Journey to the Center of the Earth

In Edinburgh, Scotland, in 1880, Professor Sir Oliver Lindenbrook (James Mason), a geologist at the University of Edinburgh, is given a piece of volcanic rock by his admiring student, Alec McEwan (Pat Boone). Deciding that the rock is unusually heavy, Lindenbrook, mostly thanks to the carelessness of his lab assistant, Mr. Paisley (Ben Wright), discovers a plumb bob inside bearing a cryptic inscription. Lindenbrook and Alec discover that it was left by a scientist named Arne Saknussemm, who had, almost 300 years earlier, found a passage to the center of the Earth by descending into SnÃ¦fellsjÃ¶kull, a volcano in western Iceland. After translating the message, Lindenbrook immediately sets off with Alec to follow in the Icelandic pioneer's footsteps.
Professor GÃ¶teborg of Stockholm (Ivan Triesault), upon receiving correspondence from Lindenbrook regarding the nature of the message, opts to try to reach the Earth's center first. Lindenbrook and McEwan chase him to Iceland. There, GÃ¶teborg and his assistant kidnap and imprison them in a cellar. They are freed by a local Icelander, Hans Bjelke (PÃ©tur Ronson), and his pet duck Gertrud. They find GÃ¶teborg dead in his room at an inn. Lindenbrook finds some potassium cyanide crystals in GÃ¶teborg's goatee and concludes that he has been murdered.
GÃ¶teborg's widow, Carla (Arlene Dahl), who initially believed Lindenbrook was trying to capitalize on the work of her deceased husband, learns the truth from her husband's diary. She provides the equipment and supplies GÃ¶teborg had gathered, including much sought after Ruhmkorff lamps, but only on condition that she go along. Lindenbrook grudgingly agrees, and the four explorers and the pet duck are soon journeying into the Earth.
They are aided by marks left by Arne Saknussemm showing the path he took 300 years before. However, they are not alone. GÃ¶teborg's murderer, Count Saknussemm (Thayer David) thinks that, as Arne Saknussemm's descendent, only he has the right to be there. He trails the group secretly with his servant. When Alec becomes separated from the others, he almost trips over Saknussemm's dead servant. When Alec refuses to be his replacement, Saknussemm shoots Alec in the arm. Lindenbrook locates Saknussemm from the reverberations of the sound of the shot, and sentences him to death. However, no one is willing to execute him, so they reluctantly take him along.
The explorers eventually come upon a subterranean ocean. They construct a raft from the stems of giant mushrooms to cross it but, not before narrowly escaping a family of dimetrodons. Their raft begins circling in a mid-ocean whirlpool. The professor deduces that this must be the center of the Earth because the magnetic forces from north and south meeting there are strong enough to snatch away even gold in the form of wedding rings and tooth fillings. Completely exhausted, they reach the opposite shore.
While the others are asleep, a hungry Saknussemm catches and eats Gertrud the duck. When Hans finds out, he rushes at the count, but is pulled off by Lindenbrook and McEwan. Reeling back, Saknussemm loosens a column of stones and is buried beneath them. Right behind the collapse, the group comes upon the sunken city of Atlantis. They also find the remains of Arne Saknussemm. The hand of his skeleton points toward a passage to the surface. They decide that they will have to break a giant rock blocking their way using gunpowder left by Saknussemm. A giant chameleon attacks them, but it is killed by released lava. They climb into a large sacrificial altar bowl which floats atop the lava to the passage, and are driven upward at great speed by the lava, reaching the surface through a volcanic shaft. Lindenbrook, Carla and Hans are thrown into the sea, while Alec lands naked in a tree in the orchard of a convent.
When they return to Edinburgh, they are hailed as national heroes. Lindenbrook, however, declines the accolades showered upon him, stating that he has no proof of his experiences, but he encourages others to follow in their footsteps. Alec marries Lindenbrook's niece Jenny (Diane Baker), and Lindenbrook and Carla, having fallen in love, kiss.

Question: How does the professor know they are at the center of the earth?