trig

I gathered the different pieces of the spell in my head, linked them together, infused them with a moderate effort of will, and then with a murmured word released that energy down into the hair in my fingers. Then I popped the hair into my mouth, broke the chalk circle with a brush of my foot, and rose.
Harry always used an object as the indicator for his tracking spells—his amulet, a compass, or some kind of pendulum. I hadn’t wanted to hurt his feelings, but that kind of thing really wasn’t necessary. I could feel the magic coursing through the hair, making my lips tingle gently. I got out a cheap little plastic compass and a ten-foot length of chalk line. I set it up and snapped it to mark out magnetic north.
Then I took the free end of the line and turned slowly, until the tingling sensation was centered on my lips. Lips are extremely sensitive parts of the body, generally, and I’ve found that they give you the best tactile feedback for this sort of thing. Once I knew which direction Thomas was, I oriented the chalk line that way, made sure it was tight, and snapped it again, resulting in an extremely elongated V shape, like the tip of a giant needle. I measured the distance at the base of the V. Then
I turned ninety degrees, walked five hundred paces, and repeated the process.
Promise me you won’t tell my high school math teacher about it, but after that I sat down and applied trigonometry to real life.
The math wasn’t hard. I had the two angles measured against magnetic north. I had the distance between them in units of Molly-paces. Molly-paces aren’t terribly scientific, but for purposes of this particular application, they were practical enough to calculate the distance to Thomas.
Using such simple tools, I couldn’t get a measurement precise enough to know which door to kick down, but I now knew that he was relatively nearby —within four or five miles, as opposed to being at the North Pole or something. I move around the city a lot, because a moving target is a lot harder to hit. I probably covered three or four times that on an average day.
I’d have to get a lot closer before I could pinpoint his location any more precisely than that. So I turned my lips toward the tingle and started walking.
Thomas was in a small office building on a big lot.


Brief Cases, Bombshells, Page 228-229