"Spring-piston airguns are one of the newer types of powerplants, coming into being about 1840. The first spring-piston guns were long guns that used volute springs instead of helically coiled springs to drive the piston. Take your belt and wind it into a tight coil on a flat surface like a table. Then pull on the center of the belt and it will rise, creating a conical spiral. The flat belt will now be an elongated spiral with a wide end and a narrow end. That is the shape of a volute spring.
Nachtrag (16.12.): Den zweiten Teil kann man hier finden.