Rev Borlase on the Cornwall Tsunami of 1761

As many people know, there have been a few tsunamis in the history of Cornwall. The most widely reported of these was the Cornwall tsunami of 1755, caused by an earthquake at Lisbon. The Rev William Borlase, MA, FRS (1695-1772) was a Cornish geologist, antiquary and naturalist, who wrote extensively about Cornwall during his lifetime and he also wrote of other tsunamis caused by earthquakes at Lisbon in subsequent years.

The two further tsunamis in Cornwall, both occurred in 1761, one in March and another in July.  Both these events were reported to The Royal Society at the time.  Below are extracts from the letters written by Rev Borlase: Cornish Tsunami, Rev Borlase

Cornwall Tsunamis in 1761

Tsunami 31 March 1761

“March 31, 1761, about 5 in the afternoon, there was an uncommon motion of the tide in Mount’s Bay, Cornwall. It was full sea that day about half an hour after 12. After the tide had ebbed about 4 hours and half, instead of continuing to retreat gradually, as usual, till it had completed the 6 hours ebb, on a sudden it advanced as it is usually at the time of the moon, at an hour and half before high water. It then retreated ear to the point of low water, then it advanced again, and retreated, making 5 advances, and as many recesses, in the space of one hour; viz. from about 5 to 6 o’clock; which was the whole time that these uncommon stretches of the tide continued. But the first motion was most considerable, the sea advancing the first time to a quarter ebb; whereas the 2nd advance was but as far as the sea reaches at half ebb. At the first surge the waters rose at this place 6 feet perpendicular. At the pier of St. Michael’s Mount, 3 miles to the east of Penzance, the tide was observed, at the same time, to rise and fall about 4 feet. At Newlyn, a mile west of Penzance, the tide rose to the same height nearly as at Penzance. At Mousehole pier, 3 miles s.w. of Penzance, it was only observed that the sea was in great agitation, and the fishing-boats in danger. At the islands of Scilly, the sea was judged to rise about 4 feet; but the agitation continued longer than in Mount’s bay, viz. more than 2 hours.”

Tsunami 28 July 1761

This letter was written by Rev Borlase to the Rev Thomas Birch, D. D., who was Secretary to the Royal Society:

“On Tuesday, July 28, 1761, the day quite calm, the sky lowering and cloudy, thunder at times all day, the tide in Mount’s bay was considerably agitated. Between Penzance and Marazion, there is a level of sands, on which there is good travelling when the tide is out; but when the tide is full, the sands are covered. At 10 a.m. the driver of a plough, belonging to William Tregennin, laden with tin, for Penzance coinage, driving as usual on the then bare sands, found himself and the plough on a sodden surrounded by the sea. The horses were frightened and plunged, the oxen stood still, the driver and his boy could neither recollect how they should help the cattle or secure themselves; several people saw them at a distance, but dared not to approach; and in a few minutes when all was given up for lost, the sea retired and left them, safely to pursue their journey. Mr. B. came to Chandour, a small village in the western extremity of these sands, about 11, and found several persons standing on the shore, intent on the several extraordinary fluxes and refluxes of the tide at that time, and was informed, that the first agitation, when the plough was surprised by the sea, the water must have risen about 6 feet perpendicular. During his stay he observed the sea flowing and retreating several times, and by his watch it was 7 minutes flowing, the water rising about a foot and half, or somewhat more, and the like time nearly in retiring. About half past 11 he was obliged to more homewards, and he passed by the brim of the water, observed that the sea advanced and retired, and was not settled; but the alterations were then small, and scarcely perceptible In the more western parts of this bay, the agitations were very apparent; and, by the papers, the like agitations were felt in the harbours of Falmouth, Fowy, and Plymouth.”

This latter report was also published in The London Magazine, or Gentleman’s Monthly Intelligence in 1763.

Image: P11061

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