By Henry C. Otter, Esq., R.N., Captain of H.M.S. Porcupine.
The Edinburgh New Philosophical Journal Volume 7 1858 p272-276
My interest in the Sound of Harris is evidenced elsewhere in my writings so it will come as no surprise to learn that I was delighted to discover this piece on the peculiarity of one particular aspect of that stretch of water.
The basic principles governing the tides were well-known in the mid-nineteenth century but the details regarding their variation were far from a complete understanding. Indeed, I read an extract from a paper of 1996 that indicates that, in several regards, our comprehension is fuller but remains far from being total.
Tides.—The law of the tidal stream in the Sound of Harris is very remarkable…
It may be generally stated, that in summer, in neap tides, the stream comes from the Atlantic during the whole of the day, and from the Minch during the whole of the night.
In winter, the reverse takes place, the Minch stream flows during the day, the Atlantic during the night.
‘Neap’ refers to tides where the tidal range between high-water and low-water is small. They occur in the week leading to a Full Moon and the week leading to a New Moon.
In spring tides, both in summer and winter, the stream comes in from the Atlantic during the greater part of the time the water is rising, but never exceeds 5¼ hours, and flows back into the Atlantic during the greater part of the fall of the tide.
‘Spring’ refers to tides where the tidal range between high-water and low-water is large. They occur in the week following a Full moon and the week following a New Moon.
The stream from the Atlantic is therefore denominated the flood stream, that from the Minch the ebb stream.
Captain Otter then proceeds to give full details of the variations in the tidal flows according to the season, the particular part of the Sound and the strength and direction of the wind. It is not the details themselves that need concern us, we merely note that they were observed in all their complexity.
Following the article itself there are appended the following Notes to Captain Otter’s Paper on the Tides in the Sound of Harris. By James Stark, M.D. F.R.S.E.
An interesting subject of inquiry is the probable cause of the flow of the current through the Sound of Harris.
To James Stark, a Fellow of the Royal Society of Edinburgh, a description in itself is insufficient for, as a man of Science, he seeks causes.
As the tidal wave in its progress from the south flows up both sides of the Western Isles, as far as the Sound of Harris, at the same time, so that at both the eastern and western extremity of the Sound the time of high-water is attained at the same hour, it is evident that the peculiar flow of the current through the Sound cannot be due to the tidal wave…
He then proposes a hypothetical cause for the effects that Captain Otter observed:
If we suppose that the sun exerts a strong attractive power over a large body of water like the Atlantic, which is undeniable, then we should expect that attraction to be greatest, and its effect in raising the level of the water most marked, when the sun was more immediately over that body of water.
This explains the variation between Summer and Winter for:
‘…the great mass of the Northern Atlantic in the same parallel of latitude as Harris, would have a higher level during the day in the summer months than it would have during the night when the sun’s attractive power was removed. As the Minch is, to a certain extent, a confined sea, the current from the Atlantic would, therefore, flow into it all day ; but when the level of the North Atlantic fell during the night, in consequence of the sun’s attractive power being removed, the current would flow from the Minch into the Atlantic. During winter, again, the sun’s rays being most powerful over the Southern Atlantic, as it is now to the south of the equator, the waters of the North Atlantic would be attracted southwards during the day, so that its level would be lower than that of the confined waters of the Minch. Consequently, during the winter months, we should expect that the stream would flow through the Sound of Harris from the Minch into the Atlantic all the day. When the sun’s attractive power, however, over the Southern Atlantic was removed during the night, the waters would fall to their level and allow the North Atlantic to regain its level; so that during the night the current during the winter season would flow through the Sound of Harris from the Atlantic.’
James Stark then extrapolates from these observations in the Sound of Harris and:
On the supposition that this explanation is the true one, it appears to me that it throws light on a phenomenon which has been long remarked, but never satisfactorily accounted for,—• viz., that during one period of the year the highest tides occur when the moon is above the horizon, but during the other half of the year when the moon is below the horizon. Now, if the moon be above the horizon during the summer when the level of the Atlantic is higher than usual from the greater attractive power of the sun, the day tide will be higher than the corresponding night tide. But if the moon be above the horizon during the day, when the Atlantic level is below its mean, as during winter, then the day tide will be lower than the corresponding night tide.
Which, if you think about it, is a pretty remarkable discovery stemming from observations in a humble little Sound in the furthest flung reaches of the British Isles!
He ends on an optimistic note hoping for further scientific evidence in support of his theory:
It would be interesting to ascertain, by actual measurements, whether there is any difference in the level of the waters in the Atlantic and Minch, and to what extent that difference exists during day and night, and during summer and winter; and I expect that this will be ascertained during the present year through the zeal of Captain Otter and Lieutenant Thomas, who are both engaged in the survey of the western coast.
One final observation of my own: Is this ‘Lieutenant Thomas…engaged in the survey of the western coast’ none other than the husband of the ‘Mrs Thomas’ who references claim to have been instrumental in the development of Harris Tweed, for she was the wife of a Captain Surveyor in the Navy and 1858 most certainly within the right time-frame?
Oh, and Captain Henry C Otter was one of 9 recipients of an Atlantic Cable Medal, First Class, which were presented on the completion of that first Transatlantic Telegraphy cable in 1858:
and lived in the Manor House, Oban, from 1845, long before he became an Admiral:
Update: I came across this interesting note in a modern account:
Navigational Point of Interest
The chart is interesting (Sound of Harris 2642) in that the lat and long lines are not drawn parallel to the edges of the chart but at about 45° – I can’t recall having used any other Admiralty charts with such an arrangement – but here it does get the various passages through the Sound on one chart. Another peculiarity concerns the tides. At springs the streams tend to run SE for the most part during the rising tide and SE during the falling tide, however, during the summer neaps there is a SE stream throughout the day and NW throughout the night whilst during winter neaps the opposite applies (NW during the day and SE during the night). As the streams may run up to 5 knots it’s worthwhile having a passage plan ready before you make for the channel.