How did Gulliver reach the floating island or describe the floating island?

In part 3, chapter 1 of Gulliver's Travels our hero has set sail once again, this time aboard the Hopewell. Unfortunately, the voyage turns out to be a bit of a disaster. First, the ship is blown off course by a storm. Then, to make matters worse, it's taken over by pirates and Gulliver is set adrift in a small canoe upon the ocean.
Eventually, Gulliver finds dry land, an island called Balnibarbi where he beds down for the night. The following morning, to his astonishment, a strange object appears in the sky. This is Laputa, a large, floating island whose movement is controlled by a magnet. After being winched up to the island, Gulliver is able to observe the size and dimensions of Laputa. It is entirely circular, with a diameter of about four and a half miles. At the center of the island is a chasm of about fifty diameters in which there is a large dome where astronomers study the heavens.
The Laputans like to study, and they're very good at math and music. But they're not in the least bit practical, devising useless experiments such as attempting to extract sunbeams from cucumbers. The fact that they literally live among the clouds, far removed from the world beneath, is symbolic. Swift is satirizing the tendency of scientists in his day to inhabit a world of rational speculation, far removed from the concerns of most people. In mocking the Laputans Swift isn't deprecating reason. He is simply saying that this isn't the sole means of getting at the truth as many scientists in his day believed, intoxicated as they were by the extraordinary discoveries that scientific pursuits had recently uncovered.