What is the triangle thing in the fireplace?

Answer

It is called a trammel.  It has hooks on it to hold pots over the fire.  The hooks are at different heights so that the pots can be kept at different distances from the fire.  That way, a woman could choose at what temperature she wanted the food to cook.  The closer to the fire, the hotter the pot would get.  It was also useful in that it could swing out so that she could tend her cooking without getting so close to the fire.  When she was finished stirring or checking for doneness, she could swing the pot back into the fireplace over the fire.