The explorers had ample opportunity for taking the longitude and latitude, which was the more important, as no European vessel had navigated these seas, which were only indicated upon the maps in accordance with the Chinese and Japanese maps published by the Jesuits.
Upon the 25th of May the frigates entered the channel of Corea, which was minutely explored, and in which soundings were taken every half hour.
As it was possible to keep close in shore, it was easy to observe some fortifications in the European style, and to note all their details.
On the 27th an island was perceived which was not to be found upon any map, and which seemed to be about twenty leagues distant from the coast of Corea. It received the name of Dagelet Island.
The course was now directed towards Japan, but it was very slow, on account of the contrary winds that prevailed.
On the 6th of June Cape Noto and the island of Tsus Sima were discovered.
"Cape Noto, upon the Japanese coast," says La Perouse, "is a point on which geographers may rely. Reckoning from it to Cape Kona on the eastern coast, the position of which was determined by Captain King, the width of the northern half of the empire may be ascertained. Our observations have the greater value for geographers as they determine the width of the Gulf of Tartary, to which I now directed my course."
Upon the 11th of June La Perouse sighted Tartary. He made land precisely at the boundary between the Corea and Manchuria. The mountains appeared to be six or seven thousand feet high. A small quantity of snow was visible on the summits. No trace of inhabitants or cultivation could be seen; nor was any river's mouth found upon a length of coast extending for forty leagues. A halt would have been desirable, to enable the naturalists and lithologists to make observations.