This estimate is corroborated to some extent by the scale of Icelandic sea-miles (vikur) given in Troil’s _Letters on Iceland_ (1780), where they are represented as nine to a degree or equal to 6⅔ miles each. Exact correspondence is of course not to be expected in standards of measurement arrived at by so rough a method as the time occupied on an average voyage. Another ‘tylft’ capable of measurement is that given in the Greenland sailing directions attributed to Ivar Bardson. Here the distance so described is that between Reykjanes (lat. 63° 24′ N., long. 22° 40′ W.) and Snæfellsnes (lat. 64° 55′ 30″ N., long. 23° 59′ 40″ W.). Calculation gives the length of a rhumb-line between the two points as 73.54 miles, according to which a ‘vika’ would be about 6.12 miles, which once more justifies the assumption that something over six rather than five miles must be the correct measurement of this unit. If the line of reasoning has been correct so far, it follows that the average rate of speed on an Icelandic voyage under favourable conditions would be something over six knots. The next thing to ascertain is the highest speed possible under exceptionally favourable circumstances. Fortunately this point is also capable of determination. It is unnecessary, and probably misleading, to enter, as Mr. Babcock does, into calculations based on the speed of modern ships. In the saga of Olaf the Saint (see this saga in _Heimskringla_, §125), one Thorar Nefjolfson accomplished what was evidently regarded as a remarkable feat by sailing from Norway (Moeri) to Eyrarbakki in Iceland in the space of four days and four nights. There is in this case no ambiguity about the meaning of eight ‘dægra’, the period recorded, for Thorar himself refers to the fact that four nights previously he was with the King in Norway. The starting-point may safely be taken as Stad, which lay in the Söndmöre district, since we know from other sources that it was the usual place of departure for Iceland, as indeed its geographical position at the extremity of the westerly trend of the coast-line from Trondhjem would render inherently probable. The geographical position of Stad is 62° 11′ N., 5° 8′ E. The distance to Eyrarbakki (63° 51′ 45″ N., 21° 7′ W.) round the most southern point of Iceland (63° 23′ 45″ N., 19° 5′ 5″ W.) comes in round figures to about 730 nautical miles. This would represent a rate of about 7·6 knots, and though this is probably too little, as the course can hardly have been so direct and we know neither the precise place of departure nor the exact times of start and finish, we shall be safe in assuming that anything appreciably over eight knots was beyond the extreme powers of an Icelandic vessel.