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The first so-called encyclopedist, supposedly, was Roman natural philosopher Pliny the Elder and his circa 77 Naturalis Historia, which purports to cover the entire field of ancient knowledge, based on the best authorities available. 
In 1710, English writer, scientist, and priest John Harris published the two-volume Lexicon Technicum, or A Universal English Dictionary of Arts and Sciences, which contained articles by such contributors as Isaac Newton.
In 1728, English globe maker apprentice Ephraim Chambers wrote a very popular two-volume Cyclopedia, which went through multiple editions and awakened publishers to the enormous profit potential of encyclopedias.
In 1732, German bookseller and publisher Johann Zedler published the 64-volume Grosses Universal-Lexicon (published 1732–1759), the first encyclopedia to include biographies of reactive people.
In 1751, editors Denis Diderot (IQ=165) (5394 articles) and Jean D'Alembert (IQ=185) (1309 articles) headed the writing of the Encyclopedie (1751-1772), which contained articles written by a group of about 100 French authors, including Jean Rousseau (IQ=150) (344 articles), Voltaire (IQ=195) (26 articles), etc., who latter came to be called “encyclopedists”.  The D’Alembert-Didertot Encyclopedie (1751-1772) is said, according to French collective intelligence theorist Pierre Levy, to mark “end of an area in which a single human being was able to comprehend the totality of knowledge” (see: "last persons to know everything"). 
In 1768, the Encyclopedia Britannica was launched by Irishmen bookseller and printer Colin Macfarquhar and engraver Andrew Bell, conceived as a conservative reaction to Diderot’s Encyclopédie, which was widely viewed as heretical.
In 1807, English polymath Thomas Young (IQ=200), under the direction and encouragement of Benjamin Thompson, finished compiling his famous two-volume Lectures on Natural Philosophy and the Mechanical Arts, wherein famously the modern definition of “energy” was first introduced. 
In 1917, the World Book Encyclopedia was launched (conceived by whom?), with an eight volume first edition.
In 1995, American mathematician, physicist, and astronomer (turned mathematics and science encyclopedist) Eric Weisstein (BA (1990) physics (Cornell); MS (1993) and PhD (1996) in planetary astronomy (CalTech)), starting from a 200-page Microsoft Word document (a collection of mathematical facts that he had been accumulating since his teenage years), uploaded it to his webspace at Caltech, under the title Eric's Treasure Trove of Sciences, has gone on to write over 17,000+ articles on science and mathematics in his MathWorld (13,000+) and ScienceWorld (4,000+) sites; some of which has been published by CRC Press in book format titled, the CRC Concise Encyclopedia of Mathematics.  His MathWorld is equivalent to more than 4,000 printed pages of encyclopedic material; ScienceWorld current stands at the equivalent of about 1,000 printed pages. 
In 2001, American internet entrepreneur (turned encyclopedia project head) Jimmy Wales, having previous making a learned attempted at launching Nupedia (2000-2003), a so-called expert written encyclopedia, famously launched the community-written Wikipedia, which has gone on to revolutionize the way encyclopedias are written.
In 2004, American hedge fund manager (turned video tutorial encyclopedist) Salman Kahn (BS mathematics, BS electrical engineering, MS electrical engineering (MIT), MBA (Harvard Business School)), starting from a tutoring request from his cousin, Nadia, working from a small office in his home, via video upload (using using Yahoo!'s Doodle notepad), founded the intuitive Khan Academy, from which he has personally produced over 2600 tutorial videos elucidating a wide spectrum of academic subjects, tending to focus on mathematics and the sciences. Bill Gates (IQ=175), notable himself for having read the entire World Book Encyclopedia by age nine, who funds Kahn's project, commented on him: "I'd say we've moved about 160 IQ points from the hedge fund category to the teaching-many-people-in-a-leveraged-way category. It was a good day his wife let him quit his job." (a hedge fund manager position he quit in 2009 to devoted more time to his project).
In 2005, Google Books was launched, which, when combined with open wiki writing and key word search technology, significantly impacted the methodology of knowledge acquisition, documentation, and re-organization employed by the encyclopedist, in the sense that in summarizing a topic in an encyclopedia manner, one must thoroughly pick through the supposed 130 million unique books in the world, in order to adequately cover a topic. 
In 2006, Thermopedia.com (site) or an “A-to-Z Guide to Thermodynamics, Heat & Mass Transfer, and Fluids Engineering Online” (trademarked : THERMOPEDIA™ in circa 2006) was launched, based on Geoffrey Hewitt and G.L. Shires 1997 International Encyclopedia of Heat and Mass Transfer. 
In 2006, American geologist and encyclopedist Cutler Cleveland, author of a number of book print energy dictionaries and encyclopedias, headed the launch of online Encyclopedia of Earth , with approximately 1,000 articles, which uses a one-person peer review sign-off process; which as of April 2008 had a listing of 700+ authors and 2,000+ articles.
In 2007, American electrochemical engineer and thermodynamicist Libb Thims, having previously tested the water as a writer at Wikipedia (2005-2007), penning some 180 articles, launched Hmolpedia, the result of the need to organize the extensive amount of material related to the three, essentially underground, interrelated fields of human chemistry, human physics, and human thermodynamics (and to some extent human mathematics); which as of 2011 had over 2,300 articles (nearly all written by Thims) and about 200+ members providing thread discussion feedback.
1. Encyclopedist – Wiktionary.
2. Levy, Pierre. (1997). Collective Intelligence: Mankind's Emerging World in Cyberspace (totality of knowledge, pg. 213). Basic Books.
3. (a) Young, Thomas. (1807). A Course of Lectures on Natural Philosophy and the Mechanical Arts: Volume One. Publisher.
(b) Young, Thomas. (1807). A Course of Lectures on Natural Philosophy and the Mechanical Arts: Volume Two. Publisher.
4. (a) Eric Weisstein (about) - MathWorld.
(b) Weisstein, Eric W. (2003). CRC Concise Encyclopedia of Mathematics. CRC Press.
(c) Eric W. Weisstein – Wikipedia.
5. (a) Encyclopedia Britannica (1911) - LoveToKnow1911.
(b) Image: from the May 1913 issue of National Geographic Magazine.
6. Hewitt, Geoffrey and Shires, G.L. (1997). International Encyclopedia of Heat and Mass Transfer (abs). CRC Press.
7. Google Books - Wikipedia.
8. (a) Salman Khan (educator) – Wikipedia.
(b) Salman Khan (2011 talk) – TED.com.
9. Eric Weisstein (about) – EricWeisstein.com.
10. (a) Encyclopedia – Wikipedia.
(b) Natural History (Pliny) – Wikipedia.