Some of you may remember the entertaining engagement I had with Dr John Hunter of the University of Tasmania, regarding his controversial reanalysis of the tide register of Thomas Lempriere in the 1840s.
Lempriere had, under the instruction of James Clark Ross, made a mark in a rock face at the Isle of the Dead, near Port Arthur in what is now Tasmania.
According to Lempriere, he made the mark at the “mean level of the ocean”. According to James Clark Ross in his sea log, Lempriere made the mark at the mean point of the ocean.
That mark is now above the mean level of the ocean.
According to John Hunter (pictured below with the mark), this is because Lempriere actually put the mark above high tide so as to avoid too much wave erosion.
After a lot of banter, John Hunter posted data that he and colleague Coleman had made in 2001-2004 by putting an electronic tide gauge in the same harbour as Lempriere, and recorded the sea level.
Here is the plot of the data (time axis in days from the beginning of the experiment, y-axis is height in mm above the mean):
The tide gauge recorded the water level every six minutes, but measured downwards to the level of the sea. In order to return the data to the more normal orientation, I computed the mean of the data, and then subtracted each data point from the mean.
Since I had a gap and intended to use Fourier Analysis, I truncated the data to just after the gap:
The Fourier Analysis was done using MathCAD 14.0 and the results are as follows:
The discrete frequencies correspond to the following resonances (all except ‘X’)
|Principal lunar semidiurnal||M2||12.42|
|Principal solar semidiurnal||S2||12|
|Larger lunar elliptic semidiurnal||N2||12.66|
|Larger lunar evectional||ν2||12.63|
|Lunar elliptical semidiurnal second-order||2″N2||12.91|
|Smaller lunar elliptic diurnal||M1||24.84|
|Smaller lunar elliptic diurnal||J1||23.1|
|Larger lunar evectional diurnal||ρ||26.72|
|Larger lunar elliptic diurnal||Q1||26.87|
However the resonance marked as “X” does not correspond to any astronomical forcing, and is distinct from the Solar Diurnal S1 as can be seen from the following close-up.
So I surmise that X corresponds to the tidal constant of the estuary. Apparently in that locale with the currents, sea floor geometry and wind directions, the sea washes in and out of the harbour every 23 hours and 37 minutes.
All of this was unknown to Thomas Lempriere, of course. The tidal constant being so close to one of the main astronomical forcings caused some very peculiar tide behaviour.
When I’ve analyzed Lempriere’s data from 1840-1842, we may be able to see how well Lempriere managed to record that data in the light of modern measurements.
More to follow.