Maury’s other mistake was even more important so far as this question of an open sea is concerned. “No one,” as I wrote in 1867, “who is familiar with the astronomical doctrine of the tides, can believe for a moment that tides could be generated in a land-locked ocean, so limited in extent as the North Polar sea (assuming its existence) must necessarily be.” To raise a tidal wave the sun and moon require not merely an ocean of wide extent to act upon, but an ocean so placed that there is a great diversity in their pull on various parts of it; for it is the difference between the pull exerted on various parts, and not the pull itself, which creates the tidal wave. Now the Polar sea has not the required extent, and is not in the proper position, for this diversity of pull to exist in sufficient degree to produce a tidal wave which could be recognized. It is certain, in fact, that, whether there is open water or not near the Pole, the tides observed by Kane and Hayes must have come from the Atlantic, and most probably by the North Atlantic channel. Captain Hall’s expedition in the _Polaris_ (really under the command of Buddington), in 1871–72, will be probably in the recollection of most of my readers. Leaving Newfoundland on June 29, 1871, it sailed up Smith’s Sound, and by the end of August had reached the 80th parallel. Thence it proceeded up Kennedy Channel, and penetrated into Robeson Channel, the northerly prolongation of Kennedy Channel, and only 13 miles wide. Captain Hall followed this passage as far as 82° 16´ north latitude, reaching his extreme northerly point on September 3. From it he saw “a vast expanse of open sea, which he called Lincoln Sea, and beyond that another ocean or gulf; while on the west there appeared, as far as the eye could reach, the contours of coast. This region he called Grant Land.” So far as appears, there was no reason at that time why the expedition should not have gone still further north, the season apparently having been exceptionally open. But the naval commander of the expedition, Captain Buddington, does not seem to have had his heart in the work, and, to the disappointment of Hall, the _Polaris_ returned to winter in Robeson Channel, a little beyond the 81st degree. In the same month, September, 1871, Captain Hall died, under circumstances which suggested to many of the crew and officers the suspicion that he had been poisoned.[23] In the spring of 1870 the _Polaris_ resumed her course homewards. They were greatly impeded by the ice. A party which got separated from those on board were unfortunately unable to regain the ship, and remained on an ice-field for 240 days, suffering fearfully. The ice-field, like that on which the crew of the _Hansa_ had to take up their abode, drifted southwards, and was gradually diminishing, when fortunately a passing steamer observed the prisoners (April 30, 1872) and rescued them. The _Polaris_ herself was so injured