what are the two main reasons why the four basins never overflow

"the water is continually exhaled by the sun in the daytime, which effectually prevents their overflowing. Besides, as it is in the power of the monarch to raise the island above the region of clouds and vapours, he can prevent the falling of dews and rain whenever he pleases. For the highest clouds cannot rise above two miles, as naturalists agree, at least they were never known to do so in that country."