What is the shape of an hour-glass with the following characteristics:
1)it will be used with water, not sand
2)two identical glass shapes are fused with a tiny hole connecting them and adequate venting to prevent any changes in air pressure from impeding water flow
3)the rate of flow through the hole between the top and bottom portion is proportional to water pressure
4)the horizontal cross section of the hourglass is a circle at every level, and it is symmetric around a vertical axis
5)the water level is proportional to the time elapsed