28 
THE TIDES : WITH SPECIAL REFERENCE TO THOSE OF 
FREMANTLE AND PORT HEDLAND. 
By 
H. B. Curlewis, B.A., F.R.A.S., Acting Government Astronomer. 
Introduction and Scope of the Paper. 
(Read Oct. 13, 1914.) 
A short summary of what was known about the tides in early 
times and a brief reference to the main theories that have been 
advanced to account for their behaviour will be given at the outset 
of this paper in order that there may be a proper understanding 
of some of the points which are discussed in connection therewith. 
As far back as the first century B.C., it was realised that there 
was a dependence between the moon and the tides, but the reason 
for this connection was not known until the seventeenth century, 
when Sir Isaac Newdon explained it as due to the force of gravita- 
tion exercised by the moon upon the waters of the ocean, and later 
on postulated that the sun must exert a similar influence. 
Naturally Newton was not satisfied with the bare statement of 
this fact but elaborated a mathematical theory which is known as 
the “Equilibrium Theory/’ It demands, in brief, that the tidal cone 
of water should be under the moon — reference to the sun is omitted 
because mention of it only complicates matters and makes a simple 
explanation impossible — but the sun’s attraction has to be considered 
equally with the moon’s. It was called the Equilibrium Theory be- 
cause the moon is supposed to act for an appreciable time upon any 
single part of the ocean and therefore that portion might be treated 
as if it were at rest or in a state of equilibrium. It is rather peculiar 
that the most trustworthy records then available, namely those of 
Cadiz, clearly place the high water under the moon. However, 
when Newton came to investigate the tides of the English coasts, 
he found so many discordances to this necessary postulate that he 
became dissatisfied with and mistrustful of his original theory, 
though he never lived to make any material alteration to it. About 
a hundred years later, however, Laplace formulated his famous 
Dynamical Theory which, as far as the then known tides were 
concerned, brought about satisfactory harmony between theory and 
actual fact. And yet, had tidal records from America, India, and 
the southern hemisphere been available, it is highly probable that 
the dynamical theory w T ould never have been evolved, for the great 
majority of these tides do not conform to it. Whereas the equilibrium 
theory does offer a reasonable explanation of what actually takes 
place or at any rate enormously reduces the number of anomalies. 
