37. From these sixty-six days, we must first deduct sixteen,
as Pertinax was murdered on the 28th of March, and Severus
most probably elected on the 13th of April (see Hist.
August. p. 65, and Tillemont, Hist. des Empereurs, tom. iii.
p. 393, Note 7). We cannot allow less than ten days after
his election, to put a numerous army in motion. Forty days
remain for this rapid march, and as we may compute about
eight hundred miles from Rome to the neighbourhood of
Vienna, the army of Severus marched twenty miles every day,
without halt or intermission.