, 
;as in the Port Hedland tides, the spring and neap tides occur one 
«or two days after the changes of the moon. Reference has already 
been made to good examples of spring and neap tides in October 
< (vide Plate IX., Fig. 2, and Plate X.). At the time of the Equinoxes 
the highest springs and the lowest neaps of the year will generally be 
found. The ready explanation being that the sun is then on the equa- 
tor and consequently the new and full moon must be on the equator 
;also, and thus towards the end of March and September the highest 
tides may be looked for simply because the two great tide-generating 
bodies are exerting their respective pulls along exactly the same line. 
So far these tidal curves have borne out what would be expected 
to take place according to the Equilibrium Theory except that the 
height of the tide is considerably greater than what would be looked 
for under ideal conditions. There are, however, a number of factors 
which singly or collectively would account for this apparent anomaly. 
The chief of which would be, firstly, the depth of water; secondly, 
the speed of the wave — this is simply a corollary of the first ; thirdly, 
the configuration of the coastline, and fourthly, the direction of ocean 
currents caused by the earth’s rotation and by the distribution set 
up by the difference in temperature between equatorial and polar 
waters. (There are probably other causes, but these must suffice.) 
Now it is a fact well known to everyone that an ordinary ocean 
roller gradually becomes steeper as it approaches a shore which 
shelves upward — Cottesloe Beach for instance. And so with the tide- 
wave: in the open ocean it has a maximum of about three feet, but 
when the ocean bed shelves upwards, then the height of the tide- 
wave is increased. The shallow, shelving ocean floor along the North- 
West coast is undoubtedly the main cause of the high tides there. 
At many places the height is still further augmented by the shape 
of the coast-line, as for instance at Derby, situated at the head of 
King Sound, where tides of 36ft. occur. However, there is not 
sufficient data available to adequately discuss the reasons for the 
high tides along the North-West and Kimberley coasts. 
Returning once again to the tide-curve for June, at first glance 
the crest of the wave or high water comes fairly well under tlfe 
moon, namely, coincides with the moon’s transit at new and full, as 
the equilibrium theory demands, and gradually lags behind as first 
and last quarter is approached. A closer inspection shows that the 
; high water is twelve to nine hours after the moon’s transit. If 
(3) 
