destroyed in 802; by 807 the invaders had penetrated inland as far as Roscommon, and four years later they had made their way round the west coast of Ireland as far as Cork. In 821 the Howth peninsula was plundered and during the next few years the rich monasteries of North Ireland were destroyed. By the year 834 the Northmen had visited nearly the whole of the island and no place was safe from their raids. About this time there came a change in the character of the attacks in that large fleets began to anchor in the loughs and harbours and estuaries with which the coast of Ireland abounds. Thence they made lengthy raids on the surrounding country, often staying the whole winter through, instead of paying summer visits only as they had done hitherto. At the same time they often strengthened their base by the erection of forts on the shores of the waters in which they had established themselves.
When the Viking raids were resumed in England in 835 it is fairly certain that they were the work of Danish and not of Norwegian invaders. The Norsemen had found other fields of activity in Ireland, while the Danes who had already visited the chief estuaries of the Frankish coast now crossed to England. At first their attacks were directed towards the southern shores of Britain, but by 841 they had penetrated into Lindsey and East Anglia. London and Rochester were sacked in 842. In 851 the Danes wintered in Thanet and four years later they stayed in Sheppey. The Danish fleet in this year numbered some 350 ships. It was probably this same fleet, somewhat reduced in numbers, which in 852 sailed round Britain and captured Dublin. With the winterings in Thanet and Sheppey the Viking invasions of England had reached the same stage of development as in Ireland. We have passed from the period of isolated raids to that of persistent attacks with a view to permanent conquest.
The mainland of Western Europe was also exposed during these years to attacks of a twofold character. In the first place, trouble arose on the boundary between Southern Denmark and Frankish territory owing to the desire of the Danish kings to extend their authority southward: in the second, constant raids were made along the whole of the shores of Europe from Frisia to Aquitaine.
The friction between the Danes and their neighbours on the south was continuous through the last years of the eighth and the greater part of the ninth century. Charles the Great by his campaigns against the Saxons and Nordalbingians had advanced towards the Danish boundary on the Eider, and the Danes first gave offence in 777 when their king Sigefridus (Old Norse Sigurðr) gave shelter to the Saxon patriot Widukind. Gradually the Frankish power advanced, and in 809 a fort was established at Itzehoe (Esesfeld) on the Stör, north of the Elbe. The Danes also made advances on their side and in 804 their king Godefridus (O.N. Guðröðr) collected a fleet and army at Slesvík (Schleswig). In 808 after a successful
