When I typed ‘gambling’ in the toolbars of those four search engines, I got a introduction for one book about gambling, a link to online casino, a link to Wikipedia to explain what gambling is, and a link to a council on problem gambling. Actually, I would choose JSTOR which is introduced at last homework, because its article of gambling are based on survey or research, it looks like more credible.
I think that Invisible Web is interesting. It is a web, but we are hard to find it in the internet. So if we want to find Invisible Web, we have to find searchable databases containing invisible web pages in the course of routine searching in most searching in most general web directories.