It had been a couple of weeks since Harold had spoken to Rose, with the business of measurements and whatnot - though he had to admit as the term wore on, he'd had less time to plan out Hamlet and more time had to be spent getting the students up to snuff; owing to the exacting nature of the discipline, he'd arranged for some preparatory examinations for the spring term, prior to the O.W.L. and N.E.W.T. examinations themselves, and a few of the students had wanted - or, rather, warranted - some extra practice sessions.
But he found himself pleasantly surprised with an unexpectedly spare evening, and while ruminating over a particularly fine cup of Keemun, he resolved to visit the library and look up that tome on Tudor-era arithmantic research that he'd heard about.
The library was in rather an unexpected state of affairs; Harold could muster nothing more than a surprised, "Well..." upon seeing the place, with all the shelves stripped bare of their literary residents - though he couldn't deny how wonderfully clean all the shelves were. It was as though a fresh coat of paint and varnish had been laid across the place, though it smelled considerably less awkward.
As he approached the Restricted section - wondering if he should find any books on any shelves there, like as not he wouldn't, he found Rose muttering to herself - one thirty-sixth times the number of... and he'd vaguely heard the words Rhaetian and Predennek, and the calculation just came to him.
Part of him wanted to say nothing and just let it pass, but a larger part of him wanted to help.
"One thirty-sixth times the Phi quotient of the runes' force, plus thirteen - I believe that's the formula for converting Rhaetian to Predennek, though I must admit that, well, it has been a while." He smiled, warmly. "I must say the shelves have never looked so clean since I've been at Hogwarts - even as a child."
He had a think for a moment, before sort of just speaking to himself. "Conversion of Rhaetian to Predennek... calculating the arithmantic outflow of the sympathetic union... I'd want a chronometer for that." Then, as if snapping himself out of a daydream he looked at Rose. "Would you happen to be in need of a chronometer? I have one up in the lab that I was showing to my class. I can pop up and fetch it if it would help."