PROBABILITY OF EXTRAORDINARILY HIGH SPRING-TIDES. 359 
On THE PROBABILITY or EXTRAORDINARILY HIGH 
SPRING-TIDES aspour tHE DECEMBER 
SOLSTICE or 1893. 
By Joun TEBBUTT, F.B.AS,, &e. 
[Read before the Royal Society of N. S. Wales, November 1, 1893 | 
A BRIEF paper by me on the High Tides of June 15 - 17th 1889, 
was read before the Royal Society on July 3rd of that year, and 
published in the twenty-third volume of the Society’s Journal. It 
treated of the extraordinary tides which occurred at and near 
Sydney, at the period referred to, and of the astronomical con- 
ditions which combined to produce them. It also referred to 
another instance of the registration of a very high tide by the 
Fort Denison gauge on May 26, 1880, and stated the astronomical 
conditions to which that tide was also due. During a few 
moments of leisure from my ordinary observatory work, my 
attention was drawn to a consideration of this interesting subject, 
and as a result, I found that the approaching summer solstice 
would present conditions much more favourable for the production 
of high tides than even those referred to in my former paper. 
It is well known that there are two astronomical conditions 
which are especially favourable for the production of high tides, 
namely, the conjunction or opposition of the moon and her passage 
through her perigee. Speaking of the earth generally, we expect 
that if the new or full moon occur simultaneously or nearly so 
with her perigeal passage, there will be an unusually high tide. 
But if we speak of Sydney in particular, there is another condition 
which sometimes coincides with those just mentioned, and which 
has a very marked effect on the magnitude of the tide wave. I 
refer to the moon’s maximum declination. When I treated of 
the conditions which combined to produce the tidal phenomena 
that formed the subject of my former paper, I showed the close 
