Birger and Gerda explored the boat, making friends with some of the passengers, and then found seats with Erik on the forward deck, where they could see the wooded shore of the lake.  They passed many an island with its pretty villas peeping out among the green trees, and saw gay pleasure parties sailing or rowing on the quiet water.

In a short time the boat sailed slowly into the peaceful waters of the Soedertelje Canal.  This is the first of the short canals which form links between the lakes and rivers of Southern Sweden, thus making a shorter waterway from Stockholm to Goeteborg; and while the trip is about three hundred and seventy miles long, only fifty miles is actual canal, more than four-fifths of the distance being covered by lakes and rivers, with a fifty-mile sail on the Baltic Sea.

The principal difficulty in making this waterway across Sweden lay in the fact that the highest of the lakes is about three hundred feet above the sea level, and the boats have to climb up to it from the Baltic Sea, and then climb down to Goeteborg.  This climbing is accomplished by means of locks in the canals between the different lakes.  In some canals there is only one lock, but in others there are several together, like a flight of stairs.  There are seventy-six locks in all.

The boat sails into a lock and great gates are closed behind it.  Then water pours in and lifts the boat slowly higher and higher until it is on a level with the water in the lock above.  The gates in front of the boat are opened, it sails slowly into the next lock, the gates close behind it; and that lock in turn is filled to the level of the one above.

The boat now wound along between the high green banks of the Soedertelje Canal until it entered the first of the locks.  Birger and Erik ran to the rail to watch the opening and closing of the gates, and the lowering of the boat to the level of the Baltic Sea; but Gerda preferred to talk with some old women who came on board with baskets full of kringlor,—­ring-twisted cakes.

The cakes looked so good, and everyone who bought them seemed to find them so delicious, that at last she ran to ask her father for some money; and when the boat had passed the lock and was once more on its way, she presented a bagful of cakes to Birger and Erik.

“The Vikings had no such easy way as this of getting from Lake Maelar out into the Baltic Sea,” said Lieutenant Ekman, coming up to find the children, and helping himself generously to the kringlor.

Gerda looked at the gnarled and sturdy oaks that lined the banks of the canal like watchful sentinels.  “The Vikings must have loved the lakes and bays of the Northland,” she said.  “Perhaps they begged All-father Odin to let their spirits come back and make their homes in these trees.”

“No doubt they did,” replied her father, gravely enough.  “I suppose when the trees wave their arms and shake themselves so violently they are saying to each other something like this:  ’See how these good-for-nothing children go in good-for-nothing boats over this good-for-nothing ditch.’”