
1) Three-book Newton work
Found on
https://www.crosswordclues.com/clue/principia
[alga] Principia is a genus of alga that has been placed in the coralline stem group on the basis of its slightly differentiated thallus; it forms an `intermediate` between Hortonella, Neoprincipia and Archaeolithophyllum.{reflist} ...
Found on
http://en.wikipedia.org/wiki/Principia_(alga)

• (n. pl.) First principles; fundamental beginnings; elements; as. Newton`s Principia.
Found on
http://thinkexist.com/dictionary/meaning/principia/

HQ building.
Found on
http://www.encyclo.co.uk/local/20764

The name for the central headquarters building in an army camp. In Republican times in temporary camps was connected with the praetorium, whereas later in permanent camps they were distinct buildings. The literal, administrative and religious centre of the later Roman forts, situated in the centre of the latera praetorii, fronting onto the via prin…...
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http://www.encyclo.co.uk/visitor-contributions.php
Prin·cip'i·a noun plural [ Latin
principium . See
Principle .] First principles; fundamental beginnings; elements; as. Newton's
Principia .
Found on
http://www.encyclo.co.uk/webster/P/162

(Latin) building for the administrative offices of a Roman fort; the principia could hold records, storerooms and the soldiers` pay.
Found on
https://www.encyclo.co.uk/local/10135

headquarters building of a Roman fort
Found on
https://www.encyclo.co.uk/local/20434

headquarters building The literal, administrative and religious centre of the later Roman forts, situated in the centre of the latera praetorii, fronting onto the via principalis and facing down the length of the via praetoria, the building was thus visible as soon as one entered the porta praetoria. The building consisted of three distinct parts; ...
Found on
https://www.encyclo.co.uk/local/20768
No exact match found.