Fifth
That to renew the air, it is not necessary the Vessel should appear above water; but approaching the Surface two tubes project, through one of which the mephitic air is discharged, through the other fresh air is drawn into the Vessel, which operation can be performed in 3 or 4 minuets, to continue again three hours under water; in this manner a crew can conseal themselves under water during the day, on renewing the air 4 times, hence might lie many days in the Neighbourhood of an enemy unperceived,