Water bottles can either be vertical, like Kemmerer bottles, or horizontal, like the Van Dorn bottles. Both are designed for grabbing a sample of water at a known depth in a body of water. Vertical bottles are best for sampling wells or other areas where you need a “narrow” sample. If you were sampling where a river empties into a lake, you would probably want to use a Kemmerer to take samples where the river first enters the lake, where the river water mixes, and again where there is only lake water. Horizontal bottles are best for very shallow water or if lakes are well stratified. If there is a distinct thermocline or chemocline, a horizontal Van Dorn bottle can get a fairly accurate sample above, below and right at the line where the water mixes.