Nelly was a good guy.
That was the conclusion Andrew had reached through their daily meetings. His experiments went better than expected every time, and the kind senior student had a lot to do with it.
Of course, Andrew's growing grasp of Transfiguration had helped too. Magic wasn't quite as magical as it seemed. It had developed step by step, following principles of its own; witches and wizards couldn't simply dream up whatever they wanted.
After nearly eighty hours of research over two weeks—every spare moment he had—Andrew finally had a rough understanding of what the Bubble-Head Charm could do.
The charm drew air from the surrounding area to the point where it was cast, then wove it together with magic. Fresh air continually filled the gaseous inner layer while carrying away the air the wearer exhaled. The transparent-looking shell was the crucial part, created by a magical effect akin to compression.
It could withstand powerful magical attacks. With Nelly's help—after Nelly had cast a Shield Charm—Andrew had tried all sorts of ways to break it and found only a few that worked. In use, it also completely shut out harmless environmental intrusions, and it provided a fair degree of insulation.
Giving it a snug fit and imitating its other functions wasn't especially hard. The real difficulty was compression.
Changing an object's size through Transfiguration was difficult. After working at it for ages, Andrew finally concluded that unless he developed a deeper understanding of Transfiguration or his magic grew stronger with age, the Bubble-Head Charm he made could only reach a third of the original's effectiveness.
"That's incredible... using Transfiguration to reproduce the Bubble-Head Charm's effects..."
Nelly was doing his best to comfort Andrew. He had taken quite a few Galleons—almost twice the agreed payment by now—and after spending all