‘Box’ valve (piston valve of square section), patented by Stölzel and Blühmel, seen from above (arrows with continuous tails show direction of airstream when valve is in normal position, arrows with broken tails show airstream when valve is depressed): a
‘Box’ valve (piston valve of square section), patented by Stölzel and Blühmel, seen from above (arrows with continuous tails show direction of airstream when valve is in normal position, arrows with broken tails show airstream when valve is depressed): a