On Thursday, August 27, 1778, Captain Cook was in the Chukchi Sea, near the coast of Siberia, at about 69°N. He wrote, “Having but little wind, I went with the boats to examine the state of the ice”.1 There follows a long description of the ice, including several ice floes whose sides had been eroded by waves, leaving an underwater shelf that extended out beyond the sidewalls of the floe, “exactly like a shoaled round an elevated rock. We measured the depth of Water upon one and found it to be 15 feet, so that the ships might have sailed over it. If I had not measured the depth of Water, I would not have beleived that there was a sufficient weight of ice above the surface to have sunk the other so much below it. The annexed drawing is a view of one of these pieces of ice and will ilustrate what I have said on the subject”.2
Which led me to wonder, where is the “annexed drawing”?
I work at the Polar Science Center, University of Washington, Seattle, and in November 2012, I undertook some research into the above question. My conclusions were that the annexed drawing
This research led me to having an e-mail exchange with Chris Beckett, the Curator of Historical Papers at the British Library, London. He told me I could order a scan of the original handwritten pages of Cook's journal from August 27, 1778, through the British Library Imaging Services. The relevant folio references are f. 526 and f. 527. He apologised for not “trawling through the rather unwieldy original drawings we have” but did check at the back of the Cook volume. Unfortunately, “there is no drawing hiding there”.
I returned to Beaglehole, and to his footnote for the entry.10 It is an entry from Charles Clerke’s journal of August 28 describing the same ice floe that Cook had described. Clerke gives more measurements.
Above is a drawing of an idealized ice floe with Clerke’s dimensions. I have taken the exposed portion of the ice floe to be circular with radius six feet, giving a circumference of 12π feet (=37.7 ft.), which does not exceed 40 feet. I have taken the height of the ice surface above the water line to be four feet, and have labeled the width of the under-water projecting part as w, with the further assump-tion that it encircles the entire floe. The thickness of the projecting part is taken to be four feet.
Assuming the ice floe is in hydrostatic equilibrium, it is a simple matter to calculate the width w, which I did in my notes of December 5, 2012. The answer is w = 5 feet 9 inches, a reasonable value. (I assumed a water density of 1.03 grams/cm3 and an ice density of 0.91 grams/cm3).
Also in the same footnote, Beaglehole wrote, “This sort of underwater platform is characteristic of old ice. Cook’s hypothesis of ‘Wind, or rather Waves’ grinding, undermining, and washing away does not allow enough for the melting and weakening of the ice during the summer months. But he was not in error in thinking that the ice was the product of more than one winter”.
While it is true that there is melting and weakening of the ice during the summer months, as Beaglehole pointed out, that by itself does not explain the existence of underwater shelves projecting from ice floes—but waves do.
The annexed drawing appears to be lost, though I am hoping it is somewhere in the “unwieldy” collection at the British Library.
My thanks to Dave Nicandri and Robin Inglis for introducing me to the Cook literature.
Originally published in Cook's Log, page 16, volume 40, number 2 (2017).
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