The sounding machine of which the annexed figure is a representation, is intended for use at considerable depths while the ship is proceeding on her course. It is therefore necessarily capable of indicating the depths independently of the length of sounding line or wire used. It possesses the further great practical advantage, that when it has served its purpose once it is immediately available for another sounding.

It indicates primarily the extent to which a given volume of air at atmospheric pressure has been condensed by the column of water to which it has been subjected. As the law of the compression of air is known, the depth reached by the instrument is at once deduced.