From Alan Crease (with thanks):
Dictionary of Latin quotations, proverbs, maxims, and mottos, classical and medieval, Riley, Henry T.
Published London: H.G. Bohn 1866
[Sternĭtur, exănĭmisque tremens procumbit humi bos. VIRG. — "The ox is felled, and, quivering, lies expiring on the ground." Porson is said to have exclaimed, on letting Bos's Ellipses fall upon some volumes of Hume's History of England, "Procumbit Humi Bos!"]
I doubt if I could repeat the act and raise a laugh amongst a circle of my acquaintances. Not that I have a copy of Bos's Ellipses, in fact I'd never heard of it. It does seem to be a work of incredible erudition and painstaking research that went through at least 6 editions. It makes you wonder how these works were ever composed and brought to publication without the aid of computers and databases. And presumably, a lot if it by candlelight.