Sift the sugar and mix the ground almonds (½ pound of shelled almonds will produce a little more than 2 cups of ground almonds).
Whip the egg whites stiff with the baking powder. Fold in the almond mixture. Continue to fold gently until smooth. The mixture should be stiff enough to hold its shape.
Form into small round balls and place about 1 inch apart on a greased cookie sheet covered with waxed paper. Bake in a slow oven preheated to about 275 degrees 20 minutes, or until lightly browned. Remove from oven. Invert the cookie sheet on a clean flat surface and lift off. Wet the back of the waxed paper with a cloth wrung out in very cold water. Macaroons will slip off easily.