Domestic slavery conspired with these evils to prevent the healing power of nature from closing these yawning wounds. Gibbon estimates the number of slaves throughout the empire, in its latter days, at a number equal to that of the freemen; in other words, one half of the whole inhabitants were in a state of servitude;[17] and as there were 120,000,000 souls under the Roman sway, sixty millions were in that degraded condition. There is reason to believe that the number of the slaves was still greater than this estimate, and at least double that of the freemen; for it is known by an authentic enumeration, that, in the time of the Emperor Claudius, the number of citizens in the empire was only 6,945,000 men, who, with their families, might amount to twenty millions of souls; and the total number of freemen was about double that of the citizens.[18] In one family alone, in the time of Pliny, there were 4116 slaves.[19] But take the number of slaves, according to Gibbon's computation, at only half the entire population, what a prodigious abstraction must this multitude of slaves have made from the physical and moral strength of the empire! Half the people requiring food, needing restraint, incapable of trust, and yet adding nothing to the muster-roll of the legions, or the persons by whom the fixed and immovable annual taxes were to be made good! In what state would the British empire now be, if we were subjected to the action of similar causes of ruin? A vast and unwieldy dominion, exposed on every side to the incursions of barbarous and hostile nations, daily increasing in numbers, and augmenting in military skill; a fixed taxation, for which the whole free inhabitants of every municipality were jointly and severally responsible, to meet the increasing military establishment required by these perils; a declining, and at length extinct, agriculture in the central provinces of the empire, owing to the deluge of cheap grain from its fertile extremities, wafted over the waters of the Mediterranean; multitudes of turbulent freemen in cities, kept quiet by daily distribution of provisions at the public expense, from the imperial granaries; and a half, or two-thirds, of the whole population in a state of slavery--neither bearing any share of the public burdens, nor adding to the strength of the military array of the empire. Such are the discoveries of modern philosophy, as to the causes of the decline and ultimate fall of the Roman empire, gleaned from a few facts, accidentally preserved by the ancient writers, apparently unconscious of their value! It is a noble science which, in so short a time, has presented such a gift to mankind.