The method Vizet provided for using the Patronus Charm was best practiced underwater at the beginning.
Harry had a deep appreciation for that line of practice. When Lupin had taught him the Patronus Charm, he had used a similar method.
Once everything was ready, Cedric, Draco, and Harry followed the rope into the bookcase.
Only after Draco entered the water did he realize the lake bottom's water temperature was actually this low. No wonder Harry had been shivering so hard.
A bone-deep chill swept over them in an instant. Draco was not the only one who felt it; Cedric and Harry clenched their teeth as well.
They failed to withstand that bone-piercing cold. The three of them moved almost in unison, hunching over like boiled prawns and shaking violently like chaff in a sieve, unable to control themselves no matter what they did.
"Ex... Expec..."
Harry and the others tried hard to recite the incantation, but their teeth seemed completely out of control. The moment they opened their mouths, their teeth clacked together, making them unable to get out a complete spell.
After they began to adapt to the lake bottom's temperature and could just barely pronounce the incantation, they discovered that the currents surging around them were also interfering with their completion of the Patronus Charm.
The three of them stayed underwater for quite a while. Silver light appeared at Harry's chest, followed closely by Cedric's and Draco's.
Using the faint warmth produced at their chests, they hurriedly tugged on the rope and left the lake bottom.
After spending so long at the bottom of the lake, when they returned to the classroom and pressed their bodies against the cold floor, they actually felt a trace of warmth.
"D-did... did you see that!" Harry's teeth were still chattering. "That's