Deja Dead Page 0,57

That’s a spike that’s left at the edge of the bone where it finally breaks. Also, individual tooth marks may be left on the cut surface.”

I dug Gagnon’s radius back out, found a partial false start on the breakaway spur, and angled the fiber-optic beam across it.

“Here, look at this.”

He leaned over and squinted into the eyepiece, fiddling with the focus knob.

“Yeah. I see it.”

“Look at the kerf floor. What do you see?”

“It looks lumpy.”

“Right. Those lumps are bone islands. They mean that the teeth were set at alternating angles from the saw blade. That kind of set causes a phenomenon known as blade drift.”

He raised his head from the microscope and looked at me blankly. The eyepiece had left double rings, grooving his face like that of a swimmer with tight goggles.

“When the first tooth cuts into the bone it tries to align itself to the plane of the blade. It seeks the midline, and the blade goes along with that. When the next tooth enters the bone it tries to do the same thing, but it’s set in the opposite direction. The blade readjusts. This happens as each tooth comes along, so the forces acting on the blade change constantly. As a result, it sort of drifts back and forth in the kerf. The more set to the teeth, the more the blade is forced to drift. A very wide set causes successive teeth to drift so much that material is actually left in the midline of the kerf. Bone islands. Lumps.”

“So they tell you the teeth were angled.”

“Actually, they tell more than that. Since each directional change of a tooth is caused by the introduction of a new tooth, the distance between these directional changes can tell the distance between teeth. Since islands represent the widest points of bone drift, the distance from island to island is equal to the distance between two teeth. Let me show you something else.”

I withdrew the radius and inserted the ulna so that the cut surface at its wrist end was illuminated, then I stepped back from the microscope.

“Can you see those wavy lines on the cut surface?”

“Yeah. Looks kind of like a washboard, only curvy.”

“That’s called harmonics. Blade drift leaves those peaks and valleys on the wall of the cut just as it leaves bone islands on the floor. The peaks and islands correspond to the wide points in drift; the valleys and narrow aspects of the floor correspond to the points in drift when the blade is closest to the midline.”

“So you can measure these peaks and valleys like you do the islands?”

“Exactly.”

“How come I don’t see anything farther down in the kerf?”

“Drift occurs mostly at the beginning or end of a cut, when the blade is free, not embedded in bone.”

“Makes sense.” He looked up. The goggles were back.

“Can you tell anything about direction?”

“Of blade stroke or blade progress?”

“What’s the difference?”

“Direction of stroke has to do with whether the blade is cutting on the push or the pull. Most Western saws are designed to cut on the push. Some Japanese saws cut on the pull. Some can cut on both. Progress has to do with the direction the blade moves through the bone.”

“Can you determine that?”

“Yup.”

“So what do you have?” he asked, rubbing his eyes and trying to look at me at the same time.

I took my time answering, kneading the small of my back, then reaching for my clipboard. I flipped through my notes, selecting relevant points.

“Isabelle Gagnon’s bones have quite a few false starts. The kerfs measure about .05 inches in width and have floors that, in most cases, have some dip to them. Harmonics are present, and there are bone islands. Both are measurable.” I flipped a page. “There’s some exit chipping.”

He waited for me to go on. When I didn’t, he said, “What does all that mean?”

“I think we’re dealing with a handsaw with alternating set teeth, probably a TPI of 10.”

“TPI?”

“Teeth per inch. In other words, tooth distance is about a tenth of an inch. The teeth are chisel type, and the saw cuts on the push stroke.”

“I see.”

“The blade drift is extreme and there’s a lot of exit chipping, but the blade seems to cut efficiently by chiseling the material clear. I think it’s probably a saw designed like a very large hacksaw. The islands mean the set has to be pretty wide, to avoid binding.”

“That narrows it to what?”

I was pretty sure I knew what had made the cuts, but wasn’t