act. # 4

QUESTION`S .


☻what are the advantage and distadvantage of using search engines ? 
Using search engines does involve a learning curve. Many beginning Internet users, because of these disadvantages, become discouraged and frustrated.


☻compare and contrast individual search engines and search metasearch engines?
Search engines obtain their data from information contained on individual web pages. This information can be gathered manually or automatically by programs commonly called spiders or crawlers….. some search engines use a combination of both. Once the data has been collected it is added to an index which is simply a huge database.
When you enter a keyword or phrase into a search engine, an integrated piece of software searches the index database to retrieve the relevant data. Even though these advanced spider and crawler programs work at lightning speed,
☻when it is appropriate to use a search engine? when its appropriate to use a search /subject directory?  
Subject Directories tell us what is available by subject and where to find it. Also called Subject Trees, Subject Indexes, Web Catalogues or Web Guides, they provide a hierarchical view of subject categories. Like the subject catalogue in the library, they are usually the work of people, often expert in the subject area, who can evaluate the content of an Internet site and classify it.

☻what is an invisible web or deep web?
The Deep Web (also called Deepnet, the invisible Web, DarkNet, Undernet, or the hidden Web) refers to World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines.
Mike Bergman, credited with coining the phrase,^[1] has said that searching on the Internet today can be compared to dragging a net across the surface of the ocean: a great deal may be caught in the net, but there is a wealth of information that is deep and therefore missed. Most of the Web's information is buried far down on dynamically generated sites, and standard search engines do not find it. Traditional search engines cannot "see" or retrieve content in the deep Web – those pages do not exist until they are created dynamically as the result of a specific search. The deep Web is several orders of magnitude larger than the surface Web
 
☻how do you find an invisible web?
The "visible web" is what you can find using general web search engines. It's also what you see in almost all subject directories. The "invisible web" is what you cannot find using these types of tools. 


☻why are these web pages now available on search engines or subject directories?
The exact number of webpages on the Internet is unknown, but a July 2000 study by the Internet Software Consortium put the number of domain names — such as beyondbooks.com or nasa.gov — at nearly 100,000,000. The search engine Google now claims to sift through more than one billion webpages. In addition to the sheer vastness of the Web, its pages appear and disappear daily. Nor is the Web arranged in any alphabetical, chronological, logical, or rational order. There is no centralized registration for pages, and there is no cataloging similar to that in a library.