What are Gulliver's thoughts on Engima about the floating island........?

Gulliver's thoughts were filled with joy because he saw signs of life and the possibility of rescue. He also felt astonishment at beholding an island in the air. 

I walked awhile among the rocks: the sky was perfectly clear, and the sun so hot, that I was forced to turn my face from it: when all on a sudden it became obscure, as I thought, in a manner very different from what happens by the interposition of a cloud.  I turned back, and perceived a vast opaque body between me and the sun moving forwards towards the island: it seemed to be about two miles high, and hid the sun six or seven minutes; but I did not observe the air to be much colder, or the sky more darkened, than if I had stood under the shade of a mountain.  As it approached nearer over the place where I was, it appeared to be a firm substance, the bottom flat, smooth, and shining very bright, from the reflection of the sea below.  I stood upon a height about two hundred yards from the shore, and saw this vast body descending almost to a parallel with me, at less than an English mile distance.  I took out my pocket perspective, and could plainly discover numbers of people moving up and down the sides of it, which appeared to be sloping; but what those people where doing I was not able to distinguish.

The natural love of life gave me some inward motion of joy, and I was ready to entertain a hope that this adventure might, some way or other, help to deliver me from the desolate place and condition I was in.  But at the same time the reader can hardly conceive my astonishment, to behold an island in the air, inhabited by men, who were able (as it should seem) to raise or sink, or put it into progressive motion, as they pleased.