"a wild irregular people in military history who inhabit the Ukraine. They are now a regular militia." (citation in list description)
In the same book, under cavalry, is slightly more information (or "information"):
"Cossacks, are a half civilized people of Russia, forming several nations, having nearly the same habits and manners; their features are broad and flat, with a pair of small, fiery and piercing eyes, rather of a small stature, very robust, active and courageous; they manage their arms with astonishing dexterity, and are very skilful in taming wild horses: they form an irregular cavalry in the Russian service. The arms of the Cossack are a brace of pistols, a sabre, a musket and a lance 10 or 12 feet long, which is fastened to the wrist, and also to the right foot, by the action of which, it is thrust forward with great force and proves fatal to the foe. Their horses are said to be quite diminutive, but are mettlesome, can walk 5 miles an hour, and are remarkably swift. A Cossack is raised with his horse and becomes so well skilled in riding, that they will leap their horses from the steepest banks into a deep and rapid river, will traverse dry and burning sands, or cross a forest almost impervious and covered with snow. Nothing can elude their activity, escape their penetration or surprise their vigilance. Irreparable disgrace would attend the Cossack, whose negligence offered an advantage to the enemy—cowardice itself could not attach so fatal a stigma; and no instance of surprise is on record: These are the troops which could strike terror into the French legions."