A drawback to the equipment pioneered by Deane and Siebe was the requirement for a constant supply of air pumped from the surface. This restricted the movements and range of the diver and was also potentially hazardous as the supply could get cut off for a number of reasons. Early attempts at creating systems that would allow divers to carry a portable breathing gas source did not succeed, as the compression and storage technology was not advanced enough to allow compressed air to be stored in containers at sufficiently high pressures. By the end of the nineteenth century, two basic templates for scuba, (self-contained underwater breathing apparatus), had emerged: open-circuit scuba where the diver's exhaust is vented directly into the water, and closed-circuit scuba where the diver's unused oxygen is filtered from the carbon dioxide and recirculated.
