from The Century Dictionary.
- noun In Rom. archæol., the channel or waterway of an aqueduct, usually of masonry and vaulted over. This was carried underground and on embankments, according to the level, but also for great distances on long arcades.
Sorry, no etymologies found.
Then I perceived very soon, that the authors who had worked upon this matter took the coloured rays upon the specus out of the prism; but none of them minded how they could get in ...
“Horrendum incultumque specus.” and the European merchants, when they come up here are glad to get away as soon as possible.
Travels in Morocco 2003
The specus or channel itself was, of course, constructed of masonry, generally of blocks of stone cemented together, and it was frequently, though not, it would appear always, lined with cement inside.
It will be seen that the specus or channel is 60 centimeters (or nearly 2 ft.) wide, and 1m. 57c.
= The same use of _ambiguum_ (which may be an Ovidian peculiarity) at _Met_ I 765-66 '_ambiguum_ Clymene precibus Phaethontis an ira/mota magis' and _Met_ XI 235-36 'est specus in medio, natura factus an arte/_ambiguum_, magis arte tamen'.
The Last Poems of Ovid 43 BC-18? Ovid
It consists of a row of large arches crossing the valley over which the water had to be carried, surmounted by a series of smaller arches, and these again by a series of still smaller ones, carrying the specus of the aqueduct.
Quare age, huc aditum ferens, perge linquere Thespiae rupis Aonios specus;
Eoque, _tempore_ interjecto, altius effossi specus, et contrahendae rursus _multitudini_ gladiatorum _spectaculum editur_, inditis pontibus pedestrem ad _pugnam_.
Tacitus and Bracciolini The Annals Forged in the XVth Century John Wilson Ross 1852
"In puteis est remedium, quale et crebri specus praebent: conceptum enim spiritum exhalant: quod in certis notatur oppidis, quae minus quatiuntur, crebris ad eluviem cuniculis cavata."
Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 — Volume 1 Alexander von Humboldt 1814
 In Phrygiâ -- juxta specus est Acherusia, ad manes, ut aiunt, pervius.
A New System; or, an Analysis of Antient Mythology. Volume II. (of VI.) Jacob Bryant 1759