the feed chute, then slowly tilt it so that the coal flows in a
uniform stream through the hopper straight down over the
center of the riffle into all the slots, then into the riffle pans, one
half of the sample being collected in each pan. Under no
circumstances shovel the sample into the riffle or dribble into
the riffle from a small-mouthed container. Do not allow the
coal to build up in or above the riffle slots. If it does not flow
freely through the slots, shake or vibrate the riffle to facilitate
even flow.