diff --git a/solidify/overlap.c b/solidify/overlap.c
index 95b2a1a..cebf03c 100644
--- a/solidify/overlap.c
+++ b/solidify/overlap.c
@@ -312,12 +312,21 @@ static void rotate(struct matrix *m, double r)
 static void do_shift(struct matrix *m, int dx, int dy)
 {
 	m->b[0] += dx;
-	m->b[1] -= dy;
+	m->b[1] += dy;
 }
 
 
 static void shift(struct matrix *m, int dx, int dy, int dir)
 {
+	/*
+	 * Wheeling "up" in each quadrant shifts in the respective direction,
+	 * wheeling "down" in the opposite direction.
+	 *
+	 * No rule without exception: we treat the "down" quadrant like the
+	 * "up" quadrant, because it would be extremely counter-intuitive to
+	 * wheel "up" to move "down".
+	 */
+
 	if (dx > 0 && dy < dx && dy > -dx)
 		do_shift(m, dir, 0);
 	if (dx < 0 && dy < -dx && dy > dx)
@@ -325,7 +334,7 @@ static void shift(struct matrix *m, int dx, int dy, int dir)
 	if (dy > 0 && dx < dy && dx > -dy)
 		do_shift(m, 0, dir);
 	if (dy < 0 && dx < -dy && dx > dy)
-		do_shift(m, 0, -dir);
+		do_shift(m, 0, dir);	/* exception ! */
 }
 
 
@@ -363,14 +372,14 @@ static gboolean scroll_event(GtkWidget *widget, GdkEventScroll *event,
 		if (center)
 			shift(&s->a->m, dx, dy, 1);
 		else
-			rotate(&s->a->m, -r);
+			rotate(&s->a->m, dx > 0 ? r : -r);
 		draw_image(darea, s, osd);
 		break;
 	case GDK_SCROLL_DOWN:
 		if (center)
 			shift(&s->a->m, dx, dy, -1);
 		else
-			rotate(&s->a->m, r);
+			rotate(&s->a->m, dx > 0 ? -r : r);
 		draw_image(darea, s, osd);
 		break;
 	default: